Muscle maximal O2 uptake at constant O2 delivery with and without CO in the blood.

Muscle maximal O2 uptake at constant with and without CO in the blood

O2 delivery

MICHAEL C. HOGAN, DONALD E. BEBOUT, ANDREW T. GRAY, PETER D. WAGNER, JOHN B. WEST, AND PIERRE E. HAAB Division of Physiology, Department of Medicine, University of California, San Diego, La Jolla, California 92093-0623; and Department of Physiology, University of Fribourg, Fribourg, Switzerland

HOGAN, MICHAEL C., DONALD GRAY, PETER D. WAGNER, JOHN HAAB. Muscle maximal 0, uptake

E. BEBOUT, ANDREW T. B. WEST, AND PIERRE E. at constant O2 delivery with and without CO in the blood. J. Appl. Physiol. 69(3): 830436, 1990.-In the present study we investigated the effects of carboxyhemoglobinemia (HbCO) on muscle maximal 0, uptake . wo Zmax) during hypoxia. 0, uptake (VOW) was measured in isolated in situ canine gastrocnemius (n = 12) working maximally (isometric twitch contractions at 5 Hz for 3 min). The muscles were pump perfused at identical blood flow, arterial PO, (Pao,,) and total hemoglobin concentration ([Hb]) with blood containing either 1% (control) or 30% HbCO. In both conditions PaO,, was set at 30 Torr, which produced the same arterial O2 contents, and muscle blood flow was set at 120 ml 100 g-l min-l, so that OZ delivery in both conditions was the same. To minimize CO diffusion into the tissues, perfusion with HbCO-containing blood was limited to the time of the contraction period. VO 2 max was 8.8 t 0.6 (SE) ml. min-’ 100 g-l (n = 12) with hypoxemia alone and was reduced by 26% to 6.5 t 0.4 ml min-’ -100 g-l when HbCO was present (n = 12; P < 0.01). In both cases, mean muscle effluent venous PO, ( PvO,,) was the same (16 & 1 Torr). Because PaO, and PvoZ were the same for both conditions, the mean capillary PO, (estimate of mean 0, driving pressure) was probably not much different for the two conditions, even though the Oa dissociation curve was shifted to the left by HbCO. Consequently the blood-tomitochondria Oa diffusive conductance was likely reduced by HbCO. Although the mechanism of this reduction cannot be identified by our data, they suggest that CO, even after shortlasting exposure at low PCO, causes some impairment to the diffusion of O2 that appears to have a larger role in reducing VO 2 max than the CO-induced leftward shift of the OZ dissociation curve. l

l

fatigue; skeletal muscle; gas exchange; exercise; acid-base ance; lactate; diffusion limitation; lactic acid; hypoxia

bal-

EXPERIMENTAL EVIDENCE in humans (22) and in isolated muscle (14, 15) has supported the hypothesis that the rate at which 0, can diffuse from the erythrocyte to the mitochondria is an important determinant of muscle maximal OZ uptake (VO, max) when OZ availability at the mitochondria is insufficient. The present study was designed to investigate this hypothesis further by studying the effect of a leftward-shifted 0, dissociation curve (ODC), induced by the presence of carbon monoxide (CO) in the blood, on hypoxic VO, maxof isolated in situ canine gastrocnemius. In theory, if other factors remain 830

