Stroke volume during orthostatic challenge: comparison of two non-invasive methods.

BACKGROUND Real time non-invasive determination of stroke volume (SV) is important to astronaut orthostatic testing. We compared simultaneous estimates of SV calculated from peripheral pulse waveforms with a more conventional non-invasive technique. METHODS Ten men and nine women completed 12-min protocols. The relative change (% delta) in beat-to-beat SV was estimated non-invasively from changes in pulse waveforms measured by application of infrared finger photoplethysmography (IFP) and thoracic impedance cardiography (TIC). The % deltaSV values were calculated from continuous measurements in the supine posture and over the first 10 s (T1), second 10 s (T2), and 3 min (T3) of 80 degrees head-up tilt (HUT). RESULTS Average % deltaSV measured by IFP at T1 (-11.7 +/- 3.7%) was statistically less than the average % deltaSV measured by TIC at T1 (-21.7 +/- 3.1%), while average % deltaSV measured by IFP at T2 (-16.2 +/- 3.9%) and T3 (-19.1 +/- 3.8%) were not statistically distinguishable from the average % deltaSV measured by TIC at T2 (-21.8 +/- 2.5%), and T3 (-22.6 +/- 2.9%). Correlation coefficients (r2) between IFP and TIC were 0.117 (T1), 0.387 (T2), and 0.718 (T3). CONCLUSION IFP provides beat-to-beat (real-time) assessment of % deltaSV after 20 s of transition to an orthostatic challenge that is comparable to TIC. IFP technology flown during space missions can be used to assess physiological status and countermeasure effectiveness for orthostatic problems that may arise in astronauts after spaceflight. While the peripherally measured IFP response is delayed, the ease of implementing this monitor in the field is advantageous.