Continuous measurement of cardiac output by inert gas throughflow: comparison with thermodilution

Impaired blood pressure compensation following hemorrhage in conscious obese Zucker rats

AIMS

Hemorrhagic shock leads to a higher risk of mortality and morbidity in obese patients, however the mechanisms for these outcomes are unclear. We hypothesized that following severe hemorrhage, blood pressure control in conscious obese Zucker rats (OZ) is impaired.

MAIN METHODS

Experiments were performed in conscious lean Zucker rats (LZ) and OZ. Blood pressure, heart rate, cardiac output, total peripheral resistance (TPR), plasma renin activity (PRA), plasma antidiuretic hormone (ADH), and blood gasses were measured before and after severe hemorrhage (35% of the total blood volume).

KEY FINDINGS

Basal blood pressure, cardiac output, TPR, PRA, and ADH levels were not different between LZ and OZ. Compared to LZ, OZ exhibited impaired baroreflex control of heart rate and showed higher levels of vascular adrenergic tone. One hour after the hemorrhage, LZ and OZ exhibited similar decreases in cardiac output. However, blood pressure, heart rate, TPR, PRA, and ADH levels were lower in OZ than in LZ.

SIGNIFICANCE

These results indicate that conscious OZ has impaired blood pressure compensation after hemorrhage due to a blunted increase in TPR. This is due at least in part to an impaired regulation of vasoconstrictor hormones. To our knowledge, the current study is the first to demonstrate that hemodynamic responses and associated hormone secretion are impaired in a conscious obese model.

Cardiac output monitoring: comparison of a new arterial pressure waveform analysis to the bolus thermodilution technique in patients undergoing off-pump coronary artery bypass surgery

OBJECTIVE

To analyze the clinical agreement between the conventional intermittent bolus thermodilution (TD) technique and a new arterial pressure waveform analysis (APCO) technique (FloTrac; Edward Lifesciences, Irvine, CA) for cardiac output (CO) estimation.

DESIGN

Prospective observational clinical study.

SETTING

Cardiac surgery operating room of a tertiary care cardiac center.

PARTICIPANTS

Twelve patients undergoing elective off-pump coronary artery bypass (OPCAB) surgery.

MEASUREMENTS AND MAIN RESULTS

CO was determined by 2 different methods: TD and APCO at 8 time points (preinduction, postinduction, poststernotomy, left internal mammary artery to left anterior descending artery anastomosis, left [obtuse marginal/diagonal] anastomosis, right [right coronary/posterior descending coronary artery] anastomosis, postprotamine administration, and poststernal closure) in 12 patients undergoing elective OPCAB surgery. The mean bias and limits of agreement (2 standard deviations) expressed in liters per minute at respective points of measurement were -0.54 +/- 1.12, -0.37 +/- 1.0, -0.42 +/- 1.50, -0.25 +/- 1.18, -0.31 + 1.28, +/-0.41 +/- 1.0, 0.06 +/- 1.50, and 0.09 +/- 1.40.

CONCLUSION

Good agreement was found between the CO values obtained by the APCO and TD techniques throughout the intraoperative period including the period of coronary artery graft surgery.

Cardiac output estimation with a new Doppler device after off-pump coronary artery bypass surgery

OBJECTIVE

To compare cardiac output (CO), stroke volume (SV), and cardiac index (CI) as estimated with a new, noninvasive Doppler device (Ultrasonic Cardiac Output Monitor [USCOM]; USCOM Ltd, Sydney, Australia) with those measured with the bolus thermodilution (TD) technique.

DESIGN

Prospective nonrandomized study.

SETTING

Postcardiac surgery recovery unit of a tertiary cardiac center.

PARTICIPANTS

Fifty patients after off-pump coronary artery bypass (OPCAB) surgery.

