Journal of Physiology (1992), 456, pp. 71-83 With 5 figures Printed in Great Britain
71
THE VENTILATORY RESPONSE TO CO2 OF THE PERIPHERAL AND CENTRAL CHEMOREFLEX LOOP BEFORE AND AFTER SUSTAINED HYPOXIA IN MAN
BY AAD BERKENBOSCH, ALBERT DAHAN, JACOB DEGOEDE AND IDA C. W. OLIEVIER From the Departments of Physiology and Anaesthesiology, University of Leiden, PO Box 9604, 2300 RC Leiden, The Netherlands
(Received 11 December 1991) SUMMARY
1.The ventilatory response to sustained hypoxia is characterized by a fast increase due to the peripheral chemoreceptors followed by a slow decline. The mechanism of this decline is unknown. 2. To investigate the characteristics of the ventilatory response to sustained hypoxia ten healthy subjects were exposed to two consecutive periods of isocapnic hypoxia (arterial saturation 78%) separated by a 5 min exposure to isocapnic normoxia. 3. The acute hypoxic response to the second exposure to hypoxia (mean increase in ventilation + s.E.M., 7-2 + 08 1 min') was significantly depressed (P = 004) compared to the first one (9 5 + 1-3 1 min-). 4. To investigate whether this depression was due to central or-peripheral effects or both we measured, in the same ten subjects, the normoxic ventilatory response to CO2 before and after a period of 25 min of hypoxia using the technique of dynamic end-tidal forcing. 5. Each response was separated into a fast peripheral and slow central component characterized by a CO2 sensitivity, time constant, time delay and an off-set. 6. A total of thirty-six prehypoxic and thirty posthypoxic responses were analysed. The ventilatory CO2 sensitivities of the peripheral and central chemoreflex loops and the overall off-set (apnoeic threshold) after 25 min of hypoxia were somewhat larger than their prehypoxic values, but this effect was not significant. 7. We argue that the hypoxic ventilatory decline in man is due to a change in the off-set of the peripheral chemoreflex loop. INTRODUCTION
In adult human beings the ventilatory response to isocapnic sustained hypoxia is a rapid increase in ventilation (VI) followed by a slow decline (Weiskopf & Gabel,
1975; Weil & Zwillich, 1976; Kagawa, Stafford, Waggener & Severinghaus, 1982; Easton, Slykerman & Anthonisen, 1986; Nishimura, Suzuki, Nishiura, Yamamoto, MS 9960
72
A. BERKENBOSCH AND OTHERS
Miyamoto, Kishi & Kawakami, 1987; Khamnei & Robbins, 1990). The rapid increase in ventilation is due to the peripheral drive from the carotid bodies. The mechanism of the slow hypoxic ventilator decline (HVD) is not known. It has been shown recently that complete recovery from a sustained hypoxic episode requires up to an hour of room air breathing (Easton, Slykerman & Anthonisen, 1988; Khamnei, 1989). In man the HVD is related to the magnitude of the initial stimulation of ventilation with hypoxia. When the peripheral drive is increased with almitrine the magnitude of 1EVD is also increased (Georgopoulos, Walker & Anthonisen, 1989b). The reverse is true when the peripheral drive is decreased by somatostatin (Filuk, Berezanski & Anthonisen, 1988). These observations led Robbins' group in Oxford to suggest that the peripheral drive from the carotid bodies adapts during sustained hypoxia. They performed experiments in which additional hypoxic pulses were given during HVD and showed that the peripheral sensitivity to hypoxia diminished (Basoom, Clement, Cunningham, Painter & Robbins, 1990). To obtain more information about a possible adaptation of the peripheral chemoreceptors we examined the effect of an episode of sustained hypoxia on the ventilatory response of the peripheral chemoreflex to CO2. METHODS
Subjects Ten subjects (aged 22-27 years; 6 males, 4 females), who gave their informed consent, took part in the experimental protocol approved by the Leiden University Ethics Committee. They were all naive to respiratory physiology and unaware of the purpose of the study. They were healthy and had no history of cardiovascular or respiratory disease. Each subject was familiarized with the experimental set-up and procedure before the study started. All subjects were asked to refrain from stimulant and depressant substances at least 12 h prior to the experiments.
Experimental design To study the dynamics of the response to hypoxia or carbon dioxide we used the technique of 'dynamic end-tidal forcing' (DEF). With this technique we are able to perform steps in the endtidal P.2 (PET 02) at a background of constant end-tidal Pco, (PET, co2) or steps in PET CO at a constant PETO by manipulating the inspired carbon dioxide and oxygen concentrations independently of the ventilatory response (Swanson & Bellville, 1975; Dahan, DeGoede, Berkenbosch & Olievier, 1990). After arrival at the laboratory a subject rested for 45 min in a comfortable chair. Subsequently a face mask was fitted and the experiment started. The subjects were encouraged to read and/or listen to music through headphones. Each subject participated in two protocols and was studied on 4 days. Protocol I (02 experiments). Experiments consisted of isocapnic steps into and out of hypoxia. After a 5-10 min period of steady-state ventilation during which the PET CO was slightly raised above resting values the PET 02 was rapidly lowered to attain arterial 02 saturation (Sa, 02) of 80%. The Sa, 02 was kept constant at this value for 25 min after which the PET was increased in a wPsket on02 stepwise fashion to the normoxic value. After 5 min of normoxia, hypoxia was reintroduced lasting for another 25 min (Fig. 1). Protocol II (CO2 experiments). Experiments consisted of two parts. In the first part a C02 run was performed. Such a run started with a 5-10 min period of 'steady-state' ventilation during which the PPET, COwa was slightly raised above resting values (5-5-6-2 kPa among subjects). The PET,
was
maintained constant at a normoxic level (14-5 kPa). The PET, Co2 was then increased about 1-1.5 kPa in a stepwise manner, maintained constant for approximately 7 min after which it was returned to its original value and kept there for another 7 min. Subsequently the subject was freed from the face mask and allowed to rest for 30 min. Then followed the second part of the experiment. After fitting the face mask again the PET C02 and PET 02 were brought to the same value as in the
VENTILATORY RESPONSE TO CO2 AFTER HYPOXIA
73
previous CO2 run. When the ventilation reached a steady-state the PET, 0was lowered to about 6 kPa. The inspired oxygen concentration was manipulated to maintain an arterial oxygen saturation (Sa. 0) of about 80 % throughout the hypoxic exposure which lasted 25 min. PET, CO2 was kept constant during this period. Subsequently the PETO2 was returned to the normoxic value and after 5 min a CO2 run as in the first part of the experiment was performed (Fig. 1). PETCO2 I
Protocol I
PET,
I
25
2
I
5
25min
Protocol II
PETCO2| PET 02
Time (min)
. 2
7
7
I 2
I 25
I 5
I| 7
7
Fig. 1. Experimental design of protocol I (02 experiments) and protocol II (CO2
experiments). All participants performed protocol II twice in one morning session and returned for a second session on another day. One subject became ill and missed the experiments of protocol I which were performed twice on different morning sessions. The order of the protocols was random.
Apparatus The subjects were seated and an oronasal face mask was fitted. They were instructed to breathe through their mouths to minimize changes in airway resistance during the experiment. The airway gas flow was measured with a pneumotachograph (Fleisch no. 3, Switzerland) connected to a differential pressure transducer (Hewlett Packard model 270, USA) and electronically integrated to yield a volume signal. This signal was calibrated with a motor-driven piston pump (stroke volume 1000 ml, at a frequency of 20 min-). Corrections were made for the changes in gas viscosity due to changes in 02 concentrations of the inhaled gas mixture. The pneumotachograph was connected to a T-piece. One arm of the T-piece received a gas mixture with a flow of 40 1 min-' from a gas mixing system, consisting of three mass flow controllers (Bronkhorst High Tech BV, F202/F203, The Netherlands) with which the flow of 02, CO2 and N2 could be set individually at a desired level. A PDP 11/23 computer provided control signals to the mass flow controllers, so that the composition of the inspiratory gas mixture could be adjusted to force the PET CO and PET 0 to follow a specific pattern in time. The 02 and CO2 concentrations of the inspired and expired gasses were measured with a gas monitor (Datex Multicap, Finland). A pulse oximeter (Nellcor 100, USA) continuously measured arterial oxygen saturation via a finger probe. All signals were recorded on a strip-chart recorder, digitized and processed by computer. The tidal volume, inspiratory time, expiratory time, respiratory frequency, inspired minute ventilation, percutaneous arterial oxygen saturation, end-tidal 02 and end-tidal CO2 tensions were stored on a breath-to-breath basis.
