Matthias Boentert

Objective: This study aimed to validate the single breath count test (SBCT) against volitional measures of respiratory muscle function in healthy subjects and people with neuromuscular disorders (NMD; n = 100 per group). Methods: Testing comprised upright and supine SBCT, forced vital capacity (FVC), maximum inspiratory pressure (MIP), and sniff nasal inspiratory pressure (SNIP). Predictability of FVC by SBCT was assessed using logarithmic regression analysis. Receiver operating characteristics curves were used to identify SBCT thresholds for lung restriction (FVC < 80% predicted), inspiratory muscle weakness (MIP < 60 cmH2O), and indication for non-invasive ventilation (NIV) in NMD patients. Results: In both groups, SBCT showed moderate correlation with FVC. In patients, SBCT values were also correlated with MIP and SNIP. Strength of correlations was similar with supine and upright SBCT which accounted for 23.7% of FVC variance in healthy individuals (44.5% in patients). Predictive thresholds of upright SBCT were < 27 for MIP < 60 cmH2O (sensitivity 0.61/specificity 0.86), <39 for NIV indication (0.92/0.46), and <41 for FVC < 80% predicted (0.89/0.62). Conclusion: The SBCT is positively correlated with spirometry. It predicts both lung restriction and NIV indication in NMD patients. The SBCT allows for remote monitoring and may substitute for spirometry/manometry if appropriate devices are unavailable.

Background. Following COVID-19, patients often present with ongoing symptoms comparable to chronic fatigue and subjective deterioration of exercise capacity (EC), which has been recently described as postacute COVID-19 syndrome. Objective. To objectify the reduced EC after COVID-19 and to evaluate for pathologic limitations. Methods. Thirty patients with subjective limitation of EC performed cardiopulmonary exercise testing (CPET). If objectively limited in EC or deteriorated in oxygen pulse, we offered cardiac stress magnetic resonance imaging (MRI) and a follow-up CPET. Results. Eighteen male and 12 female patients were included. Limited relative EC was detected in 11/30 (36.7%) patients. Limitation correlated with reduced body weight-indexed peak oxygen (O2) uptake (peakV̇O2/kg) (mean 74.7 (±7.1) % vs. 103.6 (±14.9) %, p < 0.001 ). Reduced peakV̇O2/kg was found in 18/30 (60.0%) patients with limited EC. Patients with reduced EC widely presented an impaired maximum O2 pulse (75.7% (±5.6) vs. 106.8% (±13.9), p < 0.001 ). Abnormal gas exchange was absent in all limited EC patients. Moreover, no patient showed signs of reduced pulmonary perfusion. Using cardiac MRI, diminished biventricular ejection fraction was ruled out in 16 patients as a possible cause for reduced O2 pulse. Despite noncontrolled training exercises, follow-up CPET did not reveal any exercise improvements. Conclusions. Deterioration of EC was not associated with ventilatory or pulmonary vascular limitation. Exercise limitation was related to both reduced O2 pulse and peakV̇O2/kg, which, however, did not correlate with the initial severity of COVID-19. We hypothesize that impaired microcirculation or limited peripheral O2 utilization might be causative for prolonged deterioration of EC following acute COVID-19 infection.