Beyond VO2 Peak: Hemodynamic Profiling After Bariatric Surgery Using the Fick Principle

Background: Bariatric surgery is a well-established intervention for severe obesity, resulting in substantial weight loss and cardiometabolic benefits. However, the physiological mechanisms driving changes in functional capacity postoperatively remain incompletely characterized. Methods: Fourteen patients scheduled for bariatric surgery underwent serial assessments preoperatively and at 3 and 6 months postoperatively. Evaluations included body composition analysis, cardiopulmonary exercise testing (CPET), and non-invasive measurement of cardiac output (CO) and arteriovenous oxygen difference (AVDiff) using the inert gas rebreathing (IGR) method. Patients were stratified into four hemodynamic response profiles (Q1–Q4) based on directional changes in CO and AVDiff. Repeated measures changes were analyzed using Friedman and Wilcoxon signed-rank tests; correlations were assessed using Spearman’s rank method. Results: Following surgery, patients exhibited significant reductions in BMI, body fat percentage, and CO at both rest and peak exercise (all p < 0.05). VO2 peak was preserved or modestly improved. Notably, the majority of patients exhibited a response pattern of reduced CO accompanied by increased AVDiff (Q1), both at rest and during exertion. At three months, VO2 rest correlated positively with both CO rest and AVDiff rest, while VO2 peak correlated with AVDiff peak. At six months, VO2 rest and VO2 peak both correlated positively with CO peak. Conclusions: Aerobic performance following bariatric surgery is not solely determined by peak oxygen uptake but also depends on the interaction between cardiac output and oxygen extraction, as reflected by AVDiff. These findings align with the Fick principle (VO2 = CO × AVDiff) and emphasize the importance of comprehensive physiological profiling—beyond VO2 peak alone—in understanding adaptation after bariatric surgery.