Data analysis The experiments of protocol I were evaluated by taking mean values of breath-to-breath ventilation over identical time segments of each run. We calculated mean values for the final 2 min of normoxic ventilation before the introduction of hypoxia (period A), for minutes 2 to 4 after the introduction of hypoxia (period B), the last 2 min of hypoxic ventilation (period C), the last 2 min of normoxic ventilation before the reintroduction of hypoxia (period D) and minutes 2 to 4 after the reintroduction of hypoxia (period E) (see Fig. 1). We defined the difference in ventilation between period B and A and period E and D as the acute hypoxic response (AHR).
74
A. BERKENBOSCH AND OTHERS
For the analysis of the dynamic response of the ventilation to a square-wave change in P we used a two-compartment model (Dahan et al. 1990):
,
dVC + PC = SC[PET,CO2(t-TC)-Bc]( dt
(t-TP)-BP],] dVP + VP = Sp[PETCO dt2
(2)
rC = Tonx +(I -x)0rof,
(3) (4) VI = IC +I'p +Ct. In the equations Tc and VP denote the contributions of the central and peripheral chemoreceptors to the ventilation VI. The parameters Sc, rc, Tc and Bc are the CO2 sensitivity, time constant, transport delay time and off-set of the central chemoreflex loop. Similarly Sp, rip, Tp and BP represent the C02 sensitivity, time constant, transport delay time and off-set of the peripheral chemoreflex loop. To model the central time constant of the ventilatory on-transient to be different from the off-transient r0 is written according to eqn (3) in which x = 1 when PET is high and x = 0 when PET, C2 is low. Since in some experiments a small drift in ventilation was present we included a drift term Ct in eqn (4). In the model it is assumed that there is no interaction between the peripheral and central chemoreflex loops. From eqns (1), (2) and (4) it follows that in the steady state: or
VI = (SP+ SC)PCO2-SpBp-ScBc V7 = S(PC-B),
(5) (6)
in which
B- SPBP+ScBc
(7)
and S=S,P+SC. (8) It is important to realize that the DEF technique can only separate the change in ventilation, due to a change in PETCO2 into parts belonging to the central and peripheral chemoreflex loops. Therefore the parameters BP and BC in eqns (1) and (2) cannot be estimated individually. To make the system identifiable it is customary to use the parameter B for both loops. This off-set B is equal to the extrapolated PET, CO2 of the steady-state ventilatory response curve to C02 at zero ventilation (apnoeic threshold). The parameters of the model were estimated by fitting the data to the model with a least squares method. To obtain optimal time delays a 'grid search' was applied and all combinations of Tc and Tp with increments of I s and with Tc > Tp were tried until a minimum in the residual sum of squares was found. The minimal time delays were somewhat arbitrarily chosen to be 1 s and rp was constrained to be at least 0 3 s. Statistical analysis To detect the significance of differences between the two treatments analysis of variance was performed on the estimated parameters of the individual DEF runs of the C02 experiments and on the AHR of the 02 experiments using a mixed model. Since the data were unbalanced the variance components were estimated from the data and the fixed effects were estimated through weighted means analysis (Milliken & Johnson, 1984). RESULTS
Protocol I A total of eighteen responses were performed. Three responses were discarded because the difference in PET, CO2 between the first and second hypoxic bout was more than 0O15 kPa. The mean differences in PET, CO and PET, 2 between the first and second hypoxic bout were 004 kPa (S.D. 005) and -0 01 kPa (S.D. 028) respectively. The mean value of the AHR of the first exposure to hypoxia was 9.51 min-' (s.E.M. 1-3) compared to 7-2 1 min-' (S.E.M. 08) of the second exposure.
VENTILATORY RESPONSE TO CO2 AFTER HYPOXIA
75
20 r 1-1
/
15 1
/
I 10 F v // /~~~~~~~
-oc0 0) en
I
*
5
C 0
5 15 10 First AH R (I min- )
20
Fig. 2. Scatter diagram of the initial increase in ventilation (AHR) of the first and second induction of hypoxia.
100 Sa 02
,
(%) 50.
30 .
i_
(kPa)
10 I PC02
I
(kPa)
I
I iam
40 T (I minm 0) 0
0
Time (min)
53
Fig. 3. Recording of the second part of an experiment of protocol II. Breath-by-breath ventilation (VI) response to a decrease in oxygen saturation (Sa,o2) shows the initial increase followed by the slow decline. Po2 and Pco, denote the 02 and CO2 partial pressure in inspired and expired gas.
Analysis of variance revealed that this difference just reached the level of significance (P = 0-04). A scatter diagram of the mean values of each subject is shown in Fig. 2. Protocol II A total of forty prehypoxic CO2 responses and forty posthypoxic CO2 responses were performed. All subjects completed the experiments without discomfort. Due to technical problems three prehypoxic responses and seven posthypoxic responses were lost. One prehypoxic and three posthypoxic responses were not suitable for analysis. In Fig. 3 a plot of the second part of an experiment is shown. It started with a 5 min period of steady-state ventilation and subsequently the PET, 02 was lowered
A. BERKENBOSCH AND OTHERS
76
to 6-5 kPa by reducing the inspired oxygen concentration. After 25 min the PET, 02 was returned to its original level and 5 min later a square-wave change in PET, CO2was performed and the ventilatory responses to CO2 measured. The ventilatory response to the sudden imposition of hypoxia showed an initial fast increase followed by a slow decline. After the step out of hypoxia there was a fast return of the ventilation towards the prehypoxic level. Before hypoxia
50 1-
008~
c'
0 0
21
Time (min)
After hypoxia
50
8T
_
L
Xu
. -
as_ 0
Q~ R
L
0
0
Time (min)
19
Fig. 4. Ventilatory responses to a square-wave change in PET Cc2 before (upper panel) and after 25 min period of hypoxia (lower panel). The dots represent the breath-by-breath ventilation. The curve through the data points is the model fit; it is the sum of the slow central, the fast peripheral component and the trend term (not shown separately). The estimated parameter values for prehypoxic response are: B, 4-95 kPa; Sc, 886 1 minkPa-1;Sp, 2721min-'kPa-1;ron, 310 s;T0rf, 166 S;TP, 4-5 s; T, 15s;1T, 15S; and C,011 1 min2. The estimated parameter values for the posthypoxic response are: B, 4 97 kPa; Se, 740 1 min kPa-1; Sp 2 82 1 min' kPa-1; font 155 5; Toff, 292 s; rP, 15X2 s; Tc, 13 s; Tp, 6 s; and C, 0-05 1 min .
Figure 4 shows the model fits to a prehypoxic and posthypoxic CO2 run of the same subject. It illustrates that the contribution of peripheral and central chemoreflex loop to the ventilatory response to CO2 are not substantially changed. The mean values of the central and the peripheral CO2 sensitivity together with the apnoeic
-|w|
VENTILATOR Y RESPONSE TO CO2 AFTER HYPOXIA
35
-
'c 30
-
I--
m
_ 20 x
0
15-
-= 100
co 0 O
-
v~~~~ 1*X~~~~ 9~~~~~ A(~~~~~
-r-
10 15 20 25 30 35 Sc prehypoxia (I min- kPa-1) 5
.x
I~~~~~~~
6
A
0
4
0.Q
2 -
A
-114
x
91~~~~~~~~
0
v
o 4
3
I
A
I
3
4 5 B prehypoxia (kPa)
6
,,"~/
O
8
,,'
,,'/,
.
-C
Cu -5
00,~~~~~~
Cu 0-
//
A
A/
,&
0
10 -
*Z
//
25-
77
// A ,, o/~~~~ ,' // t~~~
,,fi's~
-
C-0
0
-
I
0
2
Sp
6 8 10 prehypoxia (I min- kPa-1) 4
Fig. 5. Scatter diagrams of the central ventilatory C02 sensitivity (Sc), the peripheral ventilatory C02 sensitivity (S,,) and the apnoeic threshold (B) before and after 25 min of hypoxia. The symbols for the subjects are the same as in Fig. 2. Different symbols denote different subjects.
