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.
Breath-hold functional MRI (bh-fMRI) is a widely used method for mapping cerebrovascular reactivity (CVR). Normalization of BOLD signal changes to end-tidal CO2 changes (Delta petCO2) is suggested to improve between-subject comparability. The aim was to characterize magnitude and variability of Delta petCO2 during short breath-hold maneuvers. Twenty healthy volunteers participated in this study. Ten of these participants performed five breathing paradigms: baseline breathing, and end-expiratory breath-holds of 6, 9, and 12 s, as well as a deep breathing maneuver, each repeated seven times. petCO2was recorded continuously via nasal cannula. Within-and between-subject variability were quantified using the coefficient of variation (CV). Differences in Delta petCO2 between paradigms were assessed using repeated-measures ANOVA. Ten additional participants underwent the 9 s breath-hold paradigm and spirometry; associations between Delta petCO2 and pulmonary volumes, age, and baseline petCO2were assessed using Pearson correlation. Within-subject within-paradigm CVs were low (3.4-4.7%), indicating high reproducibility. Between-subject CVs were substantially higher (19.7-35.3%). Delta petCO2 differed significantly between the 6 and 9 s paradigms (p = 0.014, pFDR = 0.017) but not between the 9 and 12 s paradigms (p = 0.209, pFDR = 0.209). The deep breathing induced a reproducible petCO2 decrease. A very strong inverse correlation was observed between Delta petCO2 and inspiratory vital capacity (IVC) (r =-0.797 (95%CI:- 0.950--0.336), p = 0.006, pFDR = 0.030). Short breath-hold maneuvers induce reproducible, yet variable CO2 responses within individuals. Pulmonary anatomy is a key determinant of Delta petCO2. Inadvertent deep breathing prior to breath-holding may induce petCO2decreases of non-negligible magnitude, with potential implications for bh-fMRI interpretation.
