In vitro anesthetic washin and washout via bubble oxygenators: influence of anesthetic solubility and rates of carrier gas inflow and pump blood flow

The uptake and elimination of volatile anesthetic agents administered to patients under conditions of hemodilution and hypothermia during cardiopulmonary bypass have not been determined. To define the limitations imposed by oxygenators, we defined washin and washout curves for volatile anesthetic agents administered to bubble oxygenators primed with diluted blood (without connection to a patient). There was rapid equilibration of anesthetic partial pressure between delivered gas and blood (85-90% within 16 minutes). Increasing the gas inflow to the oxygenator from 3 to 12 L/min hastened washin and washout slightly, while increasing the pump blood flow from 3 to 5 L/min had no effect. Rates of washin and washout of anesthetics differed as a function of their blood/gas solubilities: enflurane greater than isoflurane greater than halothane during washin; isoflurane greater than enflurane greater than halothane during washout. However, these differences were small. Oxygenator exhaust partial pressures of anesthetic correlated with simultaneously obtained blood partial pressures, suggesting that monitoring exhaust gas may be useful clinically.     

Desflurane pharmacokinetics during cardiopulmonary bypass

OBJECTIVE: To describe the washin and washout of desflurane when first administered during cardiopulmonary bypass (CPB) for cardiac surgery. DESIGN: A single-arm prospective study. SETTING: University-affiliated hospital operating room. PARTICIPANTS: Ten adult patients presenting for cardiac surgery. INTERVENTIONS: Consenting patients presenting for cardiac surgery received anesthesia with midazolam and fentanyl. Patients were cooled to 32 degrees C on CPB, then desflurane 6% was administered and blood samples drawn repeatedly from the arterial and venous bypass cannulae as well as from the membrane oxygenator inlet and exhaust from 2 to 32 minutes of desflurane administration. Just before rewarming, final (maximum) washin samples were taken. On rewarming, desflurane was discontinued, and blood and gas samples were taken 2 to 24 minutes thereafter. MEASUREMENTS AND MAIN RESULTS: CPB time was 116 +/- 10 minutes, and ischemic time was 81 +/- 6 minutes. Mean pump flow was 4.49 +/- 0.03 L/min, and mean arterial pressure was 70.1 +/- 1 mmHg during the study period. Arterial washin of desflurane was initially rapid; arterial concentrations reached 50% of administered concentrations within 4 minutes, but then slowed, reaching 68% of inspired concentrations at 32 minutes (desflurane concentration 4.0% +/- 0.3%). Arterial washout of desflurane was more rapid; arterial concentrations fell to 18% of the maximum concentration reached within 4 minutes, and only 8% of the maximum arterial concentration was present in blood 20 minutes later. CONCLUSION: Desflurane showed rapid initial washin and washout on CPB when administration was started at 32 degrees C and stopped at time of rewarming.     

Washin and washout of isoflurane during cardiopulmonary bypass

To help decide when an inhalational agent should be discontinued during cardiopulmonary bypass (CPB), its rate of washin and washout must be known. Isoflurane one per cent was administered to 14 patients undergoing CPB and isoflurane blood concentrations were measured to determine the time course of washin and washout of this agent. Bubble oxygenators were used for seven patients and membrane oxygenators for the remaining seven. During the administration of isoflurane, isoflurane blood concentrations rose slowly and did not reach a steady state during the time available for washin. Isoflurane blood concentrations decreased by at least 50 per cent within two minutes of turning off the vaporizer, and by 15 minutes the concentration had dropped by 75 per cent. There was a tendency for more rapid elimination of isoflurane in patients undergoing rewarming during this period. There did not appear to be an important difference between bubble and membrane oxygenators in the rate of washin and washout of isoflurane. Within 15 minutes of turning off the vaporizer only 25 per cent of the original blood concentration of isoflurane will remain. The anaesthetist must decide what concentration of isoflurane is acceptable during separation from CPB. Knowledge of the time course of isoflurane washout will allow more accurate determination of when to discontinue its administration in order to reach an acceptable concentration by the time separation from CPB occurs.     

