Eastwood P R, Hillman D R, Morton A R, Finucane K E
Department of Pulmonary Physiology, Sir Charles Gairdner Hospital, Nedlands, Western Australia, Australia.
Am J Respir Crit Care Med. 1998 Oct;158(4):1190-6. doi: 10.1164/ajrccm.158.4.9803108.
Progressive threshold loading (PTL) is frequently used to assess inspiratory muscle endurance in health and disease. We and others have noted a systematic increase in endurance with the first few exposures to the task in subjects previously naïve to PTL, which may not be related to conditioning of the muscles themselves. The purpose of this study was to investigate the mechanisms responsible for this increased endurance by examining the ventilatory responses to 3 PTL tests, each > 24 h apart, in 18 healthy subjects. During PTL, threshold pressure (Pth) was increased by approximately 10% every 2 min until task failure. Subjects were allowed to adopt any breathing pattern. Respiratory muscle strength (maximal inspiratory pressure [PImax]) was unchanged over successive tests while maximal Pth (Pthmax) during PTL increased (69 +/- 17, 77 +/- 16, and 86 +/- 11% of PImax, respectively, p < 0.05) (mean +/- SD), indicating that the increased Pthmax could not be attributed to improved respiratory muscle strength. Breathing pattern changed with successive tests, so that for comparative loads inspiratory time (TI), respiratory frequency (f ), and duty cycle (TI/Ttot) decreased. This change in breathing pattern did not alter respiratory muscle efficiency (respiratory muscle V O2/work), which was similar in each test (2.4 +/- 2.2%), but perceived effort (Borg Score), which was maximal at task failure in each test, decreased at comparative loads with successive tests. Thus, Pthmax during initial tests appeared to be limited by discomfort rather than respiratory muscle function. These findings suggest that the increased Pthmax with successive tests is a consequence of differences in the breathing pattern adopted, reflecting neuropsychological rather than respiratory muscle conditioning. Measurements from PTL should only be used to assess respiratory muscle performance after allowing time for learning.
渐进性阈值负荷（PTL）常用于评估健康和疾病状态下的吸气肌耐力。我们和其他人已经注意到，对于先前未接触过PTL的受试者，在最初几次接触该任务时耐力会系统性增加，这可能与肌肉本身的适应性无关。本研究的目的是通过检查18名健康受试者对3次PTL测试（每次间隔>24小时）的通气反应，来探究这种耐力增加的机制。在PTL期间，阈值压力（Pth）每2分钟增加约10%，直至任务失败。受试者可以采用任何呼吸模式。在连续测试中，呼吸肌力量（最大吸气压力[PImax]）保持不变，而PTL期间的最大Pth（Pthmax）增加（分别为PImax的69±17%、77±16%和86±11%，p<0.05）（平均值±标准差），这表明Pthmax的增加不能归因于呼吸肌力量的改善。随着连续测试，呼吸模式发生变化，因此对于比较负荷，吸气时间（TI）、呼吸频率（f）和占空比（TI/Ttot）降低。这种呼吸模式的变化并未改变呼吸肌效率（呼吸肌V O2/功），每次测试中呼吸肌效率相似（2.4±2.2%），但自觉用力程度（Borg评分）在每次测试任务失败时最大，在连续测试的比较负荷下降低。因此，初始测试期间的Pthmax似乎受不适限制而非呼吸肌功能限制。这些发现表明，连续测试中Pthmax的增加是所采用呼吸模式差异的结果，反映的是神经心理因素而非呼吸肌适应性。只有在留出学习时间后，PTL测量结果才应用于评估呼吸肌性能。
