Background Studies in post-COVID-19 condition following hospitalisation have focused on maximal exercise, with limited insight into submaximal responses that better reflect daily activities. We compared central haemodynamic, ventilatory, and peripheral muscle oxygen extraction responses during submaximal exercise between individuals with post-COVID-19 condition and healthy age-matched controls. Methods Twelve participants with post-COVID-19 condition, who were assessed 624 (593, 729) days following hospitalisation (median (IQR) length of stay: 7 (4.7, 11.7) days) and seven age-matched healthy controls (59 ± 11 years) completed a 4-min constant-load exercise test at 80% of peak cycling workload. Cardiac output (CO), oxygen uptake (V̇O₂), and ventilatory efficiency (V̇E/V̇CO₂) were assessed throughout exercise. Arteriovenous oxygen content difference was calculated using the Fick equation (C(a–v)O₂ = V̇O₂/CO). Results Post-COVID-19 condition participants exhibited a lower (p = 0.03) mean±SD C(a–v)O₂ compared with healthy controls (135 ± 29 vs. 171 ± 46 mL O₂·L⁻¹ blood). No significant differences were observed between post-COVID-19 condition and controls for V̇O₂ (1.9 ± 0.3 vs. 2.1 ± 0.7 L·min⁻¹), V̇E/V̇CO₂ (26.6 ± 3.0 vs. 27.5 ± 2.4), or CO (12.7 ± 3.0 vs. 11.1 ± 3.6 L·min⁻¹), indicating similar central cardiovascular and ventilatory responses between groups, despite a numerically greater cardiac output in the post-COVID-19 condition group (11 ± 4% higher). In contrast, RER and V̇E/V̇O₂ were significantly higher in healthy controls than in post-COVID-19 condition participants, indicating divergent ventilatory behaviour despite preserved V̇E/V̇CO₂. Conclusion Post-COVID-19 condition following hospitalisation is characterised by reduced peripheral oxygen extraction during submaximal exercise, alongside no significant difference in cardiovascular responses and preserved ventilatory efficiency with respect to carbon dioxide elimination, compared with healthy controls. Although RER and V̇E/V̇O₂ were higher in healthy controls, indicating some divergence in ventilatory behaviour between groups, this did not reflect impaired ventilatory efficiency in post-COVID-19 condition. These findings indicate that differences in exercise responses at workloads representative of daily living are possibly peripheral rather than central, supporting rehabilitation strategies targeting skeletal muscle oxidative function.
Background Diaphragm ultrasound (DUS) provides a non-invasive assessment of diaphragm structure and function. However, the physiological information captured may depend on both the parameter measured and the contractile condition during acquisition. Whether diaphragm thickness during maximal isometric inspiratory effort more closely reflects inspiratory pressure-generating capacity than conventional DUS indices remains unclear. Objective To investigate the association between maximal inspiratory pressure (MIP) and diaphragm thickness during maximal isometric inspiratory effort in healthy adults and compare it with conventional DUS parameters. Methods Seventy-six healthy volunteers (≥18 years; 40 males) were enrolled. Diaphragmatic excursion (DE), diaphragm thickness at rest, during maximal inspiration and maximal isometric inspiratory effort, and thickening fraction (TF) were assessed by DUS. The highest of three measurements was analyzed. Correlation and regression analyses adjusted for age, sex, and height were performed. Results Median age was 43 years and BMI 22.8 kg/m². MIP correlated with TF (r = 0.39, p = 0.035), DE during deep inspiration (r = 0.46, p = 0.011), and most strongly with diaphragm thickness during maximal isometric inspiratory effort (r = 0.95, p < 0.0001). Resting diaphragm thickness correlated inversely with MIP (r = −0.39, p = 0.034). In multivariable analysis, thickness during maximal isometric inspiratory effort remained independently associated with MIP. Conclusions In healthy adults, diaphragm thickness during maximal isometric inspiratory effort showed the strongest association with MIP among DUS parameters. These proof-of-concept findings support further investigation of this measurement as an ultrasound-based marker of inspiratory muscle strength, including setting in which conventional MIP assessment may not be feasible.
Steep (50°) head down tilt (HDT) augments the ventilatory response to steady state hypercapnia although the mechanism is unclear. We hypothesised that changes in pulmonary blood flow distribution and ventilatory homogeneity during 50° HDT in hypercapnia would result in elevated arterial (PaCO2) with no concomitant change in end-tidal CO₂ (PETCO₂). We exposed 15 healthy participants (mean (SD) age 29 (10) years, 7 female, BMI 24.7 (5.5) kg/m2) to 50° HDT for eight minutes while breathing air or normoxic, hypercapnia (5% CO₂). Ventilation (pneumotachograph), end tidal O2 and CO2 concentration, and blood pressure (Finapres) were measured continuously. Arterialised capillary earlobe blood samples were obtained at baseline and during HDT to provide an estimate of PaCO₂. In a separate protocol, we measured carbon monoxide transfer factor (TLCO) during 50° HDT while breathing air in 6 healthy participants (mean (SD) age 34.7 (12.8) years, 2 female, BMI 23.7 (2.2) kg/m2). In protocol 1, ventilation increased significantly (mean (SD) 115 (24)%, p < 0.05) compared to baseline during HDT in hypercapnia with a corresponding increase in PaCO₂ (42.9 (3.0) to 45.1 (3.5) mmHg; P = 0.028) and no change in PETCO₂. The arterial to end-tidal PCO2 difference (P(a–ET)CO2) fell significantly from −4.0 (2.4) to −1.6 (2.2) mmHg; P = 0.034). No changes were observed while breathing air. Mean (SD) TLCO increased significantly during 50° HDT from 28.4 (6.5) to 36.9 (7.3) mL/min/mmHg, p < 0.0001). The augmented hypercapnic ventilatory response during 50⁰ HDT is consistent with alterations in lung ventilation/perfusion matching.