0161-7567/90

$1.50

Copyright

the same, a leftward-shifted ODC should increase the rate at which the capillary Po2 declines as 0, is removed by the working tissue, thereby reducing the capillary-totissue PO, driving gradient along the capillary length. We postulated that if, indeed, this driving gradient is an important determinant of total 0, flux into the tissue, the faster fall in POT along the capillary length with the leftward-shifted ODC should reduce the VO, max.To minimize CO movement into the tissue, we attempted to maintain blood CO partial pressure (Pco) very low and to keep the exposure of the muscle to the blood containing CO very short. Since the early work of Haldane (7, 13), it has been shown that carboxyhemoglobinemia alters the normal sigmoidal shape of the ODC, making it more hyperbolic and increasing hemoglobin affinity for 0,. It follows that the ODCs of normal and of CO-containing blood, expressed as 0, content vs. PO,, intersect at some point. At the intersection point, the location of which depends on HbCO concentration, the PO, and 0, content are the same for both curves (seeFig. 1B). Below the intersection point, the ODC of CO-containing blood lies to the left of the normal curve (39, whereas above this point it lies to the right. Thus carboxyhemoglobinemia offers the possibility of delivering to a muscle blood having the same arterial PO, (Pao,) and 0, content (Cao,) and total hemoglobin concentration ([Hb]) but a differently shaped ODC. Accordingly, in the present study we compared muscle VO, max under two conditions: 1) hypoxemia alone, i.e., hypoxic hypoxia (HH), and 2) hypoxemia with CO (CMH). The choice of 30% carboxyhemoglobin (HbCO), based on pilot measurements on dog blood, allowed equal Cao, and Paoz under both conditions at a Pao, of 30 Torr. Because the blood flow (Q) to the muscle was kept the same during both conditions, the 0, delivery (a x Cao,) to the muscle was the same in HH and CMH. METHODS

Six adult mongrel dogs of either sex with a weight range of 15-24 kg were anesthetized with pentobarbital sodium (30 mg/kg), and maintenance doses were given as required. The dogs were intubated with a cuffed endotracheal tube. Heating pads were used to maintain esophageal temperature near 37°C. The animals were

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Physiological

Society

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MUSCLE

MAXIMAL

0,

CONSUMPTION

given heparin (1,500 U/kg) after the surgery. Ventilation was maintained with a Harvard 613 ventilator. Surgical Preparation The left gastrocnemius-flexor digitorum superficialis muscle complex (for convenience referred to as gastrocnemius) was isolated as described previously (16, 29). Briefly, a medial incision was made through the skin of the left hindlimb from midthigh to the ankle. The sartorius, gracilis, semitendinosus, and semimembranosus muscles, which overlie the gastrocnemius, were doubly ligated and cut between the ties. All vessels draining into the popliteal vein, except those from the gastrocnemius, were ligat#ed to isolate the venous outflow from the gastrocnemius. The arterial circulation to the gastrocnemius was isolated by ligating all vessels from the femoral and popliteal artery that did not enter the gastrocnemius. The left popliteal vein was cannulated, and the venous outflow was returned to the animal via a jugular catheter. The right femoral artery was catheterized for arterial blood sampling. This catheter was connected to the left femoral artery so that the isolated muscle was perfused by blood from this contralateral artery. Perfusion was accomplished either directly from the contralateral (sysself perfused) or via a Sigmamotor pump temic pressureto control flow. A pressure transducer in this line at the head of the muscle constantly monitored perfusion pressure. A carotid artery was also catheterized to monitor systemic blood pressure. The left sciatic nerve, which innervates the gastrocnemius, was doubly ligated and cut between ties. To prevent cooling and drying, all exposed tissues were covered with saline-soaked gauze and with a sheet of plastic film (Saran Wrap). After the muscle was surgically isolated, the Achilles tendon was attached to an isometric myograph (Statham 1360 transducer) to measure tension development. The hindlimb was fixed at the knee and ankle and attached to the myograph with struts to minimize movement. Weights were used at the end of each experiment to calibrate the tension myograph. Isometric muscle contractions (twitches) were elicited by stimulation of the sciatic nerve with square-wave impulses of 0.2 ms duration at 4-6 V and a frequency of 5 Hz. Pilot work in this laboratory, along with other studies (l-3, 32), has demonstrated that this degree of stimulation results in the maximal metabolic rate (VO,) for isometric twitch contractions in this muscle model. Before each contraction period, the resting muscle was passively stretched until a tension setting of -10 g force per gram muscle mass was recorded. This ensured that the initial tension development was not affected by slippage in the system that might have occurred during the prior contraction period. This resting muscle length was slightly less than the length at which the contractile response was greatest. Before the experimental procedures, the dog was made hypoxemic and 500 ml of the dog’s blood were removed and replaced with 500 ml of blood from a donor dog. Although the blood perfusing the working muscle was subsequently treated differently for the two conditions,