Oxygenator exhaust capnography as an index of arterial carbon dioxide tension during cardiopulmonary bypass using a membrane oxygenator

We have studied the relationship between the partial pressure of carbon dioxide in oxygenator exhaust gas (PECO2) and arterial carbon dioxide tension (PaCO2) during hypothermic cardiopulmonary bypass with non- pulsatile flow and a membrane oxygenator. A total of 172 paired measurements were made in 32 patients, 5 min after starting cardiopulmonary bypass and then at 15-min intervals. Additional measurements were made at 34 degrees C during rewarming. The degree of agreement between paired measurements (PaCO2 and PECO2) at each time was calculated. Mean difference (d) was 0.9 kPa (SD 0.99 kPa). Results were analysed further during stable hypothermia (n = 30, d = 1.88, SD = 0.69), rewarming at 34 degrees C (n = 22, d = 0, SD = 0.84), rewarming at normothermia (n = 48, d = 0.15, SD = 0.69) and with (n = 78, d = 0.62, SD = 0.99) or without (n = 91, d = 1.07, SD = 0.9) carbon dioxide being added to the oxygenator gas. The difference between the two measurements varied in relation to nasopharyngeal temperature if PaCO2 was not corrected for temperature (r2 = 0.343, P = < 0.001). However, if PaCO2 was corrected for temperature, the difference between PaCO2 and PECO2 was not related to temperature, and there was no relationship with either pump blood flow or oxygenator gas flow. We found that measurement of carbon dioxide partial pressure in exhaust gases from a membrane oxygenator during cardiopulmonary bypass was not a useful method for estimating PaCO2.      

Solubility of desflurane (I-653), sevoflurane, isoflurane, and halothane in human blood

The blood/gas partition coefficients for the new volatile anesthetic agent desflurane (I-653), sevoflurane, isoflurane, and halothane were determined, simultaneously, in 8 human volunteers to compare the solubilities of these agents in blood. The blood/gas partition coefficient for desflurane [0.49 +/- 0.03 (mean +/- SD)] was smallest, followed by sevoflurane (0.62 +/- 0.04), isoflurane (1.27 +/- 0.06), and halothane (2.46 +/- 0.09). Differences among the anesthetic agents were significant (P less than 0.001). The results of this study confirm that among these agents the solubility of desflurane in human blood is the smallest. The results suggest that the washin and washout of desflurane will be more rapid than that of sevoflurane, isoflurane, and halothane, and the washin and washout of sevoflurane will be more rapid than that of isoflurane and halothane.     

Monitoring oxygenator expiratory isoflurane concentrations and the bispectral index to guide isoflurane requirements during cardiopulmonary bypass

OBJECTIVE: The purpose of this study was to measure the changes in isoflurane requirements during the rewarming phase of cardiopulmonary bypass with moderate hypothermia. DESIGN: An observational study. SETTING: University hospital, single center. PARTICIPANTS: Forty patients undergoing elective coronary artery bypass surgery with cardiopulmonary bypass. INTERVENTIONS: Isoflurane requirements were quantified by measuring the concentrations in the oxygenator expiratory gas. Anesthesia was guided by bispectral index monitoring. MEASUREMENTS AND MAIN RESULTS: Isoflurane concentrations required to maintain the bispectral index between 40 and 50 during the rewarming phase of cardiopulmonary bypass were measured. There was a progressive increase in expiratory isoflurane requirements during rewarming from 30 degrees C to 37 degrees C, with a Pearson correlation coefficient of 0.78. There was a significant difference in the concentration required at 30 degrees C (0.41% +/- 0.14%) compared with 37 degrees C (1.00% +/- 0.12%). CONCLUSION: Isoflurane requirements are reduced during hypothermic cardiopulmonary bypass. Monitoring anesthetic concentrations in the oxygenator expiratory gas may be a useful adjunct to monitoring the depth of anesthesia.     

