New monitors of intravascular volume: A comparison of arterial pressure waveform analysis and the intrathoracic blood volume

Objective: Two new monitoring techniques, the analysis of arterial pressure waveform during mechanical ventilation and the determination of intrathoracic blood volume, were evaluated for preload assessment in a model of graded hemorrhage. Design: 8 anesthetized dogs bled of 10, 20, and 30 % of their blood volume, then retransfused and volume loaded with plasma expander. Central venous pressure (CVP), pulmonary capillary wedge pressure (PCWP), cardiac output, parameters of the arterial pressure waveform analysis [systolic pressure variation (SPV) and delta down (dDOWN)], and intrathoracic blood volume (ITBV) were measured at baseline and after each stage of hemorrhage and volume expansion. Results: The stroke volume index decreased significantly from 1.3 +/- 0.4 ml/kg at baseline to 0.7 +/- 0.2 ml/kg at 30 % hemorrhage and then increased after retransfusion and volume loading. The changes in the filling pressures during the various stages of hemorrhage were in the range of 1-2 mmHg. CVP decreased from 5.5 +/- 0.9 to 3.1 +/- 1.7 mmHg and PCWP from 8.0 +/- 0.8 to 5.1 +/- 1.2 mmHg at 30 % hemorrhage. Both filling pressures responded significantly to retransfusion; PCWP also changed in response to a volume load. SPV and dDOWN (expressed as percent of the systolic blood pressure during a short apnea) increased significantly from 6.7 +/- 1.7 and 5.6 +/- 3.2 %, respectively, at baseline, to 9.7 +/- 2.6 and 8.1 +/- 2.9 % after 10 % blood loss and to 13.1 +/- 3.9 and 11.1 +/- 3.8 % after 30 % hemorrhage. ITBV decreased significantly from 29.7 +/- 4.5 to 26.8 +/- 5.3 ml/kg after 10 % blood loss and to 23.1 +/- 3.0 ml/kg after 30 % hemorrhage. ITBV, SPV and dDOWN responded significantly to retransfusion and volume load. Significant correlations were found between the degree of volume change and dDOWN (r = 0.93), SPV (r = 0.96), ITBV (r = 0.95), CVP (r = 0.82), and PCWP (r = 0.90). Conclusions: The parameters of arterial pressure waveform analysis (SPV and dDOWN) and ITBV were sensitive estimates of cardiac preload during the early stages of hemorrhage. Measurement of SPV and dDOWN, being both sensitive and relatively noninvasive, has advantages over other methods of preload assessment but is limited to patients on controlled mechanical ventilation. ITBV, which supplies quantitative information about cardiac preload, is more invasive but can also be used in patients who are breathing spontaneously or who are on partial ventilatory support.