IN

PRESENCE

OF

HBCO

831

the fact that the CMH blood was removed from the animal under the same hypoxemic conditions as in the HH condition minimized any other possible differences between the two blood conditions. The blood for the CMH condition was prepared in the following manner: one-third of the 500 ml sampled from the dog was quickly equilibrated with pure CO and then mixed with the rest of this blood, yielding an HbCO concentration of -30%. The mixture was gently bubbled with pure nitrogen and tonometered to PO:! and PCO~ values both close to 30 Torr, and finally the pH was readjusted to 7.4. If equilibrium is assumed, the PCO of such blood is negligibly low, -0.075 Torr when calculated from the Haldane equation: PC0 = HbCO/HbO, X PoJM, if M has the same value of 240 as for human blood. Before muscle perfusion with this blood, it was kept well mixed in a reservoir thermostated at 37OC. During perfusion, this blood was not allowed to return into the animal’s circulation and was recollected. Samples of this blood and of the dog’s normal blood were kept on ice for determination of their ODC by the biotonometry method (20). Experimental

Protocol

Each experiment (n = 6 dogs) consisted of four separate contraction periods. Each contraction period had a duration of 3 min and was separated by 15-20 min of rest. Before the first contraction period, the blood supply to the isolated muscle was switched from self-perfusion to pump perfusion, and enough time was allowed for conditions to stabilize at a blood flow similar to that resulting from self-perfusion. The first contraction period was performed while the muscle was perfused with either 1) the dog’s normal blood made hypoxic (30-31 Torr) by ventilating the animal with -10% OZ in N2 (HH) or 2) the reservoir hypoxic blood containing HbCO (CMH). For the second contraction period the alternate condition was employed. That is, if HH was used in the first contraction period, CMH was used in the second. The same blood flow used in the first contraction period was used in the second, so the first and the second periods had identical 0, deliveries. This protocol resulted in six matched Oa delivery pairs between HH and CMH (for the first 2 contraction periods). In four experiments the initial condition was HH, and in two it was CMH. For the initial contraction period the blood flow was set at a level maintaining the perfusion pressure at -140 Torr. The third and the fourth contraction periods were treated as the first 2 (again HH first in 4 muscles, CMH in 2), so that finally the 6 experiments yielded 12 matched 0, delivery pairs between HH and CMH. In five experiments vo2 max was measured under normoxic (Pao, 80 Torr) conditions between the first and the second pair of treatments. The perfusion time with CMH blood was kept as short as possible, starting only 30 s before the contraction period began and being discontinued as soon as the contraction period ended. Measurements Arterial blood samples were obtained from the arterial line entering the muscle, and venous samples were ob-


832

MUSCLE

MAXIMAL

0,

CONSUMPTION

HH

PHa PHV Pao,, Torr

14.6t0.3 31tl

50t2 7.43t0.02 7.34t0.02

72t2 7.39t0.02 7.30t0.02

mM

PsO for Hbfunct,* Torr Hill’s n for Hbfunct,* Torr

20&l

17&l

30 3.0

21 2.4

RESULTS

Values are means t SE; n = 12. HH, hypoxic hypoxia; CMH, hypoxic carboxyhemoglobinemia; Hbfuncc = HbOz + Hb - HbCO. [Hb], hemoglobin concn; HbCO, carboxyhemoglobin; HbOz, oxyhemoglobin; pHa and pH,, arterial and venous pH; Pao, and Pace,, arterial PO, and Pco~; [La],, arterial lactate concn; [HCO?],, arterial HCO, concn; P50, 02 half-saturation pressure of Hb. * Calculated on average data (see text).

