Spirometric measurements were made with a nose clip and a mouthpiece, connected to a heated number 2 Fleisch pneumotachograph, connected to a differential pressure transducer and integrator (17212, Godart, Bilthoven, The Netherlands). Because cardiac surgery may change the geometric characteristics of the chest wall and result in changes in the VMCs of the two compartments, calibration of the RIP was repeated immediately before the measurements made on each day of observation. Volume was calculated from the instantaneous sum of the RIP values for abdominal and rib cage volumes. Details concerning the calibration procedure and the calculation of VMC have been reported. The intercepts of the regression line with the x axis and y produced 1/VMC ABand 1/VMC sub RC, respectively. Patients were asked to alternate between predominantly abdominal and thoracic breathing, while changes in the cross section of the two compartments were matched to simultaneously measured spirometric volumes (V T), From these measurements, a regression line was found by the least squares method between abdominal cross section and V Ton the x axis and between rib cage cross section and V Ton the y axis. A calibration of RIP measurements to spirometry was performed to calculate volume-motion coefficients for rib cage (VMC RC) and abdomen (VMC AB). Tidal changes in rib cage and in abdominal dimensions were measured simultaneously using a respiratory inductance plethysmograph (RIP) (Respitrace, White Plains, NY).

Sonomicrometer experiments give the most direct and convincing data about local diaphragm function but are difficult to perform in humans and have the potential disadvantage that installation of the devices into the diaphragm may induce a local or general change in diaphragm function. More recently, sonomicrometers placed in human costal hemidiaphragms on the same side as a lobectomy showed reduced shortening and even paradoxical lengthening of that part of the muscle during inspiration. A study in the 2nd week after cardiac surgery found no evidence of diaphragm dysfunction assessed by electric stimulation of the phrenic nerves. One study in humans undergoing thoracotomy for pneumonectomy or lobectomy found some changes in rib cage abdomen movement and P gaand pleural pressure changes but concluded that there was little evidence for any diaphragm dysfunction. An experimental study on sheep using sonomicrometers showed a marked reduction in diaphragm shortening after thoracotomy, similar to the finding in dogs after abdominal surgery. It is important to know whether respiratory muscle behavior also is abnormal after operations on the thorax, but data available to date have not made clear the extent to which it is. In addition, five patients exhibited positive changes in Pes at the end of inspiration that corresponded to closure of the upper airway, relaxation of inspiratory muscles, and subsequent opening of the airway with sudden exhalation, producing a grunt. In these cases tidal volume was generated largely by contraction and relaxation of abdominal muscles with probable reduction in diaphragm activity. A different ventilatory pattern was observed in the other two patients, consisting of minimal rib cage excursion and a large abdominal excursion. This change was accomplished in most cases by the use of abdominal muscles in expiration with an increase in inspiratory intercostal muscle action without an increase in diaphragm activation, that is, a shift in the normal balance of respiratory muscle use in favor of muscles other than the diaphragm. Six patients showed a postoperative pattern of breathing similar to that seen in patients who have undergone abdominal surgery: a decrease in the ratio of delta Pga/delta Pes and a shift toward rib cage expansion, with an increase in breathing rate and a decrease in tidal volume. Plots of Pes against Pga and rib cage against abdominal expansion (Konno-Mead diagrams) were constructed.

Mean delta Pga/delta Pes decreased from 0.73 preoperatively to -0.56 1 day postoperatively and recovered to 0.47 5 days postoperatively.

Muscles of inspiration