Minimum Pulse Pressure and Peripheral Temperature Needed for Pulse Oximetry During Cardiac Surgery With Cardiopulmonary Bypass Heikki Pllve,
MD, and Arno Vuori, MD
The lowest values of pulse pressure (dPP) and peripheral temperature (Tp) associated with reliable readings from three different pulse oximeters (Biox Ohmeda 3700, Datex Satlite, and Nellcor N-200) were assessed, along with the ability of the pulse oximeters to work immediately before and after total cardiopulmonary bypass (CPB). The lowest mean dPP with a reliable 0, saturation reading was 13 mm Hg
T
HE ACCURACY AND usefulness of pulse oximeters have been evaluated in several studies.‘-7 They have gained widespread use in hospitals as well as with outpatients and in emergency situations outside the hospital.8,’ Pulse oximeters work by comparing absorption of light of two wavelengths. Taking the pulsatile component of the light that accounts only for about 1% of the total absorption, they reflect “arterial blood” oxygen saturation (SpO,). However, there are also reports of misleading venous pulsations in the periphery.‘“,” Any situation that diminishes arterial pulsations in the peripheral tissues may interfere with pulse oximetry” and the accuracy of the SpO,.‘j Such situations include hypovolemia, hypothermia, and administration of vasoactive drugs. Hypoxemic patients also often have a poor peripheral circulation. However, the limits of pulse oximetry have not been fully assessed. The purpose of this study was to establish the lowest pulse pressure and peripheral temperature values associated with a reliable SpO, reading during partial cardiopulmonary bypass (CPB). Second, an attempt was made to determine whether there was a difference in the ability of the monitors to provide an accurate SpO, during a diminishing and an increasing peripheral pulse wave before and after total CPB.
and the lowest mean Tp was 23.6”C. The dPP needed for a reliable reading before total CPB did not diier significantly from that needed after total CPB. No significant differences in performance were found among the three oximeters during the study. Copyright 0 1991 by W.B. Saunders Company
(Normocap, Datex-Instrumentarium Corp) and kept at a level of normoventilation. ECG, heart rate, and rhythm, along with arterial pressure and pulse pressure (dPP) from a radial artery catheter were recorded continuously (Kone 565 monitor, Instrumentarium, Espoo, Finland). Core and peripheral temperatures were measured using esophageal and cutaneous thermistors. The Biox probe was placed on the earlobe of the patient and the Datex and Nellcor probes on the right hand of the patient together with the arterial catheter. Adequate collateral blood flow was checked by testing the temperature and color of the hand before and after the cannulation. The reliability of the pulse oximeters’ SpO, display was tested before, during, and after CPB against a hemoximeter SaO, (OSM-2, Radiometer, Copenhagen, Denmark), comparing simultaneously acquired data. To obtain the lowest dPP and peripheral temperature with a valid reading, these parameters were recorded continuously before and after total CPB. The lowest values for each patient either before or after total CPB were taken. Second, to determine whether there is a difference in the pulse oximeters’ ability to function during diminishing and increasing dPP, the moments of disappearance and reappearance of a valid reading (before and after total CPB) were assessed. The statististical analysis was made by a two-way analysis of variance. The interequipment comparisons were only made from the simultaneous data that were available for all oximeters to be compared. The results are expressed as mean and SD values. RESULTS