TABLE 1. Mean values of the means per subject with the S.E.M. of the parameters of the C02 responses Parameter Prehypoxia Posthypoxia P value B (kPa) 4-82+0-18 4-96+0-20 0-25 10-23 +163 11-90+2-45 0-13 S,(l min-' kPa-1) 3-64+0-49 4-32 +0-76 0-34 S,(l min-' kPa-1) 127-5 + 27-0 1913 + 23-7 0-12 Tn(S) 110-7+20-9 137-9+33-0 0-50 Toff (S) 7-6+1-6 9-8+ 1-6 043 Tp(S) 10-2+1-3 Tc(s) 9-6+ 18 0-95 5-8+1.0 5-4+0-7 Tp(s) 0-56 C (I mill-2) 0-052 +0-025 0-122 +0-033 0-13 B, apnoeic threshold; S., central CO2 sensitivity; S,, peripheral C02 sensitivity; r,,n and Tor,, central time constants of the 'on' and 'off' responses; rp, peripheral time constant; T0 and T,,, time delays of the central and peripheral chemoreflex loops; C, trend term.
A. BERKENBOSCH AND OTHERS
78
threshold of all ten subjects are shown in Fig. 5. There was a tendency of all three parameters to increase after the period of hypoxia but this did not reach the level of significance. The results are summarized in Table 1 which shows the mean values of the means per subject of all parameters. TABLE 2. Mean value of the means per subject with the S.E.M. of the ventilation (VP), tidal volume end-tidal 02 (PET,°2) and arterial saturation (VT), respiratory frequency (f), end-tidal CO2 (PET, (Sa °2) of different periods of the experiments on protocol II
co)'
VI(l min')
VT (1) f (min-) PET, CO2 (kPa) PET, 2 (kPa)
Sa02
%
Period B 22-75 +159
Period A 13-26 + 063
0-805+00049 16-9+0-7 5-97 +0-12 96-5+006
VI(I min-)
PET, CO2 (kPa) PET,02 (kPa) Sa' 2 (%)
1008+0-075 18-9+0-8
5-85+0412
5-97+0411
Period F 2245+ 1-85
6-07 +0410 77-6+2 9
14-83 +007
6-28 +0-06 77-2+2 9
15-34+0096
Period E 22-54+ 147
0-858+00063 18-4+1P0
1P229+0-113 19.1+0.9
1P141+00093 20-3+0.9
5-98+0-12 1479 +0-06
5-91+0412 6417 +0-09
5-96+0411 6-32+0-04
Period D
VT (1) f (min-')
1P218+00088 19-2+1-1
Period C 18-77 +1-34
97-0+0-7
77-3+3-0
77-9+2-6
DISCUSSION
Our results show that after a period of sustained hypoxia the ventilatory CO2 sensitivities of the peripheral (SP) and central (Sc) chemoreflex response and the apnoeic threshold (B) were not significantly changed, although there was a tendency of all three parameters to increase. In the same subjects we observed that the acute hypoxic response (AHR) was depressed after 25 min of hypoxia. The mean difference between the first and second AHR was 2-3 1 min'. This depressant effect is long lasting and it requires up to 1 h before recovery is complete (Easton et al. 1988; Georgopoulos, Berezanski & Anthonisen, 1989 a; Khamnei, 1989). Our measurements of the ventilatory response to CO2 were completed within 20 min after the 25 min of hypoxia so that the depressant effect must be still present. Georgopoulos et al. (1989a) studied the effects of three levels of PET, C2 on the ventilatory response to sustained hypoxia. The difference between the first and second AHR they observed was independent of the PET, C02 and comparable in magnitude with ours. They did not assess the ventilatory response to CO2 directly but presented data from which the parameters of the overall ventilatory response to CO2 can be calculated. The total CO2 sensitivity S and the apnoeic threshold B calculated by linear regression using the data of Georgopoulos et al. (1989a) are tabulated in Table 3 and show a striking similarity with our results (Table 1). The mean total CO2 sensitivity and mean B value during normoxia after a period of sustained hypoxia increased by 2-1 1 min- kPa-1 and 0I11 kPa respectively com-
VENTILATORY RESPONSE TO C02 AFTER HYPOXIA 79 pared to 24 1 min- kPa-1 and 0 14 kPa in our study. The value of S increased in nine of our ten subjects due to an increase in both Sp and SC since the ratio Sp/S, did not show a trend. However, the difference didn't quite reach the level of significance (P = 0'07). In a separate preliminary study Dahan & Ward (1989) obtained similar results. It is clear that there is no long-lasting depressant effect of hypoxia on the peripheral and central ventilatory CO2 sensitivities. An effect on the off-sets should therefore be taken into consideration. TABLE 3. Slope (S) and intercept (B) of ventilatory responses to CO2 calculated from data of Georgopoulos et al. (1989a)
S (1 min-' kPa-1)
B (kPa)
10-72 18-57 17-33 12-77 18-34
4-04 4-17 4 04 407
Normoxia Acute hypoxia Sustained hypoxia Normoxia after sustained hypoxia Second time acute hypoxia
3.93
Bascom et al. (1990) investigated changes in peripheral chemoreflex loop activity during sustained hypoxia. They found that the ventilatory response due to extra hypoxic pulses at 17 min of hypoxia were significantly less than in the period before. The contribution of the peripheral chemoreceptors, calculated from the CO2 pulse response, was only slightly depressed by hypoxia. They argued that the HVD originates from the peripheral chemoreceptors themselves before the stimuli of hypercapnia and hypoxia merged into the common signal of nerve impulses and is translated into ventilation since a central depression of the peripheral chemoreflex loop activity should affect CO2 and hypoxic pulse responses equally. However, this is not necessarily so. In the steady state the contribution of the peripheral chemoreceptors to the ventilation is according to eqn (2):
Vp = Sp(Pco2-Bp).
(9)
Using the fact that the development of the HVD is slow the contribution of the peripheral chemoreceptors to the increase in ventilation due to a CO2 pulse is:
AVP
=
SpPco2.
(10)
Due to the positive interaction between CO2 and hypoxia at the level of the peripheral chemoreceptors an extra hypoxic pulse causes a fast increase of Sp by ASp and the increase in fp is: (11) Ap = ASP(PCo-Bp). If sustained hypoxia predominantly effects BP and only has a small effect on Sp, the ventilatory responses of subsequent hypoxic pulses during HVD are much more depressed than those of the CO2 pulses. Hence the results of Bascom et al. (1990) can be explained by an effect of hypoxia on BP only. It may arise centrally due to a modulation of the central translation of the input from peripheral chemoreceptors
80
A. BERKENBOSCH AND OTHERS
into ventilation and one possibility is that the HVD is due to the central release or accumulation of an inhibitory neurotransmitter or modulator. With regard to the off-set B (apnoeic threshold) we remark that a change in peripheral chemoreflex off-set, ABp, only results in a change in B value of ABp Sp/(Sp +SJ) (see eqn (7)). Using parameter values from Table 1, a change in B value of 0-14 kPa results in a change in BP of about 0 5 kPa. Due to the positive interaction of CO2 and hypoxia at the level of the peripheral chemoreceptors, Sp increases by hypoxia so that a larger change in B is to be expected for the same change in BP. The results of Georgopoulos et al. (1989a) suggest that during sustained hypoxia there is a gradual increase in B of 0-24 kPa (see Table 3). Assuming that during hypoxia SPI/S = 0 7 (Bascom et al. 1990) a decrease in B of 0-24 kPa may be the result of a change in BP of 0-6 kPa. When BP increases with ABP the decrease in the contribution of the peripheral chemoreceptors to the ventilation at constant PC02 is, according to eqn (9):
AVP
=
SP ABP.
