Effects of hypercapnia on tracheal pressure, diaphragm and intercostal electromyograms in unanaesthetized fetal lambs.

1. The electromyographic (e.m.g.) activity of the diaphragm and intercostal muscles has been recorded during breathing movements of unanaesthetized lambs in utero (109-135 days), and compared with the changes of tracheal pressure.2. The diaphragm e.m.g. was irregular in size, shape and timing, with a variable rate of rise during inspiration, often with a flattening of integrated activity before the end of a breath and with little or no post-inspiratory activity.3. The diaphragm e.m.g. gave the most reliable measurements of breath interval and incidence: in eucapnia mean T(I) was 0.45+/-0.02 (S.E. of mean) and T(E) was 0.74+/-0.05 sec, and 58-100% of the diaphragm bursts were associated with identifiable and appropriate changes of tracheal pressure.4. During fetal hypercapnia, produced by increasing the maternal inspired CO(2) in a single change or series of step changes, tracheal pressure amplitude and its rate of change during inspiration increased progressively over the P(a, CO2) range of 37-87 mmHg.5. In eucapnia the area, amplitude and inspiratory slope of the integrated diaphragm e.m.g. were not always correlated with tracheal pressure amplitude, and in hypercapnia they increased only in the lower part of the P(a, CO2) range. Inspiratory intercostal activity increased progressively as the P(a, CO2) was raised.6. The frequency histograms of variables derived from the tracheal pressure, diaphragm and intercostal e.m.g.s were skewed to the left in eucapnia but became normalized during hypercapnia. The rate and depth of breathing became regular.7. The response to mild asphyxia was a combination of the responses to hypercapnia and hypoxia.8. The interpretation of the tracheal pressure and diaphragm e.m.g. as measures of the ;depth' of breathing and respiratory ;drive' in the fetal lamb is discussed.