MATERIALS
AND METHODS
Three pulse oximeters were evaluated: Biox 3700 (Ohmeda Corp, Boulder, CO); Nellcor N-ZOO(Nellcor Inc, Hayward, CA) with electrocardiogram (ECG) synchronization; and Datex Satlite (Datex-Instrumentarium Carp, Espoo, Finland). A total of 18 cardiac surgical patients were studied with the Biox 3700 monitor, 17 of whom were also studied with the Datex monitor, and 12 of whom with the Nellcor monitor. The mean age of the patients was 52 years (range, 20 to 6.5years). The study protocol was approved by the ethics committee of University of Turku and Turku University Central Hospital. Informed consent was obtained from the patients. Premeditation of the patients consisted of morphine, 0.2 mg/kg, and scopolamine, 6 kg/kg, given intramuscularly 1 hour before anesthesia. A high-dose narcotic anesthetic was used inducing the patients with an infusion of 50 &kg of fentanyl over 5 minutes preceded by a 1.5-mg dose of pancuronium. An additional dose of fentanyl was given by constant infusion before the start of CPB. During induction, the patients were given lorazepam, 0.08 mg/kg, IV, and they breathed 100% oxygen through a face mask. After intubation they received oxygen in air (F,O, = 0.40). After induction, additional pancuronium was given up to a total dose of 0.1 mg/kg. During the anesthesia, the end-tidal CO, was monitored
Journal of Cardiothoracic and VascularAnesthesia,
Vol 5,
No 4
Mean dPP as low as 13 to 14 mm Hg was sufficient to give reliable SpO, readings (Table 1). At the same time, the core temperature was near normal. The skin temperature and the systolic pressure were both subnormal. AI1 the pulse oximeters studied could give reliable SpO, readings during markedly diminished temperature (Table 2). The core temperature measured from the esophagus was also lower than normal at the time of the low peripheral temperature. Before total CPB, the blood pressure and dPP declined quite sharply (Fig 1). On the average, at a dPP of about 20 mm Hg the SpO, reading was still reliable. As the dPP declined to about 10 mm Hg or less, the reading either
From the Departments of Anaesthesiology and Surgery, Turku University Central Hospital, Turku, Finland. Address mprirzt requests to Heikki P&e, MD, Department of Anaesthesiology, Turku University Central Hospital, SF-20520 Turku, Finland. Copyrikht 6 1991 by W B. Saunders C0mpan.v 1053-0770/91/0504-0004$03.0000/0
(August), 1991: pp327.330
327
PALVE AND VUORI
328
Table 1. Lowest Reliable Mean Pulse Pressure Values
dPP (mm Hg)
Biox
Date*
3700
Satlite
14
14
NdXX
A
N-ZOO
13
g
8
7
Tc (“C)
36.2
36.1
36.5
Tp (“C)
25.8
27.0
26.0
RRs (mm Hg)
58
62
64
No. of patients
18
17
12
SD
n7mlHg
partial perfusion
Abbreviations: dPP, pulse pressure; SD, standard deviation; Tc, core temperature; Tp, peripheral temperature;
RRs, systolic pressure at the
moment of lowest dPP.
20
vanished momentarily (Nellcor and Datex) or became unreliable (Biox 3700) (Table 3). After total CPB, reliable readings were regained with dPPs of about 16 mm Hg (Table 4). There were no statistical differences in the monitors’ ability to function reliably when comparing the oximeters with each other, or when comparing the individual monitor’s ability to work during diminishing or increasing dPP. The arterial saturation during partial CPB, as measured by the hemoximeter, remained over 96% in each patient except for one measurement. The mean SaO, during partial perfusion was 97.6% 2 1.9% and the corresponding mean values of the Biox, Datex, and Nellcor SpO, values differed from that by -1.6% * 1.3%, -2.0% * 1.7%, and 2.2% ? 1.7%, respectively.