MEASUREMENT AND MAIN RESULTS

Both right-sided and left-sided CO were estimated with a USCOM continuous-wave (CW) Doppler device, and CO was determined with the bolus TD technique performed in triplicate. On comparing the right-sided CO, SV, and CI with those of TD, the mean bias was 0.03 L/min, 1.6 mL, and 0.02 L/min/m(2), respectively. The comparison of left-sided CO, SV, and CI with those of TD revealed a means bias of 0.14 L/min, 1.0 mL, and 0.08 L/min/m(2), respectively.

CONCLUSION

This study showed excellent agreement between the values for CO, SV, and CI as determined with USCOM and TD. Since there was only 1 time period for CO estimation in each patient with both methods, the stability of this correlation needs to be further investigated over time.

Messung des Herzzeitvolumens

Diagnosis and therapy of hemodynamic instability are of the utmost importance in the treatment of critically ill patients during surgery and in intensive care. For both diagnosis and therapy, adequate and preferably continuous hemodynamic monitoring is essential. Besides the assessment of cardiac preload and blood pressure, cardiac output represents an important clinical marker of cardiac performance and global perfusion. Since its clinical introduction by Swan and Ganz in 1970, the standard technique for measuring cardiac output has been the pulmonary arterial thermodilution technique using a pulmonary artery catheter. The ongoing discussion on the risk-benefit ratio of such a pulmonary artery catheter has led to the introduction of several less invasive methods for determining cardiac output. The aim of this review is to provide background information on these alternative methods and to discuss the individual advantages and disadvantages of each method in the context of their clinical applicability.

Physiologically precise simulation of multiple lung gas exchange during anaesthesia by simultaneous gas infusion and extraction

A lung gas exchange simulator was tested which produces simultaneous uptake and/or elimination of multiple gases by an artificial test lung with physiologically realistic gas expired and exhaust gas flows, using a combination of infusion of diluting/enriching gases into the lung with lung gas extraction. A deterministic algorithm is incorporated which calculates required gas infusion and extraction flow rates for any set of possible target gas exchange values with any given set of fresh gas flows and concentrations. Six different scenarios were simulated, comprising a range of gas exchange values for each gas species which lie within a physiologically realistic range for anaesthetized patients. For each of these experiments the system was tested for 15 consecutive measurements over 25 min by measurement of gas exchange in the system using the Haldane transformation.

RESULTS

the mean bias and standard error of the mean bias (SE, in parentheses) relative to the target value was: +0.001 (0.002) l min(-1) for O(2) uptake, -0.002 (0.005) l min(-1) for CO(2) production, -0.001 (0.002) l min(-1) for uptake of nitrous oxide and +0.3 (0.1) ml min(-1) for uptake of a volatile anaesthetic agent (isoflurane). The confidence limits of the mean bias were within 5% of the target value for all gases and scenarios with the exception of those where a low uptake of anaesthetic gas was specified. The confidence limits of the mean bias for the lower uptakes of isoflurane were within 10% of the target value for these scenarios and within 15% for the low uptake of N(2)O. Good accuracy and precision of this approach to lung gas exchange simulation were demonstrated, resulting in a versatile simulator.

Non-invasive measurement of intrapulmonary shunt during inert gas rebreathing

We tested the agreement between non-invasive measurement of intrapulmonary shunt, using oxygen uptake and pulmonary capillary blood flow measurement obtained by nitrous oxide rebreathing, with that measured using mixed venous blood sampling. Nine patients were recruited pre- and post-cardiac surgery resulting in 20 sets of measurements overall. Mean shunt fraction was 12.5%, and bias between methods (+/-95% confidence limits) was -0.7% (+/-0.8%). The standard deviation of the difference was 1.7% with limits of agreement between the two methods of +2.6% and -3.9%. Correlation coefficient r was 0.90. Agreement with the invasive standard was less accurate and precise where cardiac output was measured by bolus thermodilution (mean bias +1.6%, standard deviation of the difference 2.2%, limits of agreement between the two methods of +5.8% and -2.8%, r = 0.86). Good agreement was demonstrated between the non-invasive method and the invasive reference standard.

