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Performance evaluation of a portable bioimpedance cardiac output monitor for measuring haemodynamic changes in athletes during a head-up tilt test.

Cardiac output (CO) monitoring is useful for sports performance training but most methods are unsuitable as they are invasive or hinder performance. The performance of PhysioFlow (PF), a portable non-invasive transthoracic bioimpedance CO monitor, was evaluated and compared to a reference Doppler CO monitor, USCOM, using a head-up tilt (HUT) test. With ethics committee approval, 20 healthy well-trained athletes were subjected to HUT in a fixed order of 0°, 70°, 30° and 0° for 3 minutes each. Simultaneous haemodynamic measurements using PF and USCOM were made 30s after a change in HUT and analysed using t-tests, analysis of variance and mountain plots. Heart rate (HR) and stroke volume (SV) from both monitors changed according to physiological expectation of tilt, but PF measurements of SV were higher with a positive bias (PF vs USCOM, 0°: 87.3 vs 54.0mL, p<0.001; 70°: 76.5 vs 39.5mL, p<0.001, 30°: 81.4 vs 50.1mL, p<0.001, 0°: 88.3 vs 57.1mL, p<0.001). Relative changes in SV (∆SV) after each tilt measured using PF were lower with a negative bias (PF vs USCOM, 0° to 70°: -12.3% vs -26.3%, p =0.002; 70° to 30°: +6.4% vs +31.2%, p<0.001; 30° to 0°: +9.2% vs +15.8%, p=0.280). CO measurements using PF at 70° were erroneous. Compared to USCOM, PF overestimated SV measurements but underestimated the ∆SV between HUT. Accuracy of the PF deteriorated at 70°, implying a gravitational influence on its performance. These findings suggested that the suitability of PF for sports use is questionable.