10
t min) 1
partial
6
DISCUSSION
During cardiac surgery, hypothermia and CPB are used. The periods of partial CPB before and after surgery permit study of pulse oximeters in controlled situations of poor pulsatile peripheral perfusion and hypothermia. It has been shown that peripheral perfusion decreases markedly during CPB, and the patients often develop hypoperfusion of the peripheral tissues despite seemingly adequate central hemodynamics.14 Lawson et alI5 found pulse oximeter readings in normothermic patients with only 4% to 8% of normal peripheral blood flow using laser Doppler. However, Greenblott et al,” also using a laser Doppler, could not repeat the results of the Lawson group. Further, they found that as the flow under the probe diminished, the SpO, display became inaccurate. However, these studies referred only to relative, nonpulsatile blood flow because of the laser Doppler method used. Such a flow measurement is not an optimal method to investigate working requirements for pulse Table 2. Lowest Reliable Mean Peripheral Temperature
Values
Biox
Datex
Nellcor
3700
Satlite
N-200
TP (“‘3 SD
23.7
23.9
23.6
2.5
2.5
2.1
Tc (“C)
35.2
35.2
35.1
No. of patients
17
15
11
Abbreviations: Tp, peripheral temperature; Tc, core temperature
2
SD, standard deviation;
perfusion
OC
35
30
25
20
25
15
t 5
F
1
2
15
25 (min)
Fig 1. (A) The behavior of systolic blood pressure (RR,; O-O). mean blood pressure (RRm; O-4). and dPP (x-x. lower pressure scale], as well as timing of the disappearance and reappearance of readings in one of the patients. (8) The corresponding information on the same patient concarning central (Tc) and peripheral (Tp) temperatures. (1) The moment of disappearance of display; (2) the momant of reappearance of display.
LOW PRESSURES, HYPOTHERMIA,
& PULSE OXIMETRY
329
Table 3. Relationship of Pulse Pressure and SpO, Reliability Prebypass Reliable Reading
Unreliable
Reading
Biox
Datex
dPP (mm Hg)
21.6
20.8
SD
10.2
7.4
RRs(mmHg)
63
66
62
49
51
46
9
a
7
12
10
8
No. of patients
Nelhr
Biox
Datex
17.6
10.1
9.7
N&XV
5.9
5.0
7.8
6.1
4.6
NOTE. The mean of the pulse pressure (dPP) and systolic pressure (RRs) values during partial CPB before cardiac surgery just prior to the disappearance
of the SpO, display (reliable reading) and immediately
after that (unreliable reading).
oximeters because it does not assess the pulsatile component of flow. In the present study a clinically available parameter, dPP, was studied. In order to function, pulse oximeters need pulsations at the tissue level. A minimum of about 13 mm Hg of dPP in the radial artery was found to be needed for a valid reading. This was about 27% of the initial mean value of 48 mm Hg in these patients. The results support those of Greenblott et al,” who found, in normothermic patients, that 22% to 38% of baseline flow is needed for reliable function of pulse oximeters. Considering the lowest values of dPP in this study with a reliable SpO, display before or after the total CPB, the working capacity of the pulse oximeters can be considered very good. These patients were also hypothermic, which is likely to further lessen peripheral perfusion. Peripheral perfusion diminishes during partial CPB, and it is often lower just before than immediately after total CPB.14 The reason for this might be that the patients are centrally warmed before the end of CPB but they are somewhat hypothermic before the institution of CPB. The present results are in accordance with this finding (Tables 3 and 4). Skin temperature is normally above 33°C. With a fingertip temperature of 23”C, the blood flow has diminished considerably.” Low peripheral temperature has been reported to stop pulse oximeter function.‘” However, the authors believe that the disappearance of the SpO, in this study was due less to the low temperature than to the narrow dPP. In the present study, individual fingertip temperatures as low as 20.3”C did not cause the saturation Table 4. Relationship of Pulse Pressure and SpO, Reliability Postbypass Unreliable
Reading
Reliable Reading
Biox
Data
Nellcor
Biox
Data
Nellcor
3700
Satlite
N-ZOO
3700
Satlite
N-200
16.3
dPP (mm Hg)
11.4
10.0
5.8
16.1
16.8
SD
13.4
12.1
3.6
10.9
10.1
RRs(mmHg)
49
49
53
58
62
69
7
6
6
17
16
11
No. of patients
9.7
NOTE. The mean of the pulse pressure (dPP) and systolic pressure (RRsl values during partial CPB after cardiac surgery just before the reappearance
of pulse oximeter display (unreliable reading) and imme-
diately after that (reliable reading).