(12)
Using a value of 7-8 1 min- kPa-1 for Sp and for ABP = 0-6, the value of AVp is about 4 1 min'. After the 25 min of hypoxia we observed a mean decrease in ventilation of 41 min- (Table 2). This is similar to the values observed by Easton et al. (1988), Suzuki, Nishimura, Yamamoto, Miyamoto, Kishi & Kawakami (1989), Georgopoulos et al. (1989a) and Khamnei & Robbins (1990). It is therefore tempting to conclude that the HVD is due to an increase in the off-set of the peripheral chemoreflex loop (BP). The more so as the long-lasting effect of a sustained hypoxic episode acts at least partly on the fast response, viz. on the peripheral chemoreflex loop. This would also explain why the magnitude of the HVD is larger when the drive of the peripheral chemoreceptors is increased by, for example, almitrine, since this drug increases Sp (Olievier, Berkenbosch & DeGoede, 1989). Since the depressant effect is long-lasting one would expect that the ventilation in the normoxic period just after the 25 min of hypoxia (period D) is lower than the prehypoxic normoxic ventilation. However, we found the opposite in our experiments since after hypoxia the normoxic ventilation was about 2 1 min' higher (Table 2). Several investigators have noted a gradual increase of ventilation during hypercapnia (Georgopoulos et al. 1989 a; Khamnei & Robbins, 1990). The mechanism of this gradual increase is unknown but seems to go hand in hand with an increase in both SP and S, (Table 1). This gradual increase in ventilation obscures a manifestation of the depressant effect in the normoxic period after 25 min of hypoxia. It only becomes manifest when a second hypoxic bout is given and the AHRs are compared. However, in terms of actual values the ventilation in periods B and D was not significantly different in our study (Table 2), in agreement with the results of Georgopoulos et al. (1989a). From experiments on cats it has become clear that the threshold of the chemoreceptor nerve impulse response to CO2 decreases by hypoxia (Lahiri & DeLaney, 1975). There is no adaptation of the peripheral chemoreflex response as measured in impulse activity in the carotid sinus nerve (Vizek, Pickett & Weil, 1987; Andronikou, Shirahata, Mokashi & Lahiri, 1988) or in terms of ventilation (Berkenbosch, DeGoede, Ward, Olievier & VanHartevelt, 1991). The ventilatory decline following the rapid increase in ventilation with isocapnic hypoxia, is due to
VENTILATORY RESPONSE TO C02 AFTER HYPOXIA 81 an effect of hypoxia on structures in the brain stem (VanBeek, Berkenbosch, DeGoede & Olievier, 1984). By artificial perfusion of the brain stem we have shown that brain stem hypoxia does not affect the peripheral and central CO2 sensitivities. These results in cats are in agreement with our findings in humans. The main phenomenological difference between man and cat is the time course of the development of the HVD and the speed of recovery, which appears to be much slower in man than in cats. From the results of the brain stem perfusion experiments alone it cannot be decided whether the central effect is on BP, B. or both, since BP and B. are not identifiable without further assumptions. Our studies in cats, however, strongly suggest that a hypoxia-induced increase in brain blood flow, washing out acid metabolites (CO2/H+) from central chemosensitive structures, plays an important role in the central depressant effect of hypoxia (Ward, Berkenbosch, DeGoede & Olievier, 1990; Berkenbosch, Olievier, DeGoede & Kruyt, 1991). This effect results in an increase in Be, i.e. a shift of the central ventilatory response to CO2 to higher Pco values. Recently Nishimura et al. (1987) reported that jugular venous Pco2decreased about 0-3 kPa going from normoxia to hypoxia and became constant within 6 min after the start of the induction of isocapnic hypoxia. Assuming that jugular venous Pco2 is a reliable index of brain tissue Pco2 and that ventilation follows brain tissue Pco2 instantaneously, this implies that the depressant effect of hypoxia caused by changes in brain blood flow is complete within 6 min. Since these authors also measured the relation between ventilation and jugular venous Pco2 it can be estimated from their data that the depressant effect in ventilation lies between 3 and 111 min-. The acute hypoxic response measured in a period of 1-5 min after the induction of hypoxia may therefore not be regarded as a pure peripheral chemoreflex effect since it is already contaminated with central depressant effects of hypoxia caused by increased brain blood flow. Besides these blood flow effects on B. there may also be additional effects due to central release or accumulation of an inhibitory neurotransmitter or modulator. In a recent paper Honda & Hashizume (1991) reported that in carotid body-resected patients with mildly restricted pulmonary function the apnoeic threshold B, which in these subjects is thus equal to Be, increased by 0-88 kPa on going from hyperoxia to hypoxia. This is somewhat larger than expected from the measurements of Nishimura et al. (1987) who found a decrease in jugular venous Pco2 of 0 5 kPa on going from a comparable level of hyperoxia to hypoxia. However, since Honda & Hashizume (1991) measured a small increase in slope of the ventilatory response to CO2 the amount of hypoxic depression of ventilation, calculated at a PET, C2 of 5.3 kPa, was only 3-1 1 min-. To date the picture which emerges from our study and those of others can probably best be described as follows. In the first few minutes after the induction of hypoxia the CO2 sensitivity of the peripheral chemoreceptors increases and the neuronal threshold decreases which is reflected in a decrease of the off-set of the peripheral chemoreflex loop. Simultaneously the off-set of the central chemoreflex loop increases due to an increase of brain blood flow. Thereafter, when hypoxia is sustained, the off-set of the peripheral chemoreflex loop slowly increases resulting in a hypoxic ventilatory decline. Whether this last effect is generated centrally or peripherally remains to be investigated.
82
A. BERKENBOSCH AND OTHERS
We are grateful to Mr C. N. Olievier for performing the statistical analysis.
REFERENCES
ANDRONIKOU, S., SHIRAHATA, M., MOKASHI, A. & LAHIRI,S. (1988). Carotid body chemoreceptor and ventilatory responses to sustained hypoxia and hypercapnia in the cat. Respiration
Physiology 72, 361-374. P. A. (1990). I. D., CUNNINGHAM, D. A., PAINTER, R. & ROBBINS, BASCOM, D.inA., CLEMENT,chemoreflex sensitivity during sustained, isocapnic hypoxia. Respiration Changes peripheral Physiology 82, 161-176. J., WARD, D. S., OLIEVIER, C. N. & VANHARTEVELT, J. (1991). BERKENBOSCH, A., DEGOEDE, response of the peripheral chemoreflex loop to changes in end-tidal 02. Journal of DynamicPhysiology 71, 1123-1128. Applied OLIEVIER, DEGOEDE, J. & KRUYT, E. W. (1991). Effect on ventilation of A., BERKENBOSCH,administered to C.theN.,brain stem of the anaesthetized cat. Journal of Physiology 443, papaverine 457-468. DAHAN, A., DEGOEDE, J., BERKENBOSCH, A. & OLIEVIER, I. C. W. (1990). The influence of oxygen on the ventilatory response to carbon dioxide in man. Journal of Physiology 428, 485-499. DAHAN, A. & WARD, D. S. (1989). The effect of prior hypoxia on the ventilatory response to CO2 in humans. Journal of Physiology 417, 120P. EASTON, P. A., SLYKERMAN, L. J. & ANTHONISEN, N. R. (1986). Ventilatory response to sustained hypoxia in normal adults. Journal of Applied Physiology 61, 906-911. EASTON, P. A., SLYKERMAN, L. J. & ANTHONISEN, N. R. (1988). Recovery of the ventilatory response to hypoxia in normal adults. Journal of Applied Physiology 64, 521-528. FILUK, R. B., BEREZANSKI, D. J. & ANTHONISEN, N. R. (1988). Depression of hypoxic ventilatory response in humans by somatostatin. Journal of Applied Physiology 65, 1050-1054. GEORGOPOULOS, D., BEREZANSKI, D. J. & ANTHONISEN, N. R. (1989a). Effects of CO2 breathing on response to sustained hypoxia in normal adults. Journal of Applied Physiology 66, ventilatory 1071-1078. Increased chemoreceptor output GEORGOPOULOS, D., WALKER, S. & ANTHONISEN, N. R. and ventilatory response to sustained hypoxia. Journal of Applied Physiology 67, 11571163. HONDA, Y. & HASHIZUME, I. (1991). Evidence for hypoxic depression of CO2-ventilation response in carotid body-resected humans. Journal of Applied Physiology 70, 590-593. KAGAWA, S., STAFFORD, M. J., WAGGENER, T. B. & SEVERINGHAUS, J. W. (1982). No effect of naloxone on hypoxia-induced ventilatory depression in adults. Journal of Applied Physiology 52, 1030-1034. S. (1989). Hypoxic sensitivity following a sustained period of hypoxia in man. Journal KHAMNEI, of Physiology 417, 115P. KHAMNEI, S. & ROBBINS, P. A. (1990). Hypoxic depression of ventilation in humans: alternative models for the chemoreflexes. Respiration Physiology 81, 107-126. S. & DELANEY, R. G. (1975). Stimulus interaction in the responses of carotid body LAHIRi, chemoreceptor single afferent fibres. Respiration Physiology 24, 249-266. MILLIKEN, G. A. & JOHNSON, D. E. (1984). Analysis of Messy Data, vol. I: Designed Experiments. Van Nostrand Reinhold Company, New York. NISHIMURA, M., SUZUKI, A., NISHIURA, Y., YAMAMOTO, H., MIYAMOTO, K., KISHI, F. & Y. (1987). Effect of brain blood flow on hypoxic ventilatory response in humans. KAWAKAMI, Journal of Applied Physiology 63, 1100-1106. OLIEVIER, C. N., BERKENBOSCH, A. & DEGOEDE, J. (1989). Almitrine and the peripheral ventilatory response to in hyperoxia and hypoxia. Respiration Physiology 78, 391-402. F. & KAWAKAMI, Y. (1989). No SUZUKI, A., NISHIMURA, M., YAMAMOTO, H., MIYAMOTO, K., effect of brain blood flow on ventilatory depression during sustained hypoxia. Journal of Applied Physiology 66, 1674-1678. SWANSON, G. D. & BELLVILLE, J. W. (1975). Step changes in end-tidal CO2: methods and implications. Journal of Applied Physiology 39, 377-385.
