EFFECTS OF EXTREME LATERAL POSTURE ON HEMODYNAMICS AND PLASMA ATRIAL-NATRIURETIC-PEPTIDE LEVELS IN CRITICALLY ILL PATIENTS

Objective: To quantify the hemodynamic effects of turning critically i ll, mechanically ventilated patients to the extreme left and right lat eral postures. Design: Prospective investigation. Setting: Eight-bed i ntensive care unit in a university hospital. Patients: Twelve consecut ive patients presenting with severe respiratory failure and requiring continuous positive inotropic support. Interventions: All patients wer e mechanically ventilated and placed in a kinetic treatment system. Th ey were positioned in the supine, left dependent, and right dependent postures, resting for 15 min in each position. Measurements and result s: Hemodynamic measurements, assessments of right ventricular function , and determinations of intrathoracic blood volume were performed in t hree different positions. Concentrations of atrial natriuretic peptide in plasma were quantified. In three patients, the findings were contr olled by transesophageal echocardiography. Cardiac index [median (rang e) 5.5 (3.2 - 8.1) vs 4.3 (3.2 - 7.5) 1/min per m(2), p<0.01], intrath oracic blood volume [1125 (820 - 1394) vs 1037 (821-1267) ml/m(2), p<0 .01], and right ventricular end-diastolic volume [130 (83 - 159) vs 11 4 (79 - 155) ml/m(2), p<0.05] increased significantly in the left depe ndent position compared to supine. Mean arterial pressure did not chan ge. Atrial natriuretic peptide levels rose from 140 to 203 pg/ml. In t he right dependent position, we found a marked decrease in the mean ar terial pressure [85 mmHg (supine) to 72 mmHg (right dependent), p<0.01 ]. Cardiac index and intrathoracic blood volume were unchanged, but ri ght ventricular end-diastolic volume decreased from 114 to 102 ml/m(2) (p<0.05) Additionally, atrial natriuretic peptide levels decreased si gnificantly (median Delta value: 37 pg/ml). In echocardiographic contr ols we found an increase in right ventricular end-diastolic diameters in the left dependent position and shortened diameters in the right de pendent position. Conclusions: Extreme lateral posture affects the car diovascular system in critically ill, mechanically ventilated patients : in the left dependent position a ''hyperdynamic state'' is reinforce d, while the right decubitus position impairs right ventricular preloa d and predisposes to hypotension. Echocardiography and changes in plas ma atrial natriuretic peptide values indicate that these findings are due to altered distensibility of the right ventricle caused by regiona l intrathoracic gravitational changes. We conclude that the duration a nd the angle of lateral posture should be restricted in hemodynamicall y unstable patients.