tained from the left popliteal vein as close to the gastrocnemius as possible. Arterial and venous blood samples were drawn anaerobically during each rest period and during the last 20 s of each contraction and were kept on ice. As blood samples were drawn, venous blood flow measurements were made by timed blood collections into a graduated cylinder. Barbee et al. (l), by the use of muscle contractions similar to those used in this investigation, determined that a near steady-state flow and Vo2 had been achieved by the end of 2 min. Blood lactate concentrations were determined from the arterial and venous samples using a Yellow Springs Instruments model 23L blood lactate analyzer. Blood Paz, Pco~, and pH were measured within 5-8 min with a blood gas analyzer (Instrumentation Laboratories model 813) at 37°C while [Hb], percent 0, saturation, percent CO saturation, and Oa content were measured with an Instrumentation Laboratories 282 CO-oximeter. These instruments were calibrated daily. Plasma bicarbonate concentration was calculated from the measured

SO,=f(Po,)

so 2%

lOO,A

HBCO

Two-way analysis of variance was used for the statistical analysis. In all statistical analyses, the 0.05 level of significance was used.

28t2 5.8t0.7

4.020.3

[Lala, mM

OF

Statistics

31kl

3Okl 31k2

Pace,, Torr [HCOY]a,

CMH

14.4kO.5 It1

PRESENCE

pH and PCO~ values using the Henderson-Hasselbalch equation. The Fick principle was used to calculate VO, and lactate release. The muscle was removed and weighed at the end of each experiment.

1. Blood parameters corresponding to HH and CMH conditions TABLE

[Hb], g/100 ml HbCO;,, % of HbT HbOn,;,, % of Hbf,,,t

IN

co*=

co, 20

f

Mean weight of the exercised gastrocnemius muscle (n = 6) removed after the end of the experiment was 85 t 6 (SE) $5 Table 1 presents the principal variables of Ontransport and acid-base balance in the blood perfusing the muscles during the HH and the CMH conditions. Except for the HbCO and its effects on ODC, the blood prepared in vitro closely resembles that from the animal during the HH condition: total Hb (HbT), Paoz, and pH are the same. The lactate concentration was slightly higher in the blood prepared in vitro. The most salient feature is that Hill’s coefficient (n) and the 0, half-saturation pressure of Hb (P& are both markedly lower in the CMH condition. These differences reflected the leftwardshifted nature of the ODC due to the presence of 30% HbCO. For both conditions the shape and position of the ODC have been computed, taking into account the arterial and venous acid-base status measured during the experiments; thus the n and P,, values given in Table 1 are those effective for the muscle gas exchange but they cannot be readily compared with their equivalents for standard ODCs. Figure 1 shows the ODCs corresponding to our experiments. The curves of A represent the 0, saturation of the functional hemoglobin (SOL) vs. blood PO,. Functional hemoglobin is defined as all the hemoglobin that is available for 0, binding (HbT - HbCO). This way of

(PO,,

vol. %

1B

90 80 70 60 50 40

FIG. 1. Oxyhemoglobin dissociation curves for normal dog blood and dog blood containing 30% HbCO. A: O2saturation (Sop) of functional hemoglobin vs. Paz; B: 0, content (CO,) vs. PO,. Acid-base status corresponds to that of experimental conditions (see Table 1).

30 20 10 0


MUSCLE

MAXIMAL

0,

CONSUMPTION

expressing the ODC shows that the presence of CO shifts the curve to the left over the whole range of saturation. Analysis of 0, exchange, however, requires knowledge of the relationship between 0, concentration and PO, (Fig. 1B). Pilot experiments and calculations according to Roughton and Darling (25) prompted us to choose a value of 30% for HbCO, which produced the desired combination of a moderate degree of hypoxemia and a well-defined leftward shift of the ODC. Both the curves of Fig. 1 represent our experimental conditions, but only B shows the intersection point corresponding to an 0, content of 10 ml/100 ml and Paz of 30 Torr. Table 2 presents the data relevant to 0, transport and utilization. Muscle blood flow and Cao,, and consequently 0, delivery, were not different between HH and CMH. There was no significant difference in arterial blood pressure to the muscle between the two conditions and thus none in muscular vascular resistance because blood flows were identical. The principal observation was that V02max was 26% less during CMH, the difference between the two conditions being highly significant (P < 0.01). The O2 extraction ratio (Cao, - CvoJ/Cao2, where Cvo, is venous 0, content, was therefore also 26% less in CMH. The PO, of the muscle effluent venous blood (PvoJ was the same, 16 -+ 1 Torr in both conditions, indicating a large difference in Cvo,,. Figure 2 presents 0, delivery (A) and the individual If02 max data in relative (B) and absolute (C) terms for each of the contraction periods. A slight ordering effect for VO 2max in the course of the contraction periods was observed. On average, Vo2max was 30% less during CMH for the first pair of matched O2 delivery treatments and only 20% less during the second (average decline 26%). The difference in vo2max between HH and CMH did not depend on whether the CMH treatment was first or second in the order of matched treatment pairs. As expected with repeated fatiguing contraction bouts, VO2 maxdeclined slightly throughout the study, but to a greater extent during HH than during CMH. However, varying the order of the HH and CMH within a matched pair minimized any ordering effects. It is im2. Data related to and CA4H conditions TABLE