(1989b).
CO2
KISHI,
VENTILATORY RESPONSE TO CO2 AFTER HYPOXIA
83
VANBEEK, J. H. G. M., BERKENBOSCH, A., DEGOEDE, J. & OLIEVIER, C. N. (1984). Effects of brain stem hypoxaemia on the regulation of breathing. Respiration Physiology 57, 171-188. VIZEK, M., PICKETT, C. K. & WEIL, J. V. (1987). Biphasic ventilatory response of adult cats to sustained hypoxia has central origin. Journal of Applied Physiology 63, 1658-1664. WARD, D. S., BERKENBOSCH, A., DEGOEDE, J. & OLIEVIER, C. N. (1990). Dynamics of the ventilatory response to central hypoxia in cats. Journal of Applied Physiology 68, 11071113. WEIL, J. V. & ZWILLICH, C. W. (1976). Assessment of ventilatory response to hypoxia: methods and interpretations. Chest 70, 124-128. WEISKOPF, R. B. & GABEL, R. A. (1975). Depression of ventilation during hypoxia in man. Journal of Applied Physiology 39, 911-915.
71
THE VENTILATORY RESPONSE TO CO2 OF THE PERIPHERAL AND CENTRAL CHEMOREFLEX LOOP BEFORE AND AFTER SUSTAINED HYPOXIA IN MAN
BY AAD BERKENBOSCH, ALBERT DAHAN, JACOB DEGOEDE AND IDA C. W. OLIEVIER From the Departments of Physiology and Anaesthesiology, University of Leiden, PO Box 9604, 2300 RC Leiden, The Netherlands
(Received 11 December 1991) SUMMARY
1.The ventilatory response to sustained hypoxia is characterized by a fast increase due to the peripheral chemoreceptors followed by a slow decline. The mechanism of this decline is unknown. 2. To investigate the characteristics of the ventilatory response to sustained hypoxia ten healthy subjects were exposed to two consecutive periods of isocapnic hypoxia (arterial saturation 78%) separated by a 5 min exposure to isocapnic normoxia. 3. The acute hypoxic response to the second exposure to hypoxia (mean increase in ventilation + s.E.M., 7-2 + 08 1 min') was significantly depressed (P = 004) compared to the first one (9 5 + 1-3 1 min-). 4. To investigate whether this depression was due to central or-peripheral effects or both we measured, in the same ten subjects, the normoxic ventilatory response to CO2 before and after a period of 25 min of hypoxia using the technique of dynamic end-tidal forcing. 5. Each response was separated into a fast peripheral and slow central component characterized by a CO2 sensitivity, time constant, time delay and an off-set. 6. A total of thirty-six prehypoxic and thirty posthypoxic responses were analysed. The ventilatory CO2 sensitivities of the peripheral and central chemoreflex loops and the overall off-set (apnoeic threshold) after 25 min of hypoxia were somewhat larger than their prehypoxic values, but this effect was not significant. 7. We argue that the hypoxic ventilatory decline in man is due to a change in the off-set of the peripheral chemoreflex loop. INTRODUCTION
In adult human beings the ventilatory response to isocapnic sustained hypoxia is a rapid increase in ventilation (VI) followed by a slow decline (Weiskopf & Gabel,
1975; Weil & Zwillich, 1976; Kagawa, Stafford, Waggener & Severinghaus, 1982; Easton, Slykerman & Anthonisen, 1986; Nishimura, Suzuki, Nishiura, Yamamoto, MS 9960
72
A. BERKENBOSCH AND OTHERS
Miyamoto, Kishi & Kawakami, 1987; Khamnei & Robbins, 1990). The rapid increase in ventilation is due to the peripheral drive from the carotid bodies. The mechanism of the slow hypoxic ventilator decline (HVD) is not known. It has been shown recently that complete recovery from a sustained hypoxic episode requires up to an hour of room air breathing (Easton, Slykerman & Anthonisen, 1988; Khamnei, 1989). In man the HVD is related to the magnitude of the initial stimulation of ventilation with hypoxia. When the peripheral drive is increased with almitrine the magnitude of 1EVD is also increased (Georgopoulos, Walker & Anthonisen, 1989b). The reverse is true when the peripheral drive is decreased by somatostatin (Filuk, Berezanski & Anthonisen, 1988). These observations led Robbins' group in Oxford to suggest that the peripheral drive from the carotid bodies adapts during sustained hypoxia. They performed experiments in which additional hypoxic pulses were given during HVD and showed that the peripheral sensitivity to hypoxia diminished (Basoom, Clement, Cunningham, Painter & Robbins, 1990). To obtain more information about a possible adaptation of the peripheral chemoreceptors we examined the effect of an episode of sustained hypoxia on the ventilatory response of the peripheral chemoreflex to CO2. METHODS
Subjects Ten subjects (aged 22-27 years; 6 males, 4 females), who gave their informed consent, took part in the experimental protocol approved by the Leiden University Ethics Committee. They were all naive to respiratory physiology and unaware of the purpose of the study. They were healthy and had no history of cardiovascular or respiratory disease. Each subject was familiarized with the experimental set-up and procedure before the study started. All subjects were asked to refrain from stimulant and depressant substances at least 12 h prior to the experiments.