膜式氧合器在胎羊体外循环中的运用

目的　为了改进胎羊体外循环技术 ,探讨膜式氧合器在胎羊体外循环中的应用。　方法　将健康怀孕山羊8只 ,采用 Dideco 90 1膜式氧合器和滚轴泵建立胎羊体外循环 ,常温 (37℃ )转流 6 0分钟 ,氧合器内充低氧混合气体 (8%O2 和 92 % N2 ) ,监测胎羊的血压、心率、血气、血清乳酸和胎盘血管阻力。　结果　胎羊体外循环中动脉氧分压 (PO2 )和二氧化碳分压 (PCO2 )维持在宫内生理水平 ,胎羊心搏有力 ,血压正常。但胎羊 p H值缓慢下降 (P<0 .0 5 ) ,血清乳酸值明显增高 (P<0 .0 1) ,胎盘血管阻力显著上升 (P<0 .0 1)。停体外循环后胎羊出现低氧、高碳酸血症和酸中毒。　结论　胎羊体外循环影响胎盘功能 ,膜式氧合器可以代替胎盘气体交换功能 ,体外循环中胎羊生理低水平 PO2 是否适合其需要值得探讨。     

Pharmacokinetics of desflurane, sevoflurane, isoflurane, and halothane in pigs

We tested the prediction that the alveolar washin and washout, tissue time constants, and pulmonary recovery (volume of agent recovered during washout relative to the volume taken up during washin) of desflurane, sevoflurane, isoflurane, and halothane would be defined primarily by their respective solubilities in blood, by their solubilities in tissues, and by their metabolism. We concurrently administered approximately one-third the MAC of each of these anesthetics to five young female swine and determined (separately) their solubilities in pig blood and tissues. The blood/gas partition coefficient of desflurane (0.35 +/- 0.02) was significantly smaller (P less than 0.01) than that of sevoflurane (0.45 +/- 0.02), isoflurane (0.94 +/- 0.05), and halothane (2.54 +/- 0.21). Tissue/blood partition coefficients of desflurane and halothane were smaller than those for the other two anesthetics (P less than 0.05) for all tissue groups. As predicted from their blood solubilities, the order of washin and washout was desflurane, sevoflurane, isoflurane, and halothane (most to least rapid). As predicted from tissue solubilities, the tissue time constants for desflurane were smaller than those for sevoflurane, isoflurane, and halothane. Recovery (normalized to that of isoflurane) of the volume of anesthetic taken up was significantly greater (P less than 0.05) for desflurane (93% +/- 7% [mean +/- SD]) than for halothane (77% +/- 6%), was not different from that of isoflurane (100%), but was less than that for sevoflurane (111% +/- 17%). The lower value for halothane is consistent with its known metabolism, but the lower (than sevoflurane) value for desflurane is at variance with other presently available data for their respective biodegradations.     

The relationship between oxygenator exhaust P(CO2) and arterial P(CO2) during hypothermic cardiopulmonary bypass

During cardiopulmonary bypass the partial pressure of carbon dioxide in oxygenator arterial blood (P(a)CO2) can be estimated from the partial pressure of gas exhausting from the oxygenator (P(E)CO2). Our hypothesis is that P(E)CO2 may be used to estimate P(a)CO2 with limits of agreement within 7 mmHg above and below the bias. (This is the reported relationship between arterial and end-tidal carbon dioxide during positive pressure ventilation in supine patients.) During hypothermic (28-32 degrees C) cardiopulmonary bypass using a Terumo Capiox SX membrane oxygenator, 80 oxygenator arterial blood samples were collected from 32 patients during cooling, stable hypothermia, and rewarming as per our usual clinical care. The P(a)CO2 of oxygenator arterial blood at actual patient blood temperature was estimated by temperature correction of the oxygenator arterial blood sample measured in the laboratory at 37 degrees C. P(E)CO2 was measured by connecting a capnograph end-to-side to the oxygenator exhaust outlet. We used an alpha-stat approach to cardiopulmonary bypass management. The mean difference between P(E)CO2 and P(a)CO2 was 0.6 mmHg, with limits of agreement (+/-2 SD) between -5 to +6 mmHg. P(E)CO2 tended to underestimate P(a)CO2 at low arterial temperatures, and overestimate at high arterial temperatures. We have demonstrated that P(E)CO2 can be used to estimate P(a)CO2 during hypothermic cardiopulmonary bypass using a Terumo Capiox SX oxygenator with a degree of accuracy similar to that associated with the use of end-tidal carbon dioxide measurement during positive pressure ventilation in anaesthetized, supine patients.     

