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

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Abstract
The electromyographic (EMG) activity of the diaphragm and intercostal muscles was recorded during breathing movements of unanesthetized lambs in utero (109-135 days) and compared with the changes of tracheal pressure. The diaphragm EMG 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 postinspiratory activity. The diaphragm EMG gave the most reliable measurements of breath interval and incidence: in eucapnia mean TI [tracheal inspiration] was 0.45 .+-. 0.02 (SE of mean) and TE [tracheal expiration] was 0.74 .+-. 0.05 s, and 58-100% of the diaphragm bursts were associated with identifiable and appropriate changes of tracheal pressure. During fetal hypercapnia, produced by increasing the maternal inspired CO2 in a single change or series of step changes, tracheal pressure amplitude and its rate of change during inspiration increased progressively over the PaCO2 [arterial partial pressure] range of 37-87 mm Hg. In eucapnia, the area, amplitude and inspiratory slope of the integrated diaphragm EMG were not always correlated with tracheal pressure amplitude and in hypercapnia they increased only in the lower part of the PaCO2 range. Inspiratory intercostal activity increased progressively as the PaCO2 was raised. The frequency histograms of variables derived from the tracheal pressure, diaphragm and intercostal EMG were skewed to the left in eucapnia but became normalized during hypercapnia. The rate and depth of breathing became regular. The response to mild asphyxia was a combination of the responses to hypercapnia and hypoxia. The interpretation of the tracheal pressure and diaphragm EMG as measures of the depth of breathing and respiratory drive in the fetal lamb is discussed.