Experimental design To study the dynamics of the response to hypoxia or carbon dioxide we used the technique of 'dynamic end-tidal forcing' (DEF). With this technique we are able to perform steps in the endtidal P.2 (PET 02) at a background of constant end-tidal Pco, (PET, co2) or steps in PET CO at a constant PETO by manipulating the inspired carbon dioxide and oxygen concentrations independently of the ventilatory response (Swanson & Bellville, 1975; Dahan, DeGoede, Berkenbosch & Olievier, 1990). After arrival at the laboratory a subject rested for 45 min in a comfortable chair. Subsequently a face mask was fitted and the experiment started. The subjects were encouraged to read and/or listen to music through headphones. Each subject participated in two protocols and was studied on 4 days. Protocol I (02 experiments). Experiments consisted of isocapnic steps into and out of hypoxia. After a 5-10 min period of steady-state ventilation during which the PET CO was slightly raised above resting values the PET 02 was rapidly lowered to attain arterial 02 saturation (Sa, 02) of 80%. The Sa, 02 was kept constant at this value for 25 min after which the PET was increased in a wPsket on02 stepwise fashion to the normoxic value. After 5 min of normoxia, hypoxia was reintroduced lasting for another 25 min (Fig. 1). Protocol II (CO2 experiments). Experiments consisted of two parts. In the first part a C02 run was performed. Such a run started with a 5-10 min period of 'steady-state' ventilation during which the PPET, COwa was slightly raised above resting values (5-5-6-2 kPa among subjects). The PET,
was
maintained constant at a normoxic level (14-5 kPa). The PET, Co2 was then increased about 1-1.5 kPa in a stepwise manner, maintained constant for approximately 7 min after which it was returned to its original value and kept there for another 7 min. Subsequently the subject was freed from the face mask and allowed to rest for 30 min. Then followed the second part of the experiment. After fitting the face mask again the PET C02 and PET 02 were brought to the same value as in the
VENTILATORY RESPONSE TO CO2 AFTER HYPOXIA
73
previous CO2 run. When the ventilation reached a steady-state the PET, 0was lowered to about 6 kPa. The inspired oxygen concentration was manipulated to maintain an arterial oxygen saturation (Sa. 0) of about 80 % throughout the hypoxic exposure which lasted 25 min. PET, CO2 was kept constant during this period. Subsequently the PETO2 was returned to the normoxic value and after 5 min a CO2 run as in the first part of the experiment was performed (Fig. 1). PETCO2 I
Protocol I
PET,
I
25
2
I
5
25min
Protocol II
PETCO2| PET 02
Time (min)
. 2
7
7
I 2
I 25
I 5
I| 7
7
Fig. 1. Experimental design of protocol I (02 experiments) and protocol II (CO2
experiments). All participants performed protocol II twice in one morning session and returned for a second session on another day. One subject became ill and missed the experiments of protocol I which were performed twice on different morning sessions. The order of the protocols was random.
Apparatus The subjects were seated and an oronasal face mask was fitted. They were instructed to breathe through their mouths to minimize changes in airway resistance during the experiment. The airway gas flow was measured with a pneumotachograph (Fleisch no. 3, Switzerland) connected to a differential pressure transducer (Hewlett Packard model 270, USA) and electronically integrated to yield a volume signal. This signal was calibrated with a motor-driven piston pump (stroke volume 1000 ml, at a frequency of 20 min-). Corrections were made for the changes in gas viscosity due to changes in 02 concentrations of the inhaled gas mixture. The pneumotachograph was connected to a T-piece. One arm of the T-piece received a gas mixture with a flow of 40 1 min-' from a gas mixing system, consisting of three mass flow controllers (Bronkhorst High Tech BV, F202/F203, The Netherlands) with which the flow of 02, CO2 and N2 could be set individually at a desired level. A PDP 11/23 computer provided control signals to the mass flow controllers, so that the composition of the inspiratory gas mixture could be adjusted to force the PET CO and PET 0 to follow a specific pattern in time. The 02 and CO2 concentrations of the inspired and expired gasses were measured with a gas monitor (Datex Multicap, Finland). A pulse oximeter (Nellcor 100, USA) continuously measured arterial oxygen saturation via a finger probe. All signals were recorded on a strip-chart recorder, digitized and processed by computer. The tidal volume, inspiratory time, expiratory time, respiratory frequency, inspired minute ventilation, percutaneous arterial oxygen saturation, end-tidal 02 and end-tidal CO2 tensions were stored on a breath-to-breath basis.
Data analysis The experiments of protocol I were evaluated by taking mean values of breath-to-breath ventilation over identical time segments of each run. We calculated mean values for the final 2 min of normoxic ventilation before the introduction of hypoxia (period A), for minutes 2 to 4 after the introduction of hypoxia (period B), the last 2 min of hypoxic ventilation (period C), the last 2 min of normoxic ventilation before the reintroduction of hypoxia (period D) and minutes 2 to 4 after the reintroduction of hypoxia (period E) (see Fig. 1). We defined the difference in ventilation between period B and A and period E and D as the acute hypoxic response (AHR).
74
A. BERKENBOSCH AND OTHERS
For the analysis of the dynamic response of the ventilation to a square-wave change in P we used a two-compartment model (Dahan et al. 1990):
,
dVC + PC = SC[PET,CO2(t-TC)-Bc]( dt
(t-TP)-BP],] dVP + VP = Sp[PETCO dt2
(2)
rC = Tonx +(I -x)0rof,
(3) (4) VI = IC +I'p +Ct. In the equations Tc and VP denote the contributions of the central and peripheral chemoreceptors to the ventilation VI. The parameters Sc, rc, Tc and Bc are the CO2 sensitivity, time constant, transport delay time and off-set of the central chemoreflex loop. Similarly Sp, rip, Tp and BP represent the C02 sensitivity, time constant, transport delay time and off-set of the peripheral chemoreflex loop. To model the central time constant of the ventilatory on-transient to be different from the off-transient r0 is written according to eqn (3) in which x = 1 when PET is high and x = 0 when PET, C2 is low. Since in some experiments a small drift in ventilation was present we included a drift term Ct in eqn (4). In the model it is assumed that there is no interaction between the peripheral and central chemoreflex loops. From eqns (1), (2) and (4) it follows that in the steady state: or
VI = (SP+ SC)PCO2-SpBp-ScBc V7 = S(PC-B),
(5) (6)
in which
B- SPBP+ScBc
(7)
and S=S,P+SC. (8) It is important to realize that the DEF technique can only separate the change in ventilation, due to a change in PETCO2 into parts belonging to the central and peripheral chemoreflex loops. Therefore the parameters BP and BC in eqns (1) and (2) cannot be estimated individually. To make the system identifiable it is customary to use the parameter B for both loops. This off-set B is equal to the extrapolated PET, CO2 of the steady-state ventilatory response curve to C02 at zero ventilation (apnoeic threshold). The parameters of the model were estimated by fitting the data to the model with a least squares method. To obtain optimal time delays a 'grid search' was applied and all combinations of Tc and Tp with increments of I s and with Tc > Tp were tried until a minimum in the residual sum of squares was found. The minimal time delays were somewhat arbitrarily chosen to be 1 s and rp was constrained to be at least 0 3 s. Statistical analysis To detect the significance of differences between the two treatments analysis of variance was performed on the estimated parameters of the individual DEF runs of the C02 experiments and on the AHR of the 02 experiments using a mixed model. Since the data were unbalanced the variance components were estimated from the data and the fixed effects were estimated through weighted means analysis (Milliken & Johnson, 1984). RESULTS
Protocol I A total of eighteen responses were performed. Three responses were discarded because the difference in PET, CO2 between the first and second hypoxic bout was more than 0O15 kPa. The mean differences in PET, CO and PET, 2 between the first and second hypoxic bout were 004 kPa (S.D. 005) and -0 01 kPa (S.D. 028) respectively. The mean value of the AHR of the first exposure to hypoxia was 9.51 min-' (s.E.M. 1-3) compared to 7-2 1 min-' (S.E.M. 08) of the second exposure.
VENTILATORY RESPONSE TO CO2 AFTER HYPOXIA
75
20 r 1-1
/
15 1
/
I 10 F v // /~~~~~~~
-oc0 0) en
I
*
5
C 0
5 15 10 First AH R (I min- )
20
Fig. 2. Scatter diagram of the initial increase in ventilation (AHR) of the first and second induction of hypoxia.
100 Sa 02
,
(%) 50.
30 .
i_
(kPa)
10 I PC02
I
(kPa)
I
I iam
40 T (I minm 0) 0
0
Time (min)
53
Fig. 3. Recording of the second part of an experiment of protocol II. Breath-by-breath ventilation (VI) response to a decrease in oxygen saturation (Sa,o2) shows the initial increase followed by the slow decline. Po2 and Pco, denote the 02 and CO2 partial pressure in inspired and expired gas.