vo2

maXin HH

l

PRESENCE

OF

833

HBCO

02 delivery ml. min’l lOOgo 131 A 12 11 1

iiO*max. 96 change IOO - B 90 80 70 1 ml.min’l lOO$

icoO2max

12

1 st pair

.= HH

2 nd pair

O=CrnH

C

10 8 6 4

0' .

I

I

I

2

1

3

contraction

I

4

period

2. Vozrnax as a function of order of consecutive contraction periods l-4. First and second pairs are matched for their 0, delivery (A). Points represent individual values for each of the 4 periods for the 6 animals. C: VO 2max values; B: relative change with value in HH of 100% for each pair. FIG.

portant to note that VO zrnaxwas restored to near preCMH levels after CMH. In five of the muscles, VO zrnaxwas measured at a Pao, of 80 Torr in the absence of CO between the two sets of matched treatments (with the blood pressure and the blood flow to the muscle maintained at 140 Torr and 120 ml. min-l -100 g-l, respectively). Under these conditions, O2 delivery was 18.5 ml. min-’ 100 g-l and VO, maxaveraged 12.8 t 0.9 ml. min-’ 100 g-l, a value that is significantly higher (P < 0.01) than that obtained under the HH conditions. The amount of fatigue over the 3-min contraction period, expressed as the ratio (in percent) of final to maximal tension development, was greater in the CMH condition, even though initial developed tension was not different. Lactate output from the muscle was not significantly different between the two conditions, although there was a trend for lower lactate output in the CMH condition. l

l

HH Q, ml rnino 100 g-l Muscle arterial blood pressure, Torr Cao,, ml/100 ml CvO,, ml/100 ml 0, delivery, ml. min. 100 g-l VO 2 max7 ml. min. 100 g-’ 0, extraction, % La, pm01 . min. 100 g-l Pvo,, Torr Maximal developed tension, g/g Fatigue index, %

IN

120t6 140t6 10.1t0.5

2.8t0.4 12.1kO.7

CMH 119t6 143t7 10.2t0.3

4.6t0.4" 12.2t0.7

8.8t0.6 73t3

6.5t0.4" 54t2*

67tll 16tl 105t7 89t3

47tll 16tl lOOt6

75t3*

Values are means t SE; p = 12. HH, hypoxic hypoxia; CMH, hypoxic carboxyhemoglobinemia; Q, blood flow; Caoz and Cvo2, arterial and 0, uptake; La, lactate output; venous 0, content; VOzmax, maximal PO,. Fatigue index = final/maximal muscle tension. pvo,, venous * Significantly different (P < 0.05) from HH condition.