Alveolar-to-arterial-to-venous anesthetic partial pressure differences in humans

To determine the correlation between the partial pressures of anesthetics in venous and arterial blood (Pv and Pa), and to assess whether this correlation was better than that between the partial pressure of anesthetic in alveolar gas (PA) and Pa, isoflurane (n = 4) or halothane (n = 4) was administered to eight patients undergoing surgery, and Pv, PA, and Pa were measured. PA correlated with Pa better than did Pv (R = 0.960 vs. 0.878), and there was less variability in the data. Differences between Pv and Pa increased as the relative blood flow to the hand decreased [indicated by an increasing arterial-to-venous (a-v) O2 content difference]. The difference between PA and Pa was approximately 20% of the difference between inspired gas (PI) and Pa. The differences between PA and Pa appear to be due primarily to contamination of alveolar gas by physiologic dead space gas.     

In Vivo Uptake and Elimination of Isoflurane by Different Membrane Oxygenators during Cardiopulmonary Bypass

Background: Volatile anesthetics are frequently used during cardiopulmonary bypass (CPB) to maintain anesthesia. Uptake and elimination of the volatile agent are dependent on the composition of the oxygenator. This study was designed to evaluate whether the in vivo uptake and elimination of isoflurane differs between microporous membrane oxygenators containing a conventional polypropylene (PPL) membrane and oxygenators with a new poly-(4-methyl-1-pentene) (PMP) membrane measuring isoflurane concentrations in blood.

Methods: Twenty-four patients undergoing elective coronary bypass surgery with the aid of CPB were randomly allocated to one of four groups, using either one of two different PPL-membrane oxygenators for CPB or one of two different PMP-membrane oxygenators. During hypothermic CPB, 1% isoflurane in an oxygen-air mixture was added to the oxygenator gas inflow line (gas flow, 3 l/min) for 15 min. Isoflurane concentration was measured in blood and in exhaust gas at the outflow port of the oxygenator. Between-group comparisons were performed for the area under the curve (AUC) during uptake and elimination of the isoflurane blood concentrations, the maximum isoflurane blood concentration (Cmax), and the exhausted isoflurane concentration (FE).

Results: The uptake of isoflurane, expressed as AUC of isoflurane blood concentration and a function of FE, was significantly reduced in PMP oxygenators compared to PPL oxygenators (P < 0.01). Cmax was between 8.5 and 13 times lower in the PMP-membrane oxygenator groups compared to the conventional PPL-membrane oxygenator groups (P < 0.01).     

In vivo uptake and elimination of isoflurane by different membrane oxygenators during cardiopulmonary bypass

BACKGROUND: Volatile anesthetics are frequently used during cardiopulmonary bypass (CPB) to maintain anesthesia. Uptake and elimination of the volatile agent are dependent on the composition of the oxygenator. This study was designed to evaluate whether the in vivo uptake and elimination of isoflurane differs between microporous membrane oxygenators containing a conventional polypropylene (PPL) membrane and oxygenators with a new poly-(4-methyl-1-pentene) (PMP) membrane measuring isoflurane concentrations in blood. METHODS: Twenty-four patients undergoing elective coronary bypass surgery with the aid of CPB were randomly allocated to one of four groups, using either one of two different PPL-membrane oxygenators for CPB or one of two different PMP-membrane oxygenators. During hypothermic CPB, 1% isoflurane in an oxygen-air mixture was added to the oxygenator gas inflow line (gas flow, 3 l/min) for 15 min. Isoflurane concentration was measured in blood and in exhaust gas at the outflow port of the oxygenator. Between-group comparisons were performed for the area under the curve (AUC) during uptake and elimination of the isoflurane blood concentrations, the maximum isoflurane blood concentration (C(max)), and the exhausted isoflurane concentration (F(E)). RESULTS: The uptake of isoflurane, expressed as AUC of isoflurane blood concentration and a function of F(E), was significantly reduced in PMP oxygenators compared to PPL oxygenators (P < 0.01). C(max) was between 8.5 and 13 times lower in the PMP-membrane oxygenator groups compared to the conventional PPL-membrane oxygenator groups (P < 0.01). CONCLUSIONS: The uptake of isoflurane into blood via PMP oxygenators during CPB is severely limited. This should be taken into consideration in cases using such devices.     

