Blood Pressure and Pulse Rate in the Foetal Sheep

Abstract

1. A method (the needle method) is described for the measurement of the pressure in the stream going through a vessel. 2. In the foetal sheep the needle method applied to the umbilical artery gives substantially the same results as the mercurial manometer applied to the carotid, until about half-way through the gestation period. 3. As gestation proceeds the needle method applied at the first moment at which it can be applied to the umbilical artery (or a branch) gives readings substantially lower, and increasingly lower as gestation proceeds, than does the mercurial manometer read at the first moment at which it can be read. 4. The discrepancy is due to the sum of a number of causes which are discussed, but of these the most important is an actual rise of pressure between the time of delivery and the completion of the dissections contingent on the use of the mercurial manometer. 5. The cause of this is not at present demonstrated, but either or both of two factors may be concerned: (a) a dulling of the central nervous system which weakens the depressor reflex; (b) the establishment of a greater degree of vasomotor tone consequent on the bombardment of the central nervous system with sensory stimuli. 6. The pulse rates in utero and just after delivery of the foetus into a saline bath at 39-40°C. (the umbilical circulation being unimpaired) are not significantly different. 7. The pulse rate quickens up to the 70th-80th day, after which it becomes slower as gestation proceeds. 8. If both vagi be severed, the pulse rate te to quicken throughout gestation. The pulse, therefore, comes increasingly under vagus inhibition from the 80th-90th day onwards. 9. Even after the vagi have been cut after the 120th day (it has not been tried before) adrenalin in sufficient quantity will cause a further quickening of the pulse. 10. The earliest date at which stimulation of the peripheral end of the right vagus was observed to slow the heart was the 77th day. On the 85th day peripheral stimulation of the left vagus also failed, but succeeded on the 101st day. 11. Central stimulation of the left vagus, with the right vagus intact, produced slowing on the 77th day. 12. Slowing of the heart synchronous with rise of arterial pressure has been observed on the 111th day. 13. Slowing of the heart which bears evidence of being reflex has been obtained by raising the blood pressure (clamping the cord) on the 121st day and by injection of adrenalin on the 118th day. 14. Approaching term both the carotid sinus and cardiac depressor mechanisms are functional. 15. Lowering of the blood pressure as the result of stimulation of the central end of the vagus and with both vagi severed can be demonstrated late in gestation.