reading to disappear or to become unreliable, until the dPP diminished. In individual patients, disappearance of the reading did not necessarily occur at the moment of the lowest dPP or temperature, nor did the reappearance of the display occur at the moment of the first “good” dPP value (Fig 1A). After total CPB, as myocardial activity is regained, the pulsatile flow under the probe increases. The dPP did not always increase rapidly. In this situation of increasing pulsation, reliable readings were achieved at lower dPP values comparable to those during diminishing pulsation prior to total CPB. The display was considered reliable when there was a stable SpO,, a heart rate equal to the ECG rate, and a signal strength bar greater than half of the maximum.” As reported previously, mere heart rate equality with the ECG is not enough to determine the SpO, display as accurate.‘” On the other hand, heart rate inequality does not imply that the display is inaccurate.” The readings of the Datex and Nellcor monitors were easy to determine as being either reliable or unreliable, because the stable readings disappeared rapidly as the pulsatile flow diminished to a certain value and only rarely did the monitor regain any display after that. Surprisingly, in individual cases, the Biox 3700 gave a seemingly reliable SpO, reading for some time even during total CPB with no dPP. This also has been reported previously,20~z2but the reason is unclear. The accuracy of the pulse oximeters, tested against the OSM-2 hemoximeter, was found to be good for all three. Because testing at the very moment of sudden disappearance and reappearance proved to be impossible, the accuracy was tested before and after these events, respectively. The bias and precision of the Ohmeda Biox 3700 ear and finger probes have been found to be identical.‘,” The radial arterial catheter in the ipsilateral hand has been shown not to affect the reliability of the SpO, values.23 The C-lock system of the Nellcor N-200 connects the ECG with the peripheral pulsations to make it easier for the monitor to distinguish between artefact tissue movement and artery-derived pulsation. The present patients were anesthetized and paralyzed and there were no motion artefacts. Theoretically, in cases of very poor dPP, a monitor with C-lock could work reliably longer. In this study, the C-lock system of the Nellcor N-200 made the oximeter convenient to use and the determination of the display inaccuracy very easy. However, it did not widen the range of reliability toward a cooler periphery and a poorer dPP. In conclusion, the three pulse oximeters studied were reliable when the dPP was very low. There were no differences among the individual monitors. The reliability of the SpO, reading was easiest to determine with the Datex and Nellcor monitors. The ECG-connected pulse detection system of Nellcor did not widen the physiological limits of reliable functioning. There were no differences in the ability of the monitors to read accurate SpO, values in situations of diminishing or increasing perfusion.
PALVE AND VUORI
330
REFERENCES
1. Cote CJ, Goldstein EA, Cote MA, et al: A single-blind study of pulse oximetry in children. Anesthesiology 68:184-188, 1988 2. Yelderman M, New W: Evaluation of pulse oximetry. Anesthesiology 59:349-352, 1983 3. Morris RW, Buschman A, Warren DL, et al: The prevalence of hypoxemia detected by pulse oximetry during recovery from anaesthesia. J Clin Monitor 4:16-20, 1988 4. Cecil WT, Thorpe KJ, Fibuch EE: A clinical evaluation of the accuracy of the Nellcor N-100 and Ohmeda 3700 pulse oximeters. J Clin Monitor 4:31-36,1988 5. Severinghaus JW, Naifeh KH: Accuracy and response characteristics of six pulse oximeters to profound hypoxia. Anesthesiology 67:551-558, 1987 6. Severinghaus JW, Naifeh KH, Koh SO: Errors in 14 pulse oximeters during profound hypoxia. J Clin Monitor 5:72-81, 1989 7. Mihm FG, Halperin