Oxygenator exhaust capnography: a method of estimating arterial carbon dioxide tension during cardiopulmonary bypass.

An in vivo study was undertaken during hypothermic (28 degrees C) cardiopulmonary bypass to compare oxygenator exhaust capnography as a means of estimating arterial carbon dioxide tension (PaCO2) with bench blood gas analysis. A total of 123 pairs of measurements were made in 40 patients. Oxygenator exhaust capnographic measurements systematically underestimated PaCO2 measured by a bench blood gas analyzer. During the cooling and stable hypothermic phases of cardiopulmonary bypass, the relationship was reasonably accurate, but became far more variable during rewarming. Oxygenator exhaust capnography could be used as an inexpensive means of continuously monitoring PaCO2 during the cooling and stable hypothermic phases of cardiopulmonary bypass but should not be used during rewarming.     

ARTERIAL WASHIN OF HALOTHANE AND ISOFLURANE IN YOUNG AND ELDERLY ADULT PATIENTS

We have studied the effect of age on washin of isoflurane andhalothane by comparing end - tidal (PE') and arterial (Pa) partialpressures of the agents in young (18–32 yr) and elderly(63–82 yr) healthy patients for 20 min after introductionof the agents, before surgery. PE' was measured by infra - redanalysis and Pa by gas chromatography. Washin of isofluraneoccurred at the same rate in the young and elderly, with nosignificant difference between young and elderly in PE' or Paas proportions of the inspired partial pressure (PI). After20 min of isoflurane administration, mean Pa/Pl in the youngwas 0.57 (95% confidence limit (CL) 0.53–0.62) and 0.55in the elderly (95% CL 0.51–0.59). Washin of halothanewas slower in the elderly than in the young, with Pa/Pl significantlyless in the elderly from 10 min after introduction of halothane.The difference between age groups, however, was small: meanPa/Pl after 20 min of halothane administration 0.45 (95% CL0.41–0.49) in the young and 0.38 (95% CL 0.35–0.41)in the elderly. Washin of isoflurane was significantly fasterthan that of halothane in both young and elderly subjects. Forisoflurane, the PE'-Pa gradient was small relative to Pa anddid not differ significantly between young and elderly. Forhalothane, PE'-Pa in the young did not differ significantlyfrom that for isoflurane. In the elderly, PE'-Pa for halothanewas significantly greater than in the young and than PE'-Pafor isoflurane.     

Effects of sevoflurane and isoflurane on systemic vascular resistance: use of cardiopulmonary bypass as a study model

We have examined the dose-related effects of sevoflurane and isoflurane on systemic vascular resistance (SVR) during cardiopulmonary bypass (CPB) in patients undergoing elective coronary artery surgery. Fifty- two patients were allocated randomly to one of six groups to receive 1.0, 2.0 or 3.0 vol% (inspiratory) sevoflurane or 0.6, 1.2 or 1.8 vol% isoflurane, or to a control group. During hypothermic (32-33 degrees C) non-pulsatile CPB, systemic vascular resistance index (SVRI) was recorded before administration of volatile anaesthetics and every 5 min for 20 min. Sevoflurane and isoflurane concentrations were measured next to the gas inlet port and at the gas outlet port of the oxygenator. Wash-in of sevoflurane occurred more rapidly than that of isoflurane, reaching a relatively steady state for both agents from the 10th to the 20th min. There was no significant change in SVRI in patients receiving 1.0 and 2.0 vol% sevoflurane, and 0.6 and 1.2 vol% isoflurane, compared with baseline values. However, 3 vol% sevoflurane decreased SVRI at 10, 15 and 20 min, and 1.8 vol% isoflurane decreased SVRI significantly at 15 and 20 min, whereas SVRI increased at 15 and 20 min in the control group. Thus during CPB, sevoflurane had similar vasodilator effects on SVRI as isoflurane.      