DISCUSSION

It has recently been suggested that the rate at which 0, can diffuse from the erythrocyte to the mitochondria is one of the important determinants of VOW,,, as O2 supply is reduced (30), and evidence has been obtained


834

MUSCLE

MAXIMAL

0,

CONSUMPTION

supporting this hypothesis (14, 15, 22). Gutierrez and colleagues (11, 12) have also demonstrated the importance of 0, diffusion in determining VOW in resting muscle when 0, delivery is reduced below critical levels. The purpose of the present study was to investigate this hypothesis further by administering to working muscles either hypoxic blood or blood with CO-induced increased 0, affinity under conditions of equal 0, delivery and inflowing Pa o, but differently shaped ODC. Reduced VO, maxDuring CMH The important result of this Study, showing that at the same OZ delivery VO 2maxwas 26% lower during CMH and could be reversibly altered in a predictable direction, was similar in outcome to prior observations (15) and shows that O2 delivery cannot be regarded as the unique determinant of VOW max. Although many physiologists believe that O2 delivery is perhaps the principal determinant of vo2 max (28), we have suggested that VO, maxis determined by the interaction of convective 02 delivery with the diffusive movement of 0, to the mitochondria. This diffusive flux can be considered a product of the diffusing capacity of the tissue (DO& and the POT driving gradient from capillary to mitochondria (Fick’s law). One way 0, delivery interacts with the process of Ox diffusion is by determining mean capillary POT. Thus a higher h2 maxcan be obtained when the PO:! within the capillary is maintained at a higher level because of the greater pressure head for OSdiffusion. In the present study, the leftward-shifted ODC during HbCO was expected to produce a lower mean capillary PO,, which for the same Pa . o,, 0, delivery, and Do2, would result in a lower vo 2 max7 if indeed the PO, driving gradient was an important parameter. However, we found that PvoZ was the same in both HH and CMH. Had the ODC been linear in both conditions, this unexpected finding would show that the mean capillary PO, would also be identical. Even with the small curvature of the ODCs over the range covered by the O2 extractions, the identity of the Pvo, values indicates that the mean capillary Po2 must have been similar in both HH and CMH conditions. Because the fall in VOzrnax during CMH cannot be considered to be due to a change in the capillary driving pressure for 02, as estimated by mean capillary Po2, then it seemslikely that the blood-to-tissue O2 diffusive conductance (Do2) was diminished during CMH. Computations of tissue Do2 using a Bohr integration procedure identical to that used in a study on humans (22) show that apparent Do2 decreasesby about the same amount during CMH. Thus these results suggest that as v02max perfusion with HbCO blood impairs the ease with which 0, can be transferred from the blood capillary to the mitochondria. However, it is important to determine whether CO may have had some adverse effect within the tissues and thus hampered cellular respiration and reduced V02max independently of O2 supply considerations. Concerning the interaction of CO with the enzymes of the respiratory chain, it is important to realize that during CMH the estimated PCO of the blood was very low and did not exceed 0.07 Torr (see METHODS). Many studies have

IN

PRESENCE

OF

HBCO

shown that much higher PCO’S are required to hamper the cellular respiration by poisoning of the respiratory chain (4, 31). Recently, Piantadosi et al. (21) showed that PCO 1 7 Torr causes an increase of phosphocreatine and ADP with no modification of Vo2. Thus it is safe to conclude that in our CMH conditions, i.e., with PCO 100 times lower, the cellular respiration was not hampered by CO binding to the cytochrome oxidases and that the reduction of VOWmaxduring CMH was due, to a large extent, to a reduction in the DO:! between the capillary and the mitochondria. Along with the lower VO zmaxmeasured during CMH, the tension development fell 16% more in the CMH than in the HH condition, as shown in Table 2. Whatever the mechanism of fatigue was, it is important in the context of the present study to know whether CO might have had any specific fatigue-generating effect other than that associated with the reduction of Vo2max. To this end, we have compared fatigue in our current and previous experiments (14) where Tj02 maxwas reduced by hypoxia alone. Figure 3’ illustrates this comparison by showing the fatigue as a function of VOW max.It is seen that a single relationship describes both studies. Thus it appears that, in the present experiments, fatigue was not CO dependent but resulted only from the reduction in VO,, independently of how this occurred. ~0 and Blood-to-Tissue 0, Diffusive Conductance Role of myoglobin. In muscles, the role of myoglobin as an 02 carrier protein is well established (33) and its complete blockade by CO can suppress up to 50% of the 02 delivery to mitochondria (34). CO-myoglobin affinity is some 30 times larger than that for 0, with myoglobin (5). Taking into account a myoglobin PSOfor CO of 0.16 Torr and a myoglobin saturation of -30% in maximally working muscle (18), one can estimate that for a PCO of 0.07 Torr myoglobin would be -25% saturated with CO at equilibrium. Although it is difficult to know what fraction of this equilibrium saturation was reached dur100 N 90’

