Non-invasive measurement of hemodynamic response to postural stress using inert gas rebreathing

Abstract

In postural stress, an increased preload volume leads to higher stroke volume (SV) according to the Frank-Starling law of the heart. The present study aimed to evaluate the hemodynamic response to postural stress using non-invasive inert gas rebreathing (IGR) in patients with normal as well as impaired left ventricular function. Hemodynamic measurements were performed in 91 patients undergoing cardiac magnetic resonance imaging (CMR). Mean cardiac output and SV determined by IGR were 4.4±1.3 l/min and 60±19 ml in the upright position, which increased significantly to 5.0±1.2 l/min and 75±23 ml in the supine position (P<0.01). Left ventricular systolic function was normal [ejection fraction (EF) ≥55%] in 42 patients as determined by CMR. In 21 patients, EF was mildly abnormal (45-54%), in 16 patients moderately abnormal (30-44%) and in 12 patients severely abnormal (<30%). An overall trend for a lower percentage change in SV (%ΔSV) was indicated with increasing impairment of ejection fraction. In patients with abnormal EF in comparison to those with normal EF, the %ΔSV was significantly lower (13% vs. 22%; P=0.03). Non-invasive measurement of cardiac function using IGR during postural changes may be feasible and detected significant difference in %ΔSV in patients with normal and impaired EF according to the Frank-Starling law of the heart. Several clinical scenarios including cases of heart rhythm disturbances or pulmonary or congenital heart disease are worthy of further investigation.

Keywords: cardiac output; heart failure; inert gas rebreathing; postural stress; stroke volume.

Figure 1.

Box and whisker plots of the %ΔSV separated according to EF normality. (A) Trend toward a lower %ΔSV with reduced EF (P=0.17; one-way analysis of variance). (B) Significant difference between patients with normal vs. abnormal EF (P=0.03; t-test). (C) Significant difference between patients with normal vs. abnormal EF (P=0.01; t-test). %ΔSV, percentage change in stroke volume; EF, ejection fraction. %ΔHR, percentage change in heart ratio.