Respiratory muscle performance with stretch-shortening cycle manoeuvres: maximal inspiratory pressure-flow curves

Hemodynamic effects of portable non-invasive ventilation in healthy men

VitaBreath is a portable, non-invasive ventilation device (pNIV) that relieves shortness of breath in COPD by delivering fixed inspiratory and expiratory positive airway pressures (IPAP/EPAP: 18/8 cmH₂O). Fixed pressures may cause circulatory compromise. We investigated the circulatory effects of pNIV during normal breathing (NB) and after Eucapnic Voluntary Hyperpnoea trials (EVH) sustained at 80% MVV. In a balanced order sequence, 10 healthy men performed four trials on one visit: 1-min of pNIV (intervention) or 1-min quiet breathing (QB) during NB; and 1-min pNIV (intervention) or 1-min QB during recovery from 3-min EVH. Compared to QB, pNIV application was associated with greater cardiac output (CO: 1.6 ± 1.9 L.min^-1; P = 0.03). One minute into recovery from EVH, pNIV caused greater CO (2.2 ± 1.6 L.min^-1; P = 0.01) compared to QB. Mean blood pressure was not different with pNIV compared to control. pNIV increased thoracoabdominal volumes and breathing frequency during NB and recovery from EVH. pNIV application does not induce adverse hemodynamic effects in healthy men.

Mechanical properties of respiratory muscles

Striated respiratory muscles are necessary for lung ventilation and to maintain the patency of the upper airway. The basic structural and functional properties of respiratory muscles are similar to those of other striated muscles (both skeletal and cardiac). The sarcomere is the fundamental organizational unit of striated muscles and sarcomeric proteins underlie the passive and active mechanical properties of muscle fibers. In this respect, the functional categorization of different fiber types provides a conceptual framework to understand the physiological properties of respiratory muscles. Within the sarcomere, the interaction between the thick and thin filaments at the level of cross-bridges provides the elementary unit of force generation and contraction. Key to an understanding of the unique functional differences across muscle fiber types are differences in cross-bridge recruitment and cycling that relate to the expression of different myosin heavy chain isoforms in the thick filament. The active mechanical properties of muscle fibers are characterized by the relationship between myoplasmic Ca2+ and cross-bridge recruitment, force generation and sarcomere length (also cross-bridge recruitment), external load and shortening velocity (cross-bridge cycling rate), and cross-bridge cycling rate and ATP consumption. Passive mechanical properties are also important reflecting viscoelastic elements within sarcomeres as well as the extracellular matrix. Conditions that affect respiratory muscle performance may have a range of underlying pathophysiological causes, but their manifestations will depend on their impact on these basic elemental structures.

Inter-session reproducibility of peak expiratory flow with standardized expiratory maneuvers

BACKGROUND

In adults performing forceful expiratory maneuvers, the length of post-inspiratory pause prior to forced expiration may influence the subsequently measured peak expiratory flow (PEF) and increase its variability. We investigated the effects of two different lengths of breath-hold at total lung capacity (TLC) on the short-term reproducibility of PEF in healthy volunteers.

METHODS

Forty-six healthy volunteers (age 34.6+/-8.5; 23 men) performed a series of maximal forceful expirations in two different test sessions, separated by approximately 2 weeks. In each test-session, PEF was measured with two different types of maneuvers. One maneuver (P) included a brief (<2s) post-inspiratory pause at TLC prior to forced expiration, whereas the second maneuver (NP) included no pause at TLC. The speed of inspiration to TLC was fast and similar for both maneuvers. In a given test session, all volunteers performed four efforts for each type of maneuver. The highest PEF for each maneuver was used for analysis. The Bland-Altman statistical analysis was used to determine inter-session reproducibility of PEF.

RESULTS

Within-maneuver analysis of the between-test session reproducibility of PEF showed that neither maneuver systematically biased the measured PEF (mean difference 0.02L/s for the P and 0.17L/s for the NP maneuver). Inter-maneuver between-test session analysis similarly showed that neither maneuver introduced a systematic bias in the maximal PEF (mean difference ranged from -0.15 to -0.01L/s). The limits of agreement were comparable in all maneuver-pair analyses.

CONCLUSIONS

Forceful expiratory maneuvers with or without a brief (<2s) pause at TLC produce comparable PEF values in test-retest sessions.