BD: Noninvasive detection of profound arterial desaturations using a pulse oximetry device. Anesthesiology 62:85-87, 1985 8. Silverston P: Pulse oximetry at the roadside: A study of pulse oximetry in immediate care. Br Med J 298:711-713,1989 9. McGuire TJ, Pointer JE: Evaluation of a pulse oximeter in the prehospital setting. Ann Emerg Med 17:1058-1062, 1988 10. Kim J-M, Arakawa K, Benson KT, Fox DK: Pulse oximetry and circulatory kinetics associated with pulse volume amplitude measured by photoelectric plethysmography. Anesth Analg 65:13331339,1986 Il. Mark JB: Systolic venous waves cause spurious signs of arterial hemoglobin desaturation. Anesthesiology 71:158-160, 1989
12. Gabrielczyk MR, Buist RJ: Pulse oximetry and postoperative hypothermia. Anaesthesia 43:402-404, 1988 13. Ridley SA: A comparison of two pulse oximeters. Anaesthesia 43:136-140, 1988 14. Kuttila K, Niinikoski J: Peripheral tissue perfusion during coronary artery bypass grafting. Perfusion 4:25-31, 1989 15. Lawson D, Norley I, Korbon G, et al: Blood flow limits and pulse oximeter signal detection. Anesthesiology 67:599-603, 1987 16. Greenblott GB, Gerschultz S, Tremper KK: Blood flow limits and signal detection comparing five different models of pulse oximeters. Anesthesiology 70:367-368, 1989 17. Kuttila K, Niinikoski J: Peripheral perfusion after cardiac surgery. Crit Care Med 17:217-220,1989 18. Paulus DA, Monroe MC: Cool fingers and pulse oximetry. Anesthesiology 71:168-169, 1989 19. Wong DH, Tremper KK, ‘Davidson J, et al: Pulse oximetry is accurate in patients with dysrhythmias and a pulse deficit. Anesthesiology 70:1024-1025,1989 20. Pllve H, Vuori A: Pulse oximetry during low cardiac output and hypothermia states immediately after open heart surgery. Crit Care Med 17:66-69.1989 21. Kurki T, Smith T, Sanford T, Head N: Pulse oximetry during open heart surgery. Anesth Analg 67:S123,1988 22. Kurki TS, Smith NT, Sanford TJ, et al: Pulse oximetry and finger blood pressure measurement during open heart surgery. J Clin Monitor 5:221-228, 1989 23. Morris RW, Nairn M, Beaudoin M: Does the radial arterial line degrade the performance of a pulse oximeter? Anesth Intens Care 18:107-109,199O
MD, and Arno Vuori, MD
The lowest values of pulse pressure (dPP) and peripheral temperature (Tp) associated with reliable readings from three different pulse oximeters (Biox Ohmeda 3700, Datex Satlite, and Nellcor N-200) were assessed, along with the ability of the pulse oximeters to work immediately before and after total cardiopulmonary bypass (CPB). The lowest mean dPP with a reliable 0, saturation reading was 13 mm Hg
T
HE ACCURACY AND usefulness of pulse oximeters have been evaluated in several studies.‘-7 They have gained widespread use in hospitals as well as with outpatients and in emergency situations outside the hospital.8,’ Pulse oximeters work by comparing absorption of light of two wavelengths. Taking the pulsatile component of the light that accounts only for about 1% of the total absorption, they reflect “arterial blood” oxygen saturation (SpO,). However, there are also reports of misleading venous pulsations in the periphery.‘“,” Any situation that diminishes arterial pulsations in the peripheral tissues may interfere with pulse oximetry” and the accuracy of the SpO,.‘j Such situations include hypovolemia, hypothermia, and administration of vasoactive drugs. Hypoxemic patients also often have a poor peripheral circulation. However, the limits of pulse oximetry have not been fully assessed. The purpose of this study was to establish the lowest pulse pressure and peripheral temperature values associated with a reliable SpO, reading during partial cardiopulmonary bypass (CPB). Second, an attempt was made to determine whether there was a difference in the ability of the monitors to provide an accurate SpO, during a diminishing and an increasing peripheral pulse wave before and after total CPB.
