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Gas exchange during separate diaphragm and intercostal muscle breathing

Rammelkamp Center for Education and Research, MetroHealth Medical Center, and Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, Ohio 44109-1998

Submitted 17 June 2003 ; accepted in final form 5 February 2004

In patients with diaphragm paralysis, ventilation to the basal lung zones is reduced, whereas in patients with paralysis of the rib cage muscles, ventilation to the upper lung zones in reduced. Inspiration produced by either rib cage muscle or diaphragm contraction alone, therefore, may result in mismatching of ventilation and perfusion and in gas-exchange impairment. To test this hypothesis, we assessed gas exchange in 11 anesthetized dogs during ventilation produced by either diaphragm or intercostal muscle contraction alone. Diaphragm activation was achieved by phrenic nerve stimulation. Intercostal muscle activation was accomplished by electrical stimulation by using electrodes positioned epidurally at the T₂ spinal cord level. Stimulation parameters were adjusted to provide a constant tidal volume and inspiratory flow rate. During diaphragm (D) and intercostal muscle breathing (IC), mean arterial PO₂ was 97.1 ± 2.1 and 88.1 ± 2.7 Torr, respectively (P < 0.01). Arterial PCO₂ was lower during D than during IC (32.6 ± 1.4 and 36.6 ± 1.8 Torr, respectively; P < 0.05). During IC, oxygen consumption was also higher than that during D (0.13 ± 0.01 and 0.09 ± 0.01 l/min, respectively; P < 0.05). The alveolar-arterial oxygen difference was 11.3 ± 1.9 and 7.7 ± 1.0 Torr (P < 0.01) during IC and D, respectively. These results indicate that diaphragm breathing is significantly more efficient than intercostal muscle breathing. However, despite marked differences in the pattern of inspiratory muscle contraction, the distribution of ventilation remains well matched to pulmonary perfusion resulting in preservation of normal gas exchange.

electrical stimulation; respiratory muscles; artificial respiration