Christopher A. Cutler

Instrumentation for monitoring gas exchange and metabolic rate in critically ill patients

In the critically ill patient the monitoring of oxygen consumption (VO2) and carbon dioxide production (VCO2) can identify abnormalities in tissue perfusion and metabolism. A patients metabolic utilization can be calculated by indirect calorimetry, once VO2 and VCO2 are measured. This paper evaluates a compact instrument designed for monitoring VO2 and VCO2 in the critically ill adult. Accuracy was measured under controlled laboratory conditions using oxygen-enriched air, PEEP, and intermittent mandatory ventilation (IMV). Accuracy averaged 1.3% for VO2 and 0.9% for VCO2 when room air was used. Accuracy was 11.7% for VO2 and 6.8% for VCO2 when 80% oxygen was used. PEEP of 30 cm H2O had little effect on accuracy. IMV at 2 breath/min (room air) resulted in an accuracy of 4.0% and 4.1% for VO2 and VCO2, respectively.

Calculation of Metabolic Expenditure and Substrate Utilization from Gas Exchange Measurements

At least nine different equations have been published for calculating metabolic expenditure by indirect calorimetry. This study examined the differences between equations when they are used for the nutritional assessment in an intensive care unit (ICU). Oxygen consumption and carbon dioxide production were measured in 36 ICU patients and used to calculate metabolic expenditure with the nine equations. The equations produced differences in metabolic expenditure which averaged from 0.8-96 kcal/day. The largest difference produced by any two of the nine equations was 189 kcal/day. Although differences in original stoichiometric data have resulted in numerous different equations for the calculation of metabolic expenditure, these differences are not clinically important. It makes little difference which equation is used for nutritional assessment in an ICU.

A thermodilution method for quantification of bidirectional shunts

Abstract A method for measurement of bidirectional cardiovascular shunt fractions from single indicator dilution curves is presented. Linear system models corresponding to four basic shunt configurations are devised to represent the indicator dilution response of the cardiopulmonary circulation. The congenital heart defects in which bidirectional shunts exist are discussed in terms of the four anatomic configurations. Computer algorithms and digital processing techniques used for analyses of the indicator dilution curves are presented in detail. Forty-one thermodilution curves recorded during 13 pediatric cardiac catheterization procedures were analyzed and the results compared to simultaneous oximetric determinations. Comparison of L-R shunts, expressed as fractions of pulmonary plus R-L shunt flows, calculated by the two methods gave a standard error of 0.083 and a maximum difference of 0.15. Error analysis of the indicator dilution method predicted a standard deviation of 0.06. Multiple determinations were reproducible within a standard deviation of 0.051. Comparison of the right-to-left shunts, expressed as fractions of systemic plus left-to-right shunt flows gave a standard error of 0.083 and a maximum difference of 0.13. The standard deviation of multiple determinations was 0.029.