and the lowest mean Tp was 23.6”C. The dPP needed for a reliable reading before total CPB did not diier significantly from that needed after total CPB. No significant differences in performance were found among the three oximeters during the study. Copyright 0 1991 by W.B. Saunders Company
(Normocap, Datex-Instrumentarium Corp) and kept at a level of normoventilation. ECG, heart rate, and rhythm, along with arterial pressure and pulse pressure (dPP) from a radial artery catheter were recorded continuously (Kone 565 monitor, Instrumentarium, Espoo, Finland). Core and peripheral temperatures were measured using esophageal and cutaneous thermistors. The Biox probe was placed on the earlobe of the patient and the Datex and Nellcor probes on the right hand of the patient together with the arterial catheter. Adequate collateral blood flow was checked by testing the temperature and color of the hand before and after the cannulation. The reliability of the pulse oximeters’ SpO, display was tested before, during, and after CPB against a hemoximeter SaO, (OSM-2, Radiometer, Copenhagen, Denmark), comparing simultaneously acquired data. To obtain the lowest dPP and peripheral temperature with a valid reading, these parameters were recorded continuously before and after total CPB. The lowest values for each patient either before or after total CPB were taken. Second, to determine whether there is a difference in the pulse oximeters’ ability to function during diminishing and increasing dPP, the moments of disappearance and reappearance of a valid reading (before and after total CPB) were assessed. The statististical analysis was made by a two-way analysis of variance. The interequipment comparisons were only made from the simultaneous data that were available for all oximeters to be compared. The results are expressed as mean and SD values. RESULTS
MATERIALS
AND METHODS
Three pulse oximeters were evaluated: Biox 3700 (Ohmeda Corp, Boulder, CO); Nellcor N-ZOO(Nellcor Inc, Hayward, CA) with electrocardiogram (ECG) synchronization; and Datex Satlite (Datex-Instrumentarium Carp, Espoo, Finland). A total of 18 cardiac surgical patients were studied with the Biox 3700 monitor, 17 of whom were also studied with the Datex monitor, and 12 of whom with the Nellcor monitor. The mean age of the patients was 52 years (range, 20 to 6.5years). The study protocol was approved by the ethics committee of University of Turku and Turku University Central Hospital. Informed consent was obtained from the patients. Premeditation of the patients consisted of morphine, 0.2 mg/kg, and scopolamine, 6 kg/kg, given intramuscularly 1 hour before anesthesia. A high-dose narcotic anesthetic was used inducing the patients with an infusion of 50 &kg of fentanyl over 5 minutes preceded by a 1.5-mg dose of pancuronium. An additional dose of fentanyl was given by constant infusion before the start of CPB. During induction, the patients were given lorazepam, 0.08 mg/kg, IV, and they breathed 100% oxygen through a face mask. After intubation they received oxygen in air (F,O, = 0.40). After induction, additional pancuronium was given up to a total dose of 0.1 mg/kg. During the anesthesia, the end-tidal CO, was monitored
Journal of Cardiothoracic and VascularAnesthesia,
Vol 5,
No 4
Mean dPP as low as 13 to 14 mm Hg was sufficient to give reliable SpO, readings (Table 1). At the same time, the core temperature was near normal. The skin temperature and the systolic pressure were both subnormal. AI1 the pulse oximeters studied could give reliable SpO, readings during markedly diminished temperature (Table 2). The core temperature measured from the esophagus was also lower than normal at the time of the low peripheral temperature. Before total CPB, the blood pressure and dPP declined quite sharply (Fig 1). On the average, at a dPP of about 20 mm Hg the SpO, reading was still reliable. As the dPP declined to about 10 mm Hg or less, the reading either
From the Departments of Anaesthesiology and Surgery, Turku University Central Hospital, Turku, Finland. Address mprirzt requests to Heikki P&e, MD, Department of Anaesthesiology, Turku University Central Hospital, SF-20520 Turku, Finland. Copyrikht 6 1991 by W B. Saunders C0mpan.v 1053-0770/91/0504-0004$03.0000/0
(August), 1991: pp327.330
327
PALVE AND VUORI
328
Table 1. Lowest Reliable Mean Pulse Pressure Values
dPP (mm Hg)
Biox
Date*
3700
Satlite
14
14
NdXX
A
N-ZOO
13
g
8
7
Tc (“C)
36.2
36.1
36.5
Tp (“C)
25.8
27.0
26.0
RRs (mm Hg)
58
62
64
No. of patients
18
17
12
SD
n7mlHg
partial perfusion
Abbreviations: dPP, pulse pressure; SD, standard deviation; Tc, core temperature; Tp, peripheral temperature;
RRs, systolic pressure at the
moment of lowest dPP.
