Intrathoracic blood volume accurately reflects circulatory volume status in critically ill patients with mechanical ventilation

Positive pressure ventilation in patients with acute respiratory failure (ARF) may render the interpretation of central venous pressure (CVP) or pulmonary wedge pressure (PCWP) difficult as indicators of circulating volume. The preload component of cardiac (CI) and stroke index (SI) is also influenced by the increased intrathoracic pressures of positive pressure ventilation. Moreover CI and SI do not indicate volume status exclusively but also contractility and afterload. We investigated whether intrathoracic blood volume (ITBV) more accurately reflects blood volume status and the resulting oxygen transport (DO2). CVP, PCWP, cardiac (CI) and stroke index (SI) were measured, oxygen transport index (DO2I) and oxygen consumption index (VO2I) were calculated in 21 ARF-patients. Ventilatory patterns were adjusted as necessary. CI, SI and intrathoracic blood volume index (ITBVI) were derived from thermal dye dilution curves which were detected with a 5 F fiberoptic thermistor femoral artery catheter and fed into a thermaldye-computer. All data were collected in intervals of 6h. There were 224 data sets obtained. Linear regression analysis was performed between absolute values as well as between the 6 h changes (prefix Δ). The following correlation coefficients were determined: CVP/CI and PCWP/CI 0.01 and −0.142 (p<0.05); CVP/SI and PCWP/SI −0.108 and −0.228 (p<0.01); ITBVI/CI and ITBV/SI 0.488 (p<0.01) and 0.480 (p<0.01); ITBVI and DO2I 0.460 (p<0.01); ΔCVP/ΔCI and ΔPCWP/ΔCI −0.069 and−0.018; ΔCVP/ΔSI and ΔPCWP/ΔSI −0.083 and −0.009; ΔITBVI/ΔCI and ΔITBVI/ΔSI 0.715 (p<0.01) and 0.646 (p<0.01); ΔITBVI and ΔDO2I 0.707 (p<0.01). We conclude that in mechanically ventilated patients ITBV is a suitable indicator of circulating blood volume.