NEW MONITORS OF INTRAVASCULAR VOLUME - A COMPARISON OF ARTERIAL-PRESSURE WAVE-FORM ANALYSIS AND THE INTRATHORACIC BLOOD-VOLUME

Objective: Two new monitoring techniques, the analysis of arterial pre ssure 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, 2 0, and 30 % of their blood volume, then retransfused and volume loaded with plasma expander. Central venous pressure (CVP), pulmonary capill ary wedge pressure (PCWP), cardiac output, parameters of the arterial pressure waveform analysis [systolic pressure variation (SPV) and delt a down (dDOWN)], and intrathoracic blood volume (ITBV) were measured a t baseline and after each stage of hemorrhage and volume expansion. Re sults: 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 i ncreased after retransfusion and volume loading. The changes in the fi lling pressures during the various stages of hemorrhage were in the ra nge of 1-2 mmHg. CVP decreased from 5.5 +/- 0.9 to 3.1 +/- 1.7 mmHg an d PCWP from 8.0 +/- 0.8 to 5.1 +/- 1.2 mmHg at 30 % hemorrhage. Both f illing pressures responded significantly to retransfusion; PCWP also c hanged in response to a volume load. SPV and dDOWN (expressed as perce nt of the systolic blood pressure during a short apnea) increased sign ificantly from 6.7 +/- 1.7 and 5.6 +/- 3.2 %, respectively, at baselin e, 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 signi ficantly 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 o n controlled mechanical ventilation. ITBV, which supplies quantitative information about cardiac preload, is more invasive but can also be u sed in patients who are breathing spontaneously or who are on partial ventilatory support.