Date of Award

1-1985

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Medical Sciences

Supervisor

Professor L.D. Pengelly

Abstract

Saunders and his associates reported that during submaximal neuromuscular blockade, vital capacity was reduced to a greater extent than would have been predicted according to the reduction in the maximal inspiratory pressure in a simple in a simple mechanical model (J. Appl. Physiol. 44:589,1978). Disproportionate respiratory muscle weakness and a different recruitment pattern for different voluntary efforts were proposed as possible explanations.

This issue was addressed by measuring the rectified electromyogram of the diaphragm, intercostal, scalenes, and sternomastoid muscles during maximal and submaximal static inspiratory manoeuvres. Two studies were performed. The control series of experiments showed that at very low levels of static pressure, only the diaphragm was activated. Further increases in pressure then recruited the intercostal/accessory muscles, in some subjects more than in others. There was generally an orderly increase in the rectified EMG in all muscles with elevations in pressure. For particular levels of pressure to be produced at lung volumes above the relaxed end-expiratory position, the EMG was increased. Compared to the maximum EMG elicited with maximal static pressures or full inspirations, resting ventilatory requirements still leave a large myo-electric reserve.

In the submaximal neuromuscular block study, the mean maximal inspiratory pressure was decreased from 103 to 39 cm H20, but only two of the five subjects behaved in the manner observed by Saunders and his associates. There was no clear evidence that the diaphragm was less affected than the other muscles. In many cases however, a submaximal level of pressure was achieved by augmented EMG in all muscles. It is suggested that curare interferes with the conversion of electrical events into whole muscle tension and/or that the impaired chest wall volume compartments are inefficient at producing static pressure.

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