Effect of the mechanical ventilatory cycle on thermodilution right ventricular volumes and cardiac output

The purpose of this study was to evaluate right ventricular (RV) loading an d cardiac output changes, by using the thermodilution technique, during the mechanical ventilatory cycle. Fifteen critically ill patients on mechanica l ventilation, with 5 cmH(2)O of positive end-expiratory pressure, mean res piratory frequency of 18 breaths/min, and mean tidal volume, of 708 ml, wer e studied with help of a rapid-response thermistor RV ejection fraction pul monary artery catheter, allowing 5-ml room-temperature 5% isotonic dextrose thermodilution measurements of cardiac index (CI), stroke volume (SV) inde x, RV ejection fraction (RVEF), RV end-diastolic volume (RVEDV), and RV end -systolic volume (RVESV) indexes at 10% intervals of the mechanical ventila tory cycle. The ventilatory modulation of CI and RV volumes varied from pat ient to patient, and the interindividual variability was greater for the la tter variables. Within patients also, RV volumes were modulated more by the ventilatory cycle than CI and SV index. Around a mean value of 3.95 +/- 1. 18 1 . min(-1) m(-2) (= 100%), CI varied from 87.3 +/- 5.2 (minimum) to 114 .3 +/- 5.1% (maximum), and RVESV index varied between 61.5 +/- 17.8 and 149 .3 +/- 34.1% of mean 55.1 +/- 17.9 ml/m(2) during the ventilatory cycle. Th e variations in the cycle exceeded the measurement error even though the la tter was greater for RVEF and volumes than for CI and SV index. For mean va lues, there was an inspiratory decrease in RVEF and increase in RVESV, wher eas a rise in RVEDV largely prevented a fall in SV index. We conclude that cyclic RV afterloading necessitates multiple thermodilution measurements eq ually spaced in the ventilatory cycle for reliable assessment of RV perform ance during mechanical ventilation of patients.