1

normox r(

4

‘3i 80’ !i f

Hogan

I

This

et al. 1988 study

g 50. I, 4

I

I

6 8 ml. min”

I

10 100gwl

I

12

\io,max

FIG. 3. Fatigue, expressed as relative fall in muscle tension (from maximal tension to tension at the end of 3-min contraction period), as a function of i02 max. Data of Hogan et al. (14) are compared with those of this study.


MUSCLE

MAXIMAL

0,

CONSUMPTION

ing short exposures to carboxyhemoglobinemia, some blockade of the facilitated Oatransfer must be considered as a cause of the fall in tissue O2 conductance. King et al. (19), working with much higher blood CO contents (HbCO > 60%) and for much longer periods of CO exposure, found a decrease in iTo of only 16% during carboxyhemoglobinemia in muscles that were working well below Vo2 max.They also speculated on myoglobin poisoning as a possible cause of the fall in Vo2, but their experiments did not allow a separation of this effect from that of the leftward-shifted ODC. It is worth noting that Ross and Hlastala (24) could not demonstrate any change in To2 when muscle was perfused with blood that had a leftward-shifted ODC; however, their results were obtained in resting conditions when O2 delivery was not critical. . In our experiment, the normoxic measurement of vo 2maxthat was obtained in five of the muscles (between the 2.pairs of delivery-matched treatments) yielded norma1 Vo2 maxvalues (12.8 t 0.9 ml 100 8-l. min-I, which was 31% higher than in HH). This important feature not only shows that HH VO 2maxwas substantially lower than its normoxic equivalent- and that therefore 0, supply was limited-but also indicates that any CO poisoning was rapidly reversible. This reversibility was also attested to by ancillary experiments (n = 2) in which the gastrocnemius preparation was stimulated at a slightly fatiguing rate (4 Hz) for 3 min during HH, switched to CMH without interruption for 3 min, and then immediately switched back to HH for 3 min. Blood flow, Cao,, and Pao., were kept similar during all three steps. VO, fell 20%- from HH to CMH and rose again during the second HH period to the value expected if no CMH period had been interposed. Such results are also in accordance with the idea that with short exposure times and low blood PCO, myoglobin CO poisoning, if occurring, was quickly reversible. Rate of O2 release from the capillary blood. The rapid reversibility of the O2 conductance changes, as well as the relatively small degree of myoglobin CO saturation that can be assumed during treatment with HbCO, calls for some caution in considering myoglobin blockade as the unique cause of the reduced O2 conductance. Although this study does not address the issue of the location of diffusion impairment caused by HbCO, recent work of Honig and colleagues (6, 8, 9, 18) has shown, by the frozen myoglobin spectrophotometry method, that the Po2 associated with myoglobin in maximally working muscle is very low and that, consequently, any major site of 0, diffusion resistance is likely to be located along the pathway from Hb to the muscle cell membrane. One possibility is that the kinetics of 0, release from the erythrocytes is sufficiently slow as to impose a “diffusion” resistance to 0, transport, as has been suggested by Rose and Goresky (23) in the coronary circulation. It is known that in the lung the reaction kinetics can limit the rate of pulmonary 02 uptake (26), and it is also known that the velocity of O2 release by erythrocyte is slower than that of 0, uptake, as has been confirmed by Holland et al. (17). It is possible in this present study that the rate of 0, dissociation from Hb might have been

IN

PRESENCE

OF

835

HBCO

affected by the presence of HbCO and subsequently reduced during CMH. If true, this would be a factor in decreasing the overall Do2 and thereby lower Vo2max during CMH. Although the initial off-rate reaction depends only on the concentration of oxyhemoglobin (l7), which is the same in HH and CMH conditions, it might be suspected that the presence of HbCO could modify the time course of deoxygenation (27). Such a role for off-loading kinetics would be consistent with the predictions of Gutierrez (10). Effect of Leftward-Shifted