Continuous measurement of gas uptake and elimination in anesthetized patients using an extractable marker gas

Measurement of pulmonary gas uptake and elimination is often performed, using nitrogen as marker gas to measure gas flow, by applying the Haldane transformation. Because of the inability to measure nitrogen with conventional equipment, measurement is difficult during inhalational anesthesia. A new method is described, which is compatible with any inspired gas mixture, in which fresh gas and exhaust gas flows are measured using carbon dioxide as an extractable marker gas. A system was tested in eight patients undergoing colonic surgery for automated measurement of uptake of oxygen, nitrous oxide, isoflurane, and elimination of carbon dioxide with this method. Its accuracy and precision were compared with simultaneous measurements made with the Haldane transformation and corrected for predicted nitrogen excretion by the lungs. Good agreement was obtained for measurement of uptake or elimination of all gases studied. Mean bias was −0.003 l/min for both oxygen and nitrous oxide uptake, −0.0002 l/min for isoflurane uptake, and 0.003 l/min for carbon dioxide elimination. Limits of agreement lay within 30% of the mean uptake rate for nitrous oxide, within 15% for oxygen, within 10% for isoflurane, and within 5% for carbon dioxide. The extractable marker gas method allows accurate and continuous measurement of gas uptake and elimination in an anesthetic breathing system with any inspired gas mixture.

Noninvasive measurement of intrapulmonary shunting

OBJECTIVE

To investigate the accuracy and precision of a noninvasive approach to measurement of pulmonary shunt fraction using simultaneous application of 2 fundamental respiratory mixing equations: the direct Fick equation for oxygen and the shunt equation of Berggren. This can be performed without mixed venous blood sampling and requires measurement of oxygen uptake and pulmonary blood flow.

DESIGN

Comparison with invasive shunt fraction measured using mixed venous blood sampling and with estimated shunt fraction using an assumed arteriovenous O(2) content difference.

SETTING

Major teaching hospital.

PARTICIPANTS

Nine patients undergoing anesthesia for cardiac surgery.

INTERVENTIONS

Pulmonary blood flow was measured using an indirect Fick technique (nitrous oxide throughflow) and by bolus thermodilution for comparison.

MEASUREMENTS AND MAIN RESULTS

The mean shunt fraction measured by the invasive method was 0.145 (range 0.057-0.263). When pulmonary blood flow was measured using an indirect Fick technique (nitrous oxide throughflow), the absolute mean bias for noninvasive shunt fraction was -0.005 with a standard deviation of 0.012. Correlation was excellent (r(2) = 0.95, p < 0.001). Agreement was less precise when pulmonary blood flow was measured using thermodilution (mean bias + 0.001 with a standard deviation of 0.018).

CONCLUSIONS

The noninvasive method is an accurate substitute for invasive shunt fraction measurement with mixed venous blood sampling.

A new method for measurement of gas exchange during anaesthesia using an extractable marker gas

A new method for the measurement of pulmonary gas exchange during inhalational anaesthesia is described which measures fresh gas and exhaust gas flows using carbon dioxide as an extractable marker gas. The theoretical precision of the method was compared by Monte Carlo modelling with other approaches which use marker gas dilution. A system was constructed for automated measurement of uptake of oxygen, nitrous oxide, volatile anaesthetic agent and elimination of carbon dioxide by an anaesthetized patient. The accuracy and precision of the method was tested in vitro on a lung gas exchange simulator, by comparison with simultaneous measurements made using nitrogen as marker gas and the Haldane transformation. Good agreement was obtained for measurement of simulated uptake or elimination of all gases studied over a physiologically realistic range of values. Mean bias for oxygen and nitrous oxide uptake was 0.003 l min(-1), for isoflurane 0.0001 l min(-1) and for carbon dioxide 0.001 l min(-1). Limits of agreement lay within 10% of the mean uptake rate for nitrous oxide, within 5% for oxygen and isoflurane and within 1% for carbon dioxide. The extractable marker gas method allows accurate and continuous measurement of gas exchange in an anaesthetic breathing system with any inspired gas mixture.