Analysis of variance revealed that this difference just reached the level of significance (P = 0-04). A scatter diagram of the mean values of each subject is shown in Fig. 2. Protocol II A total of forty prehypoxic CO2 responses and forty posthypoxic CO2 responses were performed. All subjects completed the experiments without discomfort. Due to technical problems three prehypoxic responses and seven posthypoxic responses were lost. One prehypoxic and three posthypoxic responses were not suitable for analysis. In Fig. 3 a plot of the second part of an experiment is shown. It started with a 5 min period of steady-state ventilation and subsequently the PET, 02 was lowered
A. BERKENBOSCH AND OTHERS
76
to 6-5 kPa by reducing the inspired oxygen concentration. After 25 min the PET, 02 was returned to its original level and 5 min later a square-wave change in PET, CO2was performed and the ventilatory responses to CO2 measured. The ventilatory response to the sudden imposition of hypoxia showed an initial fast increase followed by a slow decline. After the step out of hypoxia there was a fast return of the ventilation towards the prehypoxic level. Before hypoxia
50 1-
008~
c'
0 0
21
Time (min)
After hypoxia
50
8T
_
L
Xu
. -
as_ 0
Q~ R
L
0
0
Time (min)
19
Fig. 4. Ventilatory responses to a square-wave change in PET Cc2 before (upper panel) and after 25 min period of hypoxia (lower panel). The dots represent the breath-by-breath ventilation. The curve through the data points is the model fit; it is the sum of the slow central, the fast peripheral component and the trend term (not shown separately). The estimated parameter values for prehypoxic response are: B, 4-95 kPa; Sc, 886 1 minkPa-1;Sp, 2721min-'kPa-1;ron, 310 s;T0rf, 166 S;TP, 4-5 s; T, 15s;1T, 15S; and C,011 1 min2. The estimated parameter values for the posthypoxic response are: B, 4 97 kPa; Se, 740 1 min kPa-1; Sp 2 82 1 min' kPa-1; font 155 5; Toff, 292 s; rP, 15X2 s; Tc, 13 s; Tp, 6 s; and C, 0-05 1 min .
Figure 4 shows the model fits to a prehypoxic and posthypoxic CO2 run of the same subject. It illustrates that the contribution of peripheral and central chemoreflex loop to the ventilatory response to CO2 are not substantially changed. The mean values of the central and the peripheral CO2 sensitivity together with the apnoeic
-|w|
VENTILATOR Y RESPONSE TO CO2 AFTER HYPOXIA
35
-
'c 30
-
I--
m
_ 20 x
0
15-
-= 100
co 0 O
-
v~~~~ 1*X~~~~ 9~~~~~ A(~~~~~
-r-
10 15 20 25 30 35 Sc prehypoxia (I min- kPa-1) 5
.x
I~~~~~~~
6
A
0
4
0.Q
2 -
A
-114
x
91~~~~~~~~
0
v
o 4
3
I
A
I
3
4 5 B prehypoxia (kPa)
6
,,"~/
O
8
,,'
,,'/,
.
-C
Cu -5
00,~~~~~~
Cu 0-
//
A
A/
,&
0
10 -
*Z
//
25-
77
// A ,, o/~~~~ ,' // t~~~
,,fi's~
-
C-0
0
-
I
0
2
Sp
6 8 10 prehypoxia (I min- kPa-1) 4
Fig. 5. Scatter diagrams of the central ventilatory C02 sensitivity (Sc), the peripheral ventilatory C02 sensitivity (S,,) and the apnoeic threshold (B) before and after 25 min of hypoxia. The symbols for the subjects are the same as in Fig. 2. Different symbols denote different subjects.
TABLE 1. Mean values of the means per subject with the S.E.M. of the parameters of the C02 responses Parameter Prehypoxia Posthypoxia P value B (kPa) 4-82+0-18 4-96+0-20 0-25 10-23 +163 11-90+2-45 0-13 S,(l min-' kPa-1) 3-64+0-49 4-32 +0-76 0-34 S,(l min-' kPa-1) 127-5 + 27-0 1913 + 23-7 0-12 Tn(S) 110-7+20-9 137-9+33-0 0-50 Toff (S) 7-6+1-6 9-8+ 1-6 043 Tp(S) 10-2+1-3 Tc(s) 9-6+ 18 0-95 5-8+1.0 5-4+0-7 Tp(s) 0-56 C (I mill-2) 0-052 +0-025 0-122 +0-033 0-13 B, apnoeic threshold; S., central CO2 sensitivity; S,, peripheral C02 sensitivity; r,,n and Tor,, central time constants of the 'on' and 'off' responses; rp, peripheral time constant; T0 and T,,, time delays of the central and peripheral chemoreflex loops; C, trend term.
A. BERKENBOSCH AND OTHERS
78
threshold of all ten subjects are shown in Fig. 5. There was a tendency of all three parameters to increase after the period of hypoxia but this did not reach the level of significance. The results are summarized in Table 1 which shows the mean values of the means per subject of all parameters. TABLE 2. Mean value of the means per subject with the S.E.M. of the ventilation (VP), tidal volume end-tidal 02 (PET,°2) and arterial saturation (VT), respiratory frequency (f), end-tidal CO2 (PET, (Sa °2) of different periods of the experiments on protocol II
co)'
VI(l min')
VT (1) f (min-) PET, CO2 (kPa) PET, 2 (kPa)
Sa02
%
Period B 22-75 +159
Period A 13-26 + 063
0-805+00049 16-9+0-7 5-97 +0-12 96-5+006
VI(I min-)
PET, CO2 (kPa) PET,02 (kPa) Sa' 2 (%)
1008+0-075 18-9+0-8
5-85+0412
5-97+0411
Period F 2245+ 1-85
6-07 +0410 77-6+2 9
14-83 +007
6-28 +0-06 77-2+2 9
15-34+0096
Period E 22-54+ 147
0-858+00063 18-4+1P0
1P229+0-113 19.1+0.9
1P141+00093 20-3+0.9
5-98+0-12 1479 +0-06
5-91+0412 6417 +0-09
5-96+0411 6-32+0-04
Period D
VT (1) f (min-')
1P218+00088 19-2+1-1
Period C 18-77 +1-34
97-0+0-7
77-3+3-0
77-9+2-6
DISCUSSION
Our results show that after a period of sustained hypoxia the ventilatory CO2 sensitivities of the peripheral (SP) and central (Sc) chemoreflex response and the apnoeic threshold (B) were not significantly changed, although there was a tendency of all three parameters to increase. In the same subjects we observed that the acute hypoxic response (AHR) was depressed after 25 min of hypoxia. The mean difference between the first and second AHR was 2-3 1 min'. This depressant effect is long lasting and it requires up to 1 h before recovery is complete (Easton et al. 1988; Georgopoulos, Berezanski & Anthonisen, 1989 a; Khamnei, 1989). Our measurements of the ventilatory response to CO2 were completed within 20 min after the 25 min of hypoxia so that the depressant effect must be still present. Georgopoulos et al. (1989a) studied the effects of three levels of PET, C2 on the ventilatory response to sustained hypoxia. The difference between the first and second AHR they observed was independent of the PET, C02 and comparable in magnitude with ours. They did not assess the ventilatory response to CO2 directly but presented data from which the parameters of the overall ventilatory response to CO2 can be calculated. The total CO2 sensitivity S and the apnoeic threshold B calculated by linear regression using the data of Georgopoulos et al. (1989a) are tabulated in Table 3 and show a striking similarity with our results (Table 1). The mean total CO2 sensitivity and mean B value during normoxia after a period of sustained hypoxia increased by 2-1 1 min- kPa-1 and 0I11 kPa respectively com-
VENTILATORY RESPONSE TO C02 AFTER HYPOXIA 79 pared to 24 1 min- kPa-1 and 0 14 kPa in our study. The value of S increased in nine of our ten subjects due to an increase in both Sp and SC since the ratio Sp/S, did not show a trend. However, the difference didn't quite reach the level of significance (P = 0'07). In a separate preliminary study Dahan & Ward (1989) obtained similar results. It is clear that there is no long-lasting depressant effect of hypoxia on the peripheral and central ventilatory CO2 sensitivities. An effect on the off-sets should therefore be taken into consideration. TABLE 3. Slope (S) and intercept (B) of ventilatory responses to CO2 calculated from data of Georgopoulos et al. (1989a)
S (1 min-' kPa-1)
B (kPa)
10-72 18-57 17-33 12-77 18-34
4-04 4-17 4 04 407
Normoxia Acute hypoxia Sustained hypoxia Normoxia after sustained hypoxia Second time acute hypoxia
3.93
Bascom et al. (1990) investigated changes in peripheral chemoreflex loop activity during sustained hypoxia. They found that the ventilatory response due to extra hypoxic pulses at 17 min of hypoxia were significantly less than in the period before. The contribution of the peripheral chemoreceptors, calculated from the CO2 pulse response, was only slightly depressed by hypoxia. They argued that the HVD originates from the peripheral chemoreceptors themselves before the stimuli of hypercapnia and hypoxia merged into the common signal of nerve impulses and is translated into ventilation since a central depression of the peripheral chemoreflex loop activity should affect CO2 and hypoxic pulse responses equally. However, this is not necessarily so. In the steady state the contribution of the peripheral chemoreceptors to the ventilation is according to eqn (2):
Vp = Sp(Pco2-Bp).
