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.

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 patient's 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.

Blood-gas analyzer calibration and quality control using a precision gas-mixing instrument.

We describe a new instrument that performs on-site mixing of oxygen (O2), carbon dioxide (CO2), and nitrogen (N2) to create compositions that can replace gases from standard premixed cylinders. This instrument yields accurate and predictable gas mixtures that can be used for two-point gas calibration of blood gas/pH analyzers or for liquid tonometry of either an aqueous buffer or blood used as quality-control material on blood-gas electrodes. The desired mixture of O2, CO2, and N2 is produced by microprocessor control of the sequential open-times on three solenoid valves that meter these pure gases through a common small-bore orifice. Any combination of O2 and CO2 can be chosen by dialing the front panel thumbwheels and pressing a button. Gas chromatographic evaluation of this gas-mixing instrument demonstrates its accuracy and precision to be better than +/- 0.1% absolute full scale for O2, CO2, and N2, making this instrument calibration and tonometry.

New gas-liquid equilibration method: syringe tonometer.

This new apparatus for gas-liquid equilibration (tonometry) in a transportable vessel is designed for tonometry of blood or buffer solution in a specially designed syringe. Gas enters the syringe chamber through small holes in the tip of the syringe plunger and bubbles upward through the sample. The syringe plunger is a second chamber, which is used for warming and humidifying the gas before it enters the tonometer chamber. The entire syringe is housed in a transparent, temperature-controlled environment during equilibration. After equilibration, the sample is easily entered into a blood-gas analyzer. At most, gas-liquid O2/CO2 equilibration for 2.5 mL of buffer or blood requires less than 13 min. Comparisons with a standard thin-film tonometer show good agreement for pO2 and pCO2 over the range 0-93 kPa (0-700 mmHg) and 2-20 kPa (14-150 mmHg), respectively. The syringe tonometer eliminates contamination of the sample during transfer and transport to the analyzer, thus making tonometry more technique-independent. The apparatus is simple and easy to use, with definite advantages over existing methods of tonometry.

Shunt quantification by mathematical analysis of indicator dilution curves.

Mathematical models are presented for describing and analyzing indicator dilution curves recorded in patients with intracardiac and great vessel shunts. The models treat individual segments of the circulation as linear system blocks, each having, at its output, a characteristic time response to a rapid injection of indicator at its input. These blocks are combined in feedback and feed-forward configurations to simulate left-to-right, right-to-left, and bidirectional shunts. A shunt analysis algorithm, using discrete analogs of the linear system models, was implemented in a computer program and used to analyze thermodilution curves recorded in patients with congenital heart defects. Results are presented comparing shunt fractions obtained from thermodilution curve analyses with oximetrically determined values in 20 patients. Comparing left-to-right shunts measured by the two methods, the mean systematic difference was 0.7% of pulmonary flow and the standard deviation was 7.6% of pulmonary flow. Statistical validation of the bidirectional shunt method will require acquisition and analysis of more data; however, reasonable shunt fractions were computed in five cases studied and good agreement with oximetric determinations was obtained in two cases where complete oximetric data were available.