Biologically variable pulsation improves jugular venous oxygen saturation during rewarming

BACKGROUND: Conventional pulsatile (CP) roller pump cardiopulmonary bypass (CPB) was compared to computer controlled biologically variable pulsatile (BVP) bypass designed to return beat-to-beat variability in rate and pressure with superimposed respiratory rhythms. Jugular venous O2 saturation (SjvO2) below 50% during rewarming from hypothermia was compared for the two bypass techniques. A SjvO2 less than 50% during rewarming is correlated with cognitive dysfunction in humans. METHODS: Pigs were placed on CPB for 3 hours using a membrane oxygenator with alpha-stat acid base management and arterial filtration. After apulsatile normothermic CPB was initiated, animals were randomized to CP (n = 8) or BVP (roller pump speed adjusted by an average of 2.9 voltage output modulations/second; n = 8), then cooled to a nasopharyngeal temperature of 28 degrees C. During rewarming to stable normothermia, SjvO2 was measured at 5 minute intervals. The mean and cumulative area for SjvO2 less than 50% was determined. RESULTS: No between group difference in temperature existed during hypothermic CPB or during rewarming. Mean arterial pressure, arterial partial pressure O2, and arterial partial pressure CO2 did not differ between groups. The hemoglobin concentration was within 0.4 g/dL between groups at all time periods. The range of systolic pressure was greater with BVP (41 +/- 18 mm Hg) than with CP (12 +/- 4 mm Hg). A greater mean and cumulative area under the curve for SjvO2 less than 50% was seen with CP (82 +/- 96 versus 3.6% +/- 7.3% x min, p = 0.004; and 983 +/- 1158 versus 42% +/- 87% x min; p = 0.004, Wilcoxon 2-sample test). CONCLUSIONS: Computer-controlled BVP resulted in significantly greater SjvO2 during rewarming from hypothermic CPB. Both mean and cumulative area under the curve for SjvO2 less than 50% exceeded a ratio of 20 to 1 for CP versus BVP. Cerebral oxygenation is better preserved during rewarming from moderate hypothermia with bypass that returns biological variability to the flow pattern.     

Pharmacologic EEG suppression during cardiopulmonary bypass: cerebral hemodynamic and metabolic effects of thiopental or isoflurane during hypothermia and normothermia

We have determined the effects of thiopental or isoflurane upon cerebral blood flow (CBF) and the cerebral metabolic rate for oxygen (CMRO2) when these agents are used in sufficient dose to attain a deep burst suppression pattern on the electroencephalogram (EEG) during hypothermic and normothermic cardiopulmonary bypass (CPB). Thirty-one patients undergoing coronary artery bypass graft surgery were anesthetized with fentanyl 0.1 mg X kg-1, and were randomly allocated to one of three groups: control (no further anesthetics during bypass and continuous EEG activity), thiopental treatment (EEG suppression), or isoflurane treatment (EEG suppression). Hypothermia (25-29 degrees C) was routinely induced at onset of nonpulsatile cardiopulmonary bypass. In the treatment groups, thiopental or isoflurane were used during bypass to achieve a deep burst suppression pattern. Cerebral blood flow and cerebral metabolic rate for oxygen were determined during hypothermia and upon rewarming to normothermia (37 degrees C). Pharmacologic EEG suppression with either isoflurane or thiopental was associated with lower cerebral metabolic rate than control values during both hypothermic and normothermic bypass. However, only thiopental-induced EEG suppression was associated with lower cerebral blood flow than control. Cerebral blood flow during isoflurane-induced EEG suppression was similar to control values in spite of the reduced cerebral metabolic rate.     