20
vanished momentarily (Nellcor and Datex) or became unreliable (Biox 3700) (Table 3). After total CPB, reliable readings were regained with dPPs of about 16 mm Hg (Table 4). There were no statistical differences in the monitors’ ability to function reliably when comparing the oximeters with each other, or when comparing the individual monitor’s ability to work during diminishing or increasing dPP. The arterial saturation during partial CPB, as measured by the hemoximeter, remained over 96% in each patient except for one measurement. The mean SaO, during partial perfusion was 97.6% 2 1.9% and the corresponding mean values of the Biox, Datex, and Nellcor SpO, values differed from that by -1.6% * 1.3%, -2.0% * 1.7%, and 2.2% ? 1.7%, respectively.
10
t min) 1
partial
6
DISCUSSION
During cardiac surgery, hypothermia and CPB are used. The periods of partial CPB before and after surgery permit study of pulse oximeters in controlled situations of poor pulsatile peripheral perfusion and hypothermia. It has been shown that peripheral perfusion decreases markedly during CPB, and the patients often develop hypoperfusion of the peripheral tissues despite seemingly adequate central hemodynamics.14 Lawson et alI5 found pulse oximeter readings in normothermic patients with only 4% to 8% of normal peripheral blood flow using laser Doppler. However, Greenblott et al,” also using a laser Doppler, could not repeat the results of the Lawson group. Further, they found that as the flow under the probe diminished, the SpO, display became inaccurate. However, these studies referred only to relative, nonpulsatile blood flow because of the laser Doppler method used. Such a flow measurement is not an optimal method to investigate working requirements for pulse Table 2. Lowest Reliable Mean Peripheral Temperature
Values
Biox
Datex
Nellcor
3700
Satlite
N-200
TP (“‘3 SD
23.7
23.9
23.6
2.5
2.5
2.1
Tc (“C)
35.2
35.2
35.1
No. of patients
17
15
11
Abbreviations: Tp, peripheral temperature; Tc, core temperature
2
SD, standard deviation;
perfusion
OC
35
30
25
20
25
15
t 5
F
1
2
15
25 (min)
Fig 1. (A) The behavior of systolic blood pressure (RR,; O-O). mean blood pressure (RRm; O-4). and dPP (x-x. lower pressure scale], as well as timing of the disappearance and reappearance of readings in one of the patients. (8) The corresponding information on the same patient concarning central (Tc) and peripheral (Tp) temperatures. (1) The moment of disappearance of display; (2) the momant of reappearance of display.
LOW PRESSURES, HYPOTHERMIA,
& PULSE OXIMETRY
329
Table 3. Relationship of Pulse Pressure and SpO, Reliability Prebypass Reliable Reading
Unreliable
Reading
Biox
Datex
dPP (mm Hg)
21.6
20.8
SD
10.2
7.4
RRs(mmHg)
63
66
62
49
51
46
9
a
7
12
10
8
No. of patients
Nelhr
Biox
Datex
17.6
10.1
9.7
N&XV
5.9
5.0
7.8
6.1
4.6
NOTE. The mean of the pulse pressure (dPP) and systolic pressure (RRs) values during partial CPB before cardiac surgery just prior to the disappearance
of the SpO, display (reliable reading) and immediately
after that (unreliable reading).
