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

A Comparison of the Arterial Blood Concentration of Isoflurane During Cardiopulmonary Bypass Between 2 Polypropylene Oxygenators

OBJECTIVE

The primary objective was to compare arterial blood concentration of isoflurane during cardiopulmonary bypass (CPB) between 2 polypropylene oxygenators of different designs. Secondary objectives were to compare levels of Bispectral Index Score (BIS) during CPB between the 2 oxygenators and to examine the relationships between oxygenator exhaust and arterial blood concentrations of isoflurane and BIS.

DESIGN

Single, blinded, randomized control trial.

SETTING

Teaching hospital.

PARTICIPANTS

Twenty-five patients undergoing cardiac surgery with CPB.

INTERVENTIONS

Subjects were randomly assigned (1:1) to Inspire 8F (Sorin) or Affinity Fusion (Medtronic) oxygenators.

MEASUREMENTS AND MAIN RESULTS

The mean arterial blood concentration in the Inspire 8F (Sorin) group was 59 (standard deviation [SD] 23) µg/mL, compared with 53 (SD 17) µg/mL in the Affinity Fusion (Medtronic) group with a nonsignificant mean difference of 6 (95% confidence interval = -11, 22) µg/mL (t[23] = 0.676, p = 0.50). No significant difference in BIS was found between the groups (p = 0.896). Moderate and strong, negative correlations respectively, were found between arterial and oxygenator exhaust correlations and BIS (r = -0.472, p < 0.05; r = -0.812, p < 0.001). A strong, positive correlation was found between arterial and exhaust isoflurane concentration (r = 0.810, p < 0.0005).

CONCLUSIONS

No significant difference in arterial blood concentration of isoflurane or BIS was found between the Inspire 8F (Sorin) and Affinity Fusion (Medtronic) oxygenators. A significant positive correlation was found between arterial blood and oxygenator exhaust concentrations of isoflurane, as well as significant negative correlations between both arterial and oxygenator exhaust concentrations of isoflurane and BIS.

The depth of anaesthesia associated with the administration of isoflurane 2.5% during cardiopulmonary bypass

BACKGROUND

Administering isoflurane 2.5% into the oxygenator during cardiopulmonary bypass results in no patient movement. However, doing so may result in an excessive depth of anaesthesia particularly, when hypothermia is induced. Bispectral index and arterial blood and oxygenator exhaust concentrations of volatile anaesthetics should be related to depth of anaesthesia. The primary aim of this study was to measure the depth of anaesthesia using bispectral index, resulting from administering isoflurane 2.5% into the oxygenator during cardiopulmonary bypass, and secondary aims were to examine the relationships between blood and oxygenator exhaust isoflurane concentrations and bispectral index.

METHODS

Arterial and mixed-venous blood samples were aspirated at three time points during cardiopulmonary bypass and measured for isoflurane concentration using mass spectrometry. Simultaneously, oxygenator exhaust isoflurane concentration, nasopharyngeal temperature and bispectral index were recorded.

RESULTS

When averaged across the three time points, all patients had a bispectral index score below 40 (binomial test, p < 0.001). There were no significant correlations between bispectral index score and arterial or mixed-venous blood isoflurane concentrations (r = -0.082, p = 0.715; r = -0.036, p = 0.874) and oxygenator exhaust gas concentration of isoflurane (r = -0.369, p = 0.091).

CONCLUSION

When 2.5% isoflurane was administered into the sweep gas supply to the oxygenator during cardiopulmonary bypass, all patients experienced a bispectral index score less than 40 and no significant relationship was found between either arterial or mixed-venous blood or oxygenator exhaust concentrations of isoflurane and bispectral index.

Physiological closed-loop control of mechanical ventilation and extracorporeal membrane oxygenation

A new concept is presented for cooperative automation of mechanical ventilation and extracorporeal membrane oxygenation (ECMO) therapy for treatment of acute respiratory distress syndrome (ARDS). While mechanical ventilation is continuously optimized to promote lung protection, extracorporeal gas transfer rates are simultaneously adjusted to control oxygen supply and carbon dioxide removal using a robust patient-in-the-loop control system. In addition, the cooperative therapy management uses higher-level algorithms to adjust both therapeutic approaches. The controller synthesis is derived based on the introduced objectives, the experimental setup and the uncertain models. Finally, the autonomous ARDS therapy system capabilities are demonstrated and discussed based on

Carbon dioxide production during cardiopulmonary bypass: pathophysiology, measure and clinical relevance

Carbon dioxide production during cardiopulmonary bypass derives from both the aerobic metabolism and the buffering of lactic acid produced by tissues under anaerobic conditions. Therefore, carbon dioxide removal monitoring is an important measure of the adequacy of perfusion and oxygen delivery. However, routine monitoring of carbon dioxide removal is not widely applied. The present article reviews the main physiological and pathophysiological sources of carbon dioxide, the available techniques to assess carbon dioxide production and removal and the clinically relevant applications of carbon dioxide-related variables as markers of the adequacy of perfusion during cardiopulmonary bypass.