Solubility of I-653, sevoflurane, isoflurane, and halothane in plastics and rubber composing a conventional anesthetic circuit

This study defines some characteristics of a standard anesthetic circuit that may impede anesthetic induction and recovery with I-653, sevoflurane, isoflurane, and halothane. Partition coefficients for anesthetic circuit components (masks, bellows, bags, airways, and circuit tubes) consistently ranked halothane greater than isoflurane greater than sevoflurane greater than I-653, suggesting a reverse order of washin and washout rates for an anesthetic circuit constructed from similar components. Consistent with this prediction, the concentrations of I-653 increased and decreased more rapidly than those of the other agents at any flow rate during washin (0.5, 1, or 2 L/min gas inflow rates) or washout (1, 3, or 5 L/min) in a conventional anesthetic circuit. The rates of change in I-653 concentration closely approximated the maximal possible theoretical rates. Our results suggest that absorption of I-653 by circuit components or soda lime should not hinder induction of or recovery from anesthesia.     

A comparison of anaesthetic tensions in arterial blood and oxygenator exhaust gas during cardiopulmonary bypass

This study evaluates the usefulness of the analysis of gas sampled from the exhaust port of a membrane oxygenator in the estimation of anaesthetic tension in arterial blood. Sixty-seven arterial blood samples were drawn from patients undergoing hypothermic cardiopulmonary bypass with anaesthesia maintained by either isoflurane or desflurane. Anaesthetic tensions in the oxygenator exhaust gas were measured using an infrared analyser and in arterial blood using a two-stage headspace technique with a gas chromatograph. Both measurement systems were calibrated with the same standard gas mixtures. There was no difference in anaesthetic tension measured in arterial blood and gas leaving the oxygenator exhaust (isoflurane: n = 29, range: 0.3-0.8%, 95% limits of agreement: -0.08% to 0.09%; desflurane: n = 38, range: 1.5-5.4%; 95% limits of agreement -0.65% to 0.58%). We conclude that anaesthetic tensions in arterial blood can be accurately monitored by analysis of the gas emerging from the exhaust port of a membrane oxygenator.     

In vitro validation of the Affinity NT oxygenator arterial outlet temperatures

During cardiopulmonary bypass, the rates of cooling and rewarming and the maximum temperatures attained are implicated in patient morbidity. Thus, accurate oxygenator arterial outlet temperature measurements are needed. The purpose of this study was to determine the accuracy of the arterial outlet temperature probe on the "Affinity NT" membrane oxygenator in measuring perfusate temperatures. An in vitro circuit was used. Crystalloid solution was recirculated through an Affinity NT membrane oxygenator and, to simulate the patient, a second oxygenator. Water was recirculated through the heat exchanger of the second oxygenator via a reservoir. A myocardial temperature probe was inserted in-line 4 cm distal to the Affinity NT oxygenator arterial outlet temperature probe and was considered to measure the actual temperature of the perfusate. Temperatures were simultaneously recorded from the in-line probe, arterial outlet probe, and reservoir every second. Twenty-seven trials were run using random combinations of three Affinity NT oxygenators and three in-line probes. Each trial entailed cooling an initially normothermic reservoir to 28 degrees C and then rewarming it to normothermia again. The arterial outlet temperature probe on the Affinity NT membrane oxygenator underestimated the perfusate temperatures during early rewarming (bias of 0.72 degrees C; precision of +/-1.15 degrees C) and late rewarming (bias of 0.52 degrees C; precision of +/-0.97 degrees C). An overestimation of the perfusate temperatures occurred during early cooling (bias of -0.57 degrees C; precision of +/-1.37 degrees C). Only during the late cooling phase was the arterial outlet temperature probe accurate (bias of -0.02 degrees C; precision of +/-0.3 degrees C). The perfusionist should be aware of the temperature probe monitoring characteristics of the oxygenator to safely perfuse the patient.