oximeters because it does not assess the pulsatile component of flow. In the present study a clinically available parameter, dPP, was studied. In order to function, pulse oximeters need pulsations at the tissue level. A minimum of about 13 mm Hg of dPP in the radial artery was found to be needed for a valid reading. This was about 27% of the initial mean value of 48 mm Hg in these patients. The results support those of Greenblott et al,” who found, in normothermic patients, that 22% to 38% of baseline flow is needed for reliable function of pulse oximeters. Considering the lowest values of dPP in this study with a reliable SpO, display before or after the total CPB, the working capacity of the pulse oximeters can be considered very good. These patients were also hypothermic, which is likely to further lessen peripheral perfusion. Peripheral perfusion diminishes during partial CPB, and it is often lower just before than immediately after total CPB.14 The reason for this might be that the patients are centrally warmed before the end of CPB but they are somewhat hypothermic before the institution of CPB. The present results are in accordance with this finding (Tables 3 and 4). Skin temperature is normally above 33°C. With a fingertip temperature of 23”C, the blood flow has diminished considerably.” Low peripheral temperature has been reported to stop pulse oximeter function.‘” However, the authors believe that the disappearance of the SpO, in this study was due less to the low temperature than to the narrow dPP. In the present study, individual fingertip temperatures as low as 20.3”C did not cause the saturation Table 4. Relationship of Pulse Pressure and SpO, Reliability Postbypass Unreliable
Reading
Reliable Reading
Biox
Data
Nellcor
Biox
Data
Nellcor
3700
Satlite
N-ZOO
3700
Satlite
N-200
16.3
dPP (mm Hg)
11.4
10.0
5.8
16.1
16.8
SD
13.4
12.1
3.6
10.9
10.1
RRs(mmHg)
49
49
53
58
62
69
7
6
6
17
16
11
No. of patients
9.7
NOTE. The mean of the pulse pressure (dPP) and systolic pressure (RRsl values during partial CPB after cardiac surgery just before the reappearance
of pulse oximeter display (unreliable reading) and imme-
diately after that (reliable reading).
reading to disappear or to become unreliable, until the dPP diminished. In individual patients, disappearance of the reading did not necessarily occur at the moment of the lowest dPP or temperature, nor did the reappearance of the display occur at the moment of the first “good” dPP value (Fig 1A). After total CPB, as myocardial activity is regained, the pulsatile flow under the probe increases. The dPP did not always increase rapidly. In this situation of increasing pulsation, reliable readings were achieved at lower dPP values comparable to those during diminishing pulsation prior to total CPB. The display was considered reliable when there was a stable SpO,, a heart rate equal to the ECG rate, and a signal strength bar greater than half of the maximum.” As reported previously, mere heart rate equality with the ECG is not enough to determine the SpO, display as accurate.‘” On the other hand, heart rate inequality does not imply that the display is inaccurate.” The readings of the Datex and Nellcor monitors were easy to determine as being either reliable or unreliable, because the stable readings disappeared rapidly as the pulsatile flow diminished to a certain value and only rarely did the monitor regain any display after that. Surprisingly, in individual cases, the Biox 3700 gave a seemingly reliable SpO, reading for some time even during total CPB with no dPP. This also has been reported previously,20~z2but the reason is unclear. The accuracy of the pulse oximeters, tested against the OSM-2 hemoximeter, was found to be good for all three. Because testing at the very moment of sudden disappearance and reappearance proved to be impossible, the accuracy was tested before and after these events, respectively. The bias and precision of the Ohmeda Biox 3700 ear and finger probes have been found to be identical.‘,” The radial arterial catheter in the ipsilateral hand has been shown not to affect the reliability of the SpO, values.23 The C-lock system of the Nellcor N-200 connects the ECG with the peripheral pulsations to make it easier for the monitor to distinguish between artefact tissue movement and artery-derived pulsation. The present patients were anesthetized and paralyzed and there were no motion artefacts. Theoretically, in cases of very poor dPP, a monitor with C-lock could work reliably longer. In this study, the C-lock system of the Nellcor N-200 made the oximeter convenient to use and the determination of the display inaccuracy very easy. However, it did not widen the range of reliability toward a cooler periphery and a poorer dPP. In conclusion, the three pulse oximeters studied were reliable when the dPP was very low. There were no differences among the individual monitors. The reliability of the SpO, reading was easiest to determine with the Datex and Nellcor monitors. The ECG-connected pulse detection system of Nellcor did not widen the physiological limits of reliable functioning. There were no differences in the ability of the monitors to read accurate SpO, values in situations of diminishing or increasing perfusion.
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