The lowering of stroke volume measured by means of impedance cardiography during endexpiratory breath holding.

Impedance cardiography is a reliable method for estimating stroke volume (SV). Breathing, however, causes artefacts, which can be avoided by measuring during breath holding. This study investigated whether SV determination is accurate during breath holding. Twelve healthy subjects were tested in the supine position at rest and during two levels of exercise: 100 and 200 W. Averaged SV values were monitored by means of impedance cardiography before and after endexpiratory breath holding. During breath holding, SV measurement was on a beat-to-beat basis. An obvious decrease in SV during breath holding was noticed, being significant only during exercise (mean decrease of 38% at 100 W and 58% at 200 W). The rest measurements were repeated with open and closed glottis, which yielded the same results. This indicates that the SV decrease was not caused by a Valsalva-like manoeuvre. The mean SV value calculated by means of impedance cardiography for the total breath hold period was significantly lower than the SV during breathing, both at rest (91.7 +/- 2.4%) and at 100 W (90.5 +/- 7.0%). From this study it can be concluded that averaging of the impedance signal, measured while the subject is breathing, is preferential to measuring during breath holding, because the latter condition systematically underestimates SV.