(9)
Using the fact that the development of the HVD is slow the contribution of the peripheral chemoreceptors to the increase in ventilation due to a CO2 pulse is:
AVP
=
SpPco2.
(10)
Due to the positive interaction between CO2 and hypoxia at the level of the peripheral chemoreceptors an extra hypoxic pulse causes a fast increase of Sp by ASp and the increase in fp is: (11) Ap = ASP(PCo-Bp). If sustained hypoxia predominantly effects BP and only has a small effect on Sp, the ventilatory responses of subsequent hypoxic pulses during HVD are much more depressed than those of the CO2 pulses. Hence the results of Bascom et al. (1990) can be explained by an effect of hypoxia on BP only. It may arise centrally due to a modulation of the central translation of the input from peripheral chemoreceptors
80
A. BERKENBOSCH AND OTHERS
into ventilation and one possibility is that the HVD is due to the central release or accumulation of an inhibitory neurotransmitter or modulator. With regard to the off-set B (apnoeic threshold) we remark that a change in peripheral chemoreflex off-set, ABp, only results in a change in B value of ABp Sp/(Sp +SJ) (see eqn (7)). Using parameter values from Table 1, a change in B value of 0-14 kPa results in a change in BP of about 0 5 kPa. Due to the positive interaction of CO2 and hypoxia at the level of the peripheral chemoreceptors, Sp increases by hypoxia so that a larger change in B is to be expected for the same change in BP. The results of Georgopoulos et al. (1989a) suggest that during sustained hypoxia there is a gradual increase in B of 0-24 kPa (see Table 3). Assuming that during hypoxia SPI/S = 0 7 (Bascom et al. 1990) a decrease in B of 0-24 kPa may be the result of a change in BP of 0-6 kPa. When BP increases with ABP the decrease in the contribution of the peripheral chemoreceptors to the ventilation at constant PC02 is, according to eqn (9):
AVP
=
SP ABP.
(12)
Using a value of 7-8 1 min- kPa-1 for Sp and for ABP = 0-6, the value of AVp is about 4 1 min'. After the 25 min of hypoxia we observed a mean decrease in ventilation of 41 min- (Table 2). This is similar to the values observed by Easton et al. (1988), Suzuki, Nishimura, Yamamoto, Miyamoto, Kishi & Kawakami (1989), Georgopoulos et al. (1989a) and Khamnei & Robbins (1990). It is therefore tempting to conclude that the HVD is due to an increase in the off-set of the peripheral chemoreflex loop (BP). The more so as the long-lasting effect of a sustained hypoxic episode acts at least partly on the fast response, viz. on the peripheral chemoreflex loop. This would also explain why the magnitude of the HVD is larger when the drive of the peripheral chemoreceptors is increased by, for example, almitrine, since this drug increases Sp (Olievier, Berkenbosch & DeGoede, 1989). Since the depressant effect is long-lasting one would expect that the ventilation in the normoxic period just after the 25 min of hypoxia (period D) is lower than the prehypoxic normoxic ventilation. However, we found the opposite in our experiments since after hypoxia the normoxic ventilation was about 2 1 min' higher (Table 2). Several investigators have noted a gradual increase of ventilation during hypercapnia (Georgopoulos et al. 1989 a; Khamnei & Robbins, 1990). The mechanism of this gradual increase is unknown but seems to go hand in hand with an increase in both SP and S, (Table 1). This gradual increase in ventilation obscures a manifestation of the depressant effect in the normoxic period after 25 min of hypoxia. It only becomes manifest when a second hypoxic bout is given and the AHRs are compared. However, in terms of actual values the ventilation in periods B and D was not significantly different in our study (Table 2), in agreement with the results of Georgopoulos et al. (1989a). From experiments on cats it has become clear that the threshold of the chemoreceptor nerve impulse response to CO2 decreases by hypoxia (Lahiri & DeLaney, 1975). There is no adaptation of the peripheral chemoreflex response as measured in impulse activity in the carotid sinus nerve (Vizek, Pickett & Weil, 1987; Andronikou, Shirahata, Mokashi & Lahiri, 1988) or in terms of ventilation (Berkenbosch, DeGoede, Ward, Olievier & VanHartevelt, 1991). The ventilatory decline following the rapid increase in ventilation with isocapnic hypoxia, is due to
VENTILATORY RESPONSE TO C02 AFTER HYPOXIA 81 an effect of hypoxia on structures in the brain stem (VanBeek, Berkenbosch, DeGoede & Olievier, 1984). By artificial perfusion of the brain stem we have shown that brain stem hypoxia does not affect the peripheral and central CO2 sensitivities. These results in cats are in agreement with our findings in humans. The main phenomenological difference between man and cat is the time course of the development of the HVD and the speed of recovery, which appears to be much slower in man than in cats. From the results of the brain stem perfusion experiments alone it cannot be decided whether the central effect is on BP, B. or both, since BP and B. are not identifiable without further assumptions. Our studies in cats, however, strongly suggest that a hypoxia-induced increase in brain blood flow, washing out acid metabolites (CO2/H+) from central chemosensitive structures, plays an important role in the central depressant effect of hypoxia (Ward, Berkenbosch, DeGoede & Olievier, 1990; Berkenbosch, Olievier, DeGoede & Kruyt, 1991). This effect results in an increase in Be, i.e. a shift of the central ventilatory response to CO2 to higher Pco values. Recently Nishimura et al. (1987) reported that jugular venous Pco2decreased about 0-3 kPa going from normoxia to hypoxia and became constant within 6 min after the start of the induction of isocapnic hypoxia. Assuming that jugular venous Pco2 is a reliable index of brain tissue Pco2 and that ventilation follows brain tissue Pco2 instantaneously, this implies that the depressant effect of hypoxia caused by changes in brain blood flow is complete within 6 min. Since these authors also measured the relation between ventilation and jugular venous Pco2 it can be estimated from their data that the depressant effect in ventilation lies between 3 and 111 min-. The acute hypoxic response measured in a period of 1-5 min after the induction of hypoxia may therefore not be regarded as a pure peripheral chemoreflex effect since it is already contaminated with central depressant effects of hypoxia caused by increased brain blood flow. Besides these blood flow effects on B. there may also be additional effects due to central release or accumulation of an inhibitory neurotransmitter or modulator. In a recent paper Honda & Hashizume (1991) reported that in carotid body-resected patients with mildly restricted pulmonary function the apnoeic threshold B, which in these subjects is thus equal to Be, increased by 0-88 kPa on going from hyperoxia to hypoxia. This is somewhat larger than expected from the measurements of Nishimura et al. (1987) who found a decrease in jugular venous Pco2 of 0 5 kPa on going from a comparable level of hyperoxia to hypoxia. However, since Honda & Hashizume (1991) measured a small increase in slope of the ventilatory response to CO2 the amount of hypoxic depression of ventilation, calculated at a PET, C2 of 5.3 kPa, was only 3-1 1 min-. To date the picture which emerges from our study and those of others can probably best be described as follows. In the first few minutes after the induction of hypoxia the CO2 sensitivity of the peripheral chemoreceptors increases and the neuronal threshold decreases which is reflected in a decrease of the off-set of the peripheral chemoreflex loop. Simultaneously the off-set of the central chemoreflex loop increases due to an increase of brain blood flow. Thereafter, when hypoxia is sustained, the off-set of the peripheral chemoreflex loop slowly increases resulting in a hypoxic ventilatory decline. Whether this last effect is generated centrally or peripherally remains to be investigated.
82
A. BERKENBOSCH AND OTHERS
We are grateful to Mr C. N. Olievier for performing the statistical analysis.
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