RESPIRATORY MUSCLE BLOOD-FLOW IN THE FETAL LAMB DURING APNEA AND BREATHING

We measured blood flow to the respiratory muscles of the fetal lamb us ing the radioactively-labelled microsphere technique in order to asses s whether fetal breathing is an energetically costly activity as has b een reported. Diaphragm flow ranged from 6.4-35.2 ml.min(-1).100 g(-1) during fetal apnoea and rose to 21.1-615 ml.min(-1).100 g(-1) during fetal breathing (P<0.02; n = 7). Parasternal muscle flow also increase d significantly (P<0.02) between fetal apnoea and breathing while exte rnal and internal intercostal flows did not change. Expressed as a per centage of cardiac output the diaphragm received 0.08-0.28%;, during a pnoea and 0.22-2.2% during fetal breathing. Neither placental blood fl ow nor fetal O-2 consumption increased significantly between fetal apn oea and breathing. We conclude that the levels of perfusion required b y the respiratory muscles for breathing in the fetus are inconsistent with fetal breathing costing a large proportion of the fetal O-2 budge t.