Function of the canine inspiratory muscle pump in pleural effusion: influence of body position.

Pleural effusion, a complicating feature of many diseases of the lung and pleura, adversely affects the pressure-generating capacity of the diaphragm in supine dogs. The objective of the present study was to assess the impact of body position on this effect and to evaluate the adaptation to effusion of the inspiratory muscle pump during breathing. Two experiments were performed. In the first, progressively increasing effusion was induced in anesthetized animals, and the changes in pleural (ΔPpl) and abdominal (ΔPab) pressure were measured during isolated phrenic nerve stimulation while the animals were placed in both the supine and the 45° head-up posture. In the second experiment, graded pleural effusion was also performed, and ΔPpl, ΔPab, and the electromyogram of the parasternal intercostal muscles were measured while the vagotomized animals were breathing spontaneously in the same two postures. The data showed that with effusion 1) ΔPpl during phrenic nerve stimulation was substantially lower with the animals in the head-up than in the supine posture; 2) this postural effect was primarily the result of the decrease in muscle length in the head-up posture; 3) during spontaneous breathing, however, parasternal intercostal inspiratory activity increased and ΔPpl remained unaltered while ΔPab decreased; and 4) the decrease in ΔPab and in the ΔPab/ΔPpl ratio was much larger in the head-up than in the supine posture. It is concluded that in the presence of pleural effusion, the pressure contribution of the inspiratory intercostal muscles during breathing increases and compensates for the shortening of the diaphragm, particularly in the upright posture.