Relationship between the fick principle and cardiopulmonary exercise testing in patients with hypertrophic cardiomyopathy

Background The relationship between oxygen consumption (VO2), stroke volume (SV), heart rate (HR), and peripheral extraction is illustrated in the Fick-equation. There is a need to understand the added value of non-invasive cardiac output (CO) measurements in patients with hypertrophic cardiomyopathy (HCM) to evaluate the Fick´s principle in relation to cardiopulmonary exercise testing (CPET) parameters. Methods We conducted CPET and non-invasive CO measurements during exercise via PhysioFlow in HCM patients. Percent of predicted oxygen pulse (O2p) was correlated via linear regression with PhysioFlow measurements. Results We evaluated 102, predominantly male (78%) patients with HCM (26% obstructive) aged 56 (IQR 23) years. Median peak VO2 was 22.1 (IQR 8.9) mL/min/kg, VE/VCO2 slope 29.5 (IQR 7.2) and peak O2p 13.3 (IQR 4.3) mL/beat corresponding to 98.9 (IQR 30.5) % of the predicted (O2pp). In linear regression analysis, O2pp was significantly associated with peak SV (estimate: 0.536, p <0.001) and ∆SV (estimate: 0.381, p 0.002). Further, VE/VCO2 slope was significantly associated with peak SV (estimate: -1.885, p <0.001) and ∆SV (estimate: -1.103, p 0.005). Interestingly, O2pp was unrelated to LVOT gradient at rest, confirming previous data suggesting a weak capability of LVOT gradient in HCM functional capacity stratification. When grouping patients according to their O2pp in groups of >=100% (n=50) and <100 (n=52), patients with >=100% show a higher peak SV (131.2 (IQR 46.1) vs. 109.3 (IQR 41.3) mL, p <0.001), higher ∆SV (38 (IQR 33) vs. 28 (IQR 23) mL), lower rest HR (61.0 ± 11.2 vs. 68.9 ± 12.6 beats/min, p=0.001), lower peak HR (130 (IQR 37) vs. 147 (IQR 42) beats/min, p=0.020) and a higher ∆CO (12.2 ± 5.3 L/min vs. 10.1 ± 4.9 L/min, p=0.037). Importantly patients with better O2pp showed similar peak HR values (expressed as % of the predicted) compared to their worst counterparts, suggesting a HR-independent effect of O2pp in HCM patients’ stratification. Conclusion The association of the O2pp and SV supports the relationship of the Fick´s principle with CPET parameters in patients with HCM.