Measurement of systemic carbon dioxide production during cardiopulmonary bypass: a comparison of Fick’s principle with oxygentor exhaust output

Theoretically, systemic carbon dioxide (VCO2) production should be an alternative means to systemic oxygen uptake (VO2) for estimating the global efficacy of cardiopulmonary bypass (CPB). This study compared two methods of estimating VCO2: Fick’s principle and oxygenator exhaust carbon dioxide (CO2) output. Both of these estimates were then compared with VO2. Fifty-one patients (39 male and 12 female) undergoing elective cardiac surgery requiring CPB were studied. Blood sampling was performed and measurements recorded during active cooling, environmental cooling/stable hypothermia and during rewarming. Blood samples were measured for CO2 tension from which content was estimated. VCO2 was calculated as the product of the arteriovenous difference in CO2 content and pump flow rate (Fick’s principle), or the fresh gas flow rate and concentration of the oxygenator exhaust CO2 (output technique). Over all measurements, method comparison analysis revealed a large mean bias of 41 (95% confidence intervals (CI) 32-50) mL/min with very wide limits of agreement (-23, 105 mL/min). Regression analysis found that the bias was also proportional to the size of measurement (β=0.75 (95% CI 0.55, 0.95)). Although both methods of VCO2 correlated significantly with VO2 ( p<0.01), regression analysis found that the coefficients (β) of both techniques had wide CI (Fick’s principle: β=1.37 (95% CI 1.20, 1.54); output technique: β=0.58 (95%CI 0.44, 0.71)). In conclusion, both techniques of VCO2 cannot be used interchangeably, and both are imprecisely related to VO2 as estimated by Fick’s principle.

Temperature-Related Differences in Mean Expired Pump and Arterial Carbon Dioxide in Patients Undergoing Cardiopulmonary Bypass

OBJECTIVE

To investigate the relationship between arterial carbon dioxide (PaCO(2)) and mean expired pump CO(2) during cardiopulmonary bypass (PeCPBCO(2)) in patients undergoing cardiac surgery with CPB during steady state, cooling, and rewarming phases of CPB.

DESIGN

Consenting patients, prospective study.

SETTING

University-affiliated hospital.

PARTICIPANTS

Twenty-nine patients.

INTERVENTIONS

Patients aged 22 to 81 years were enrolled. An alpha-stat acid-base regimen was performed during CPB. The PeCPBCO(2) was measured by an infrared multigas analyzer with the sampling line connected to the scavenging port of the oxygenator. Values for PaCPBCO(2) from the arterial outflow to the patient and PeCPBCO(2) during CPB at various oxygenator arterial temperatures were collected and compared. Data were analyzed by analysis of variance with 1-way repeated measures and post hoc pair-wise Tukey testing when appropriate. The differences between PaCPBCO(2) and PeCPBCO(2) were linearly regressed against temperature. A p value <0.05 was considered significant.

MEASUREMENTS AND MAIN RESULTS

Three to 5 data sets during CPB were collected from each patient. The mean gradient between PaCPBCO(2) and PeCPBCO(2) was positive 12.4 +/- 10.0 mmHg during the cooling phase and negative 9.3 +/- 9.9 mmHg during the rewarming phase, respectively. On regression of the data, the difference between PaCPBCO(2) and PeCPBCO(2) shows a good correlation with the change in temperature (r(2) = 0.79). The arterial CO(2) +/- x mmHg can be predicted by the formula PaCPBCO(2) = (-2.17x + 69.2) + PeCPBCO(2), where x is temperature in degrees C.

CONCLUSIONS

Monitoring the mean expired CO(2) value from the CPB oxygenator exhaust scavenging port with a capnography monitor provides a continuous and noninvasive data source to aid in sweep flow CPB circuit management during CPB.

Is body surface area still the best way to determine pump flow rate during cardiopulmonary bypass?

