No abstract

Inspiratory pressure support (IPS) decreases the work of breathing in patients with hyperinflation secondary to airways obstruction when weaning from mechanical ventilation.'0 Non-invasive assisted ventilation has been shown to reduce breathlessness during acute ventilatory failure resulting from COPD. " These patients have increased loads on the respiratory muscles, particularly during exercise, inducing extreme breathlessness. We hypothesised that the application of non-invasive IPS during exercise may reduce breathlessness and improve exercise tolerance by reducing the load on the respiratory muscles. This hypothesis was tested by measuring the effects of non-invasive IPS by face mask on the tolerance to treadmill exercise in eight patients with stable severe COPD. Walks with IPS were compared with those with oxygen and CPAP. MethodsEight men with stable COPD who complained of disabling breathlessness on exertion were recruited from the chest clinic (age range 60-74 years, mean (SD) FEV, 0 73 (02) 1).All patients underwent a period of familiarisation before data collection. This involved several visits to the laboratory where treadmill walks, with and without breathing equipment, were performed so that the patients became completely familiar with the use of the face masks, CPAP, IPS, the Borg scale, and the treadmill. During this training period a comfortable walking speed was found for each patient matching as closely as possible their usual walking speed. Once this had been established, all subsequent walks were performed at this speed. Patients were asked to walk on the treadmill until their perceived rating of

Reduced diaphragm contractility has been described in normal subjects after whole body endurance exercise, and it indicates low frequency fatigue (LFF); it is unknown whether LFF is of clinical importance. We therefore studied the effect of treadmill exercise to exhaustion on diaphragm contractility in six patients with severe chronic obstructive pulmonary disease (COPD) (mean FEV1, 0.71, 27% predicted). The subjects first performed a short (control), treadmill walk and then, after resting, a second walk to a state of severe dyspnea. Cervical magnetic stimulation of the phrenic nerve roots was performed at the start of the study and 20 and 30 min after each walk. The twitch transdiaphragmatic pressure (Tw Pdi) was reproducible (mean coefficient of variation, 5.3%; range, 2 to 12.5%). Mean Twi Pdis were 18.4 cm H2O at baseline and 19.6 cm H2O and 19.2 cm H2O 20 and 30 min after the control walk. At the same times after the exhaustive walk, mean Tw Pdis were 19.6 and 20.4 cm H20. Tw Pdi was not reduced by exhaustive treadmill walking (p > 0.9), and a power calculation showed that the study had a 95% chance of detecting a 10% fall at the 5% significance level. We conclude that Tw Pdi is not reduced when patients with severe COPD walk to a state of extreme breathlessness and that therefore low frequency fatigue of the diaphragm does not occur.

When patients with COPD walk to a state of intolerable dyspnea, there is excessive inspiratory muscle loading, as evidenced by slowing of the maximum relaxation rate of the inspiratory muscles, measured from esophageal pressure during a sniff (Sn Pes MRR). In this setting, inspiratory pressure support (IPS) delivered via an orofacial mask increases walking distance and reduces dyspnea, but the mechanism by which this benefit is achieved remains unclear. In this study we compared Sn Pes MRR after equidistant treadmill walking in six men with severe COPD (mean FEV1: 0.6 L, 22% predicted). After the free walk there was a mean slowing of Sn Pes MRR of 41% (p < 0.03). After the IPS-assisted walks, the slowing of Sn Pes MRR was 20% of baseline; this was significantly less than after the free walk (p < 0.05). Four subjects performed shorter walks; after free walks of one third and two thirds of maximum distance, the mean slowing of Sn Pes MRR was 23% and 28%, respectively. We conclude that when patients with COPD walk to exhaustion, IPS reduces slowing of inspiratory muscle MRR, and that this represents a considerable unloading of the inspiratory muscles. The magnitude of the reduction is approximately the same as reducing the distance walked by two thirds.

The excessive load placed on inspiratory muscles when patients with COPD exercise could lead to fatigue and contribute to exercise limitation. Slowing of maximal relaxation rate (MRR) of skeletal muscle is an early index of the fatiguing process. We investigated whether inspiratory muscle MRR slows when patients with COPD walk to exhaustion. We studied nine well-trained and motivated patients with stable severe COPD (mean FEV1: 0.7 L, 28% predicted). Each subject performed sniff maneuvers before and after walking on a treadmill until they were forced to stop because of dyspnea. Esophageal (Pes), gastric, and transdiaphragmatic pressures were measured using balloon-tipped catheters. MRR was calculated as the percent Pes drop/10 ms. In the first minute after exercise there was a mean decrease of Pes MRR of 42% (range, 21 to 65%) (p < 0.01), which returned to baseline within 3 to 5 min. The fall in MRR indicates that the inspiratory muscles of patients with COPD walking to exhaustion are sufficiently heavily loaded to initiate the fatiguing process.