ODC

More than 80 years ago, Haldane (13) observed that the consequences of CO poisoning were worse for tissue oxygenation than those of an equivalent loss of functional hemoglobin by anemia. He interpreted his observation as resulting from the greater affinity of blood for Ox when HbCO was present, which, in turn, renders the 0, extraction by the tissues more difficult. Our results show that this difficulty is, to a large extent, due to the effect of a reduced blood-to-tissue conductance. However, mainly because the muscle is heterogeneous as far as Vo2-to-perfusion mismatch is concerned, it is impossible at this time to calculate an exact value for the DO,. Therefore we cannot exclude that part of the decrease in . vo 2maxduring CMH was still a result of the reduction of the local capillary-to-tissue POT gradients. On the whole organ, however, this effect appears to be outweighed by the reduction of the conductance for 0,. It is important to realize that the fall in v02 maxfrom normoxia (12.8 t 0.9 ml. 100 8-l. min-I) to hypoxia (8.8 t 0.6) associated with the diminished 0, delivery can result from both the reduced 0, supply and its functional heterogeneity. However, these two factors cannot explain the further fall in V02max observed in the CMH condition. In conclusion, these results demonstrate that 0, delivery cannot be regarded as the unique determinant of V02max and suggest that the rate of diffusion of 0, from erythrocyte to mitochondria, especially when some impairment (such as HbCO) is present, must also be taken into consideration. This research was supported by National Heart, Lung, and Blood Institute Grant HL-17731. For this work P. Haab was supported in part by the Swiss Association of Cigarette Manufacturers. Address for reprint requests: M. C. Hogan, Dept. of Medicine, M023A, University of California, San Diego, La Jolla, CA 92093-0623. Received

30 May

1989; accepted

in final

form

25 April

1990.

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Maximal O2 uptake of boys and girls -- ages 14--17.

Effect of hemoglobin concentration on maximal O2 uptake in canine gastrocnemius muscle in situ.

The effect of priming exercise on O2 uptake kinetics, muscle O2 delivery and utilization, muscle activity, and exercise tolerance in boys.

Increased plasma O2 solubility improves O2 uptake of in situ dog muscle working maximally.

Reliability of peak O2 uptake and O2 uptake kinetics in step exercise tests in healthy subjects.

Hypoxia and CO alter O2 extraction but not peripheral diffusing capacity during maximal aerobic exercise.

Systemic and regional O2 delivery and uptake in bled dogs given hypertonic saline, whole blood, or dextran.

Effects of training on muscle O2 transport at VO2max.

Rate constant and thermochemistry for K + O2 + N2 = KO2 + N2.

O2 delivery to the pregnant uterus: its relationship to O2 consumption.

Dietary nitrate accelerates postexercise muscle metabolic recovery and O2 delivery in hypoxia.

Effects of moderate hypothermia on O2 consumption at various O2 deliveries in a sheep model.

Oxygen uptake in maximal effort constant rate and interval running.

O2 delivery at VO2max and oxidative capacity in muscles of standardbred horses.

Abnormal oxidative metabolism and O2 transport in muscle phosphofructokinase deficiency.

A 31P-NMR study of tissue respiration in working dog muscle during reduced O2 delivery conditions.

Oxygen delivery (DEL O2) dependent and independent oxygen consumption (VO2).

An analog approach to measuring O2 uptake on-line.

O2, anybody?

[O2 uptake and CO2 production during total intravenous propofol-alfentanil anesthesia under steady-state conditions. Is O2 uptake a parameter for assessing the depth of anesthesia?].

Effects of altering O2 delivery on VO2 of isolated, working muscle.

Effect of hemodilution on blood flow, O2 consumption, and performance of skeletal muscle during exercise.

The reactions of Cr(CO)6, Fe(CO)5, and Ni(CO)4 with O2 yield viable oxo-metal carbonyls.

Maximal aerobic exercise in pregnant women: heart rate, O2 consumption, CO2 production, and ventilation.