For over four decades, pump flow rate during cardiopulmonary bypass (CPB) has been estimated using body surface area (BSA). As patients presenting for heart surgery are increasingly obese, this approach may no longer be appropriate and other estimates of systemic metabolism should be used, such as body mass index and lean body mass. Mixed venous oxygen saturation (SvO2) is a robust and independent estimate of the global efficacy of CPB. The aim of this study was to determine which factors, including body surface area, body mass index and lean body mass, best predict SvO2 during CPB. Forty-eight patients undergoing elective cardiac surgery requiring CPB were studied. Patients' height, weight and skinfold thickness at four sites (biceps, triceps, subscapularis and suprailiac) were measured. Body surface area, lean body mass and body mass index were then calculated. Pump flow rate was maintained at 2.4 L/min/ m2 during CPB as per standard unit protocol. Arterial and mixed venous blood samples were taken during the cooling, stable hypothermia and rewarming phases of CPB. Nasopharyngeal temperatures and flow rates were recorded contemporaneously. The blood samples were analysed for oxygen saturation, haemoglobin concentration and partial pressures of oxygen and carbon dioxide. The values of the three time points were meaned. All potential predictor variables were then univariately correlated with mixed venous oxygen saturation (SvO2). Those correlating significantly (p < 0.1) were entered into a multivariate linear regression model. Nasopharyngeal temperature (beta = 0.615, p < 0.001) and lean body mass (beta = 0.256, p < 0.028) were the only significant predictors of SvO2 (r2 = 0.433, p2 < 0.001). Pump flow rates maintained at 2.4 L/min/m2 throughout CPB results in relative over-perfusion during hypothermia. Lean body mass may be a more sensitive estimate of systemic metabolism and, therefore, may provide a more accurate means of determining pump flow rate than body surface area in patients undergoing heart surgery.

Adding lactate to the prime solution during hypothermic cardiopulmonary bypass: a quantitative acid-base analysis

BACKGROUND

The effect of adding lactate to the cardiopulmonary bypass (CPB) prime was investigated using Stewart's quantitative acid-base approach. According to this quantitative model, serum pH and bicarbonate are determined by three independent factors: the partial pressure of carbon dioxide (PCO(2)), the total concentration of weak acids (e.g. albumin), and the strong ion difference. The apparent strong ion difference is calculated as the sum of sodium, potassium, magnesium and calcium minus chloride concentrations. The pH decreases with a smaller strong ion difference and vice versa.

METHODS

Twenty patients scheduled for coronary surgery were studied prospectively. All patients were treated identically, except for the prime, which either contained lactate or was lactate free. Just before bypass and before coming off bypass, haemoglobin, glucose, plasma osmolality and colloid osmotic pressure were determined; albumin, lactate, sodium, potassium, ionized calcium, magnesium, phosphate, arterial pH, PCO(2), bicarbonate, and base excess were measured for use in Stewart's analysis.

RESULTS

Metabolic acidosis had resolved by the end of bypass with the lactated prime. Although the strong ion gap (apparent minus effective strong ion difference) increased significantly in both groups, its composition differed significantly between the groups. The Stewart technique detected polyanionic gelatin as a weak acid component contributing to the unidentified anion fraction. Colloid osmotic pressure was maintained in both groups.

CONCLUSION

Exogenous lactate attenuates acidosis related to CPB. The oncotic and weak acid deficits produced by hypoalbuminaemia may be compensated for temporarily during CPB by polyanionic synthetic colloids such as succinylated gelatin.

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.

Use of monitoring devices during anesthesia for cardiac surgery: a survey of practices at public hospitals within the United Kingdom and Ireland

A questionnaire was sent to all 42 public hospitals, within the United Kingdom (UK) and Ireland, known to conduct elective cardiac surgery. Information was sought with regard to the availability of intraoperative monitoring equipment. A one hundred percent response rate was achieved. Pulse oximetry, inspired oxygen analysis, and expired carbon dioxide analysis were not utilized at 4, 8, and 10 hospitals, respectively. Similarly, continuous monitoring of arterial oxygen tension and oxygen fraction in the gas flow to the bypass machine was not conducted in 28 and 32 hospitals, respectively. This survey revealed that essential anesthetic monitoring devices, as defined by the United Kingdom Association of Anesthetists, are not in routine usage during the pre-bypass and post-bypass phases of anesthesia for cardiac surgery within the British Isles.

What's new in monitoring the coronary surgery patient?

Monitoring has been extensively reviewed in most textbooks of cardiothoracic surgery and anaesthesia, particularly in the recent textbooks on monitoring edited by Carol L Lake 1 and Casey D Blitt 2 and in the Journal of Clinical Monitoring. Although monitoring properly includes both pre- and postoperative periods, this review will concentrate exclusively on the operative period. I will also concentrate on new approaches or information which relate to more traditional approaches to monitoring. The emphasis in this review will not be on what we can monitor, but rather on what we should monitor. In this regard, I will analyse accuracy and identify sources of error and try to answer the following questions. Does the device or parameter measure (monitor) what we want to know? Does it improve patient outcome and safety? Is it cost-effective? Unfortunately, data are not always available to answer all these questions at present, but hopefully the discussions will make us aware of what we do and do not know, and what we should look for in the near future.