Analysis of the systemic arterial circulation by a square wave infusion technique.
Creators: name: Bottomley, Michael G.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1969
Subjects: subject: Biology, Animal Physiology.

description: An attempt has been made to analyse the physical properties of the systemic arterial circulation by a standard engineering approach (transient analysis). This involved giving a square wave infusion of blood and interpretation of the pressure response. Development of a technique to give a square wave infusion of blood was required. This resulted in an infusion-giving apparatus which was composed essentially of a syringe whose action was controlled by a large, servo-controlled motor. The square wave effect was achieved by an electro-magnetic clutch-brake combination. The latter was capable of full activation within a 30 millisecond time interval. The whole apparatus was able to give a perfect square wave within the tolerance of these limits. Approximately 5% of the animals blood volume was slowly withdrawn into the infusion giving apparatus through a cannula in the right carotid artery. On activation of the electro-magnetic clutch, this blood was returned to the rabbit in the form of a square wave over a 2 - 3 second period. During this period, the heart was transiently arrested by vagus nerve stimulation. The pressure response of the arterial system to this square wave infusion of known flow rate was monitored in the right femoral artery with a pressure transducer. By relating the input flow pattern to the output pressure response, information could be obtained about the physical properties of the system. Direct pressure-flow studies across the whole systemic arterial circulation were found to be linear in the range of 90 to 40 mm Hg pressure. A 'critical closing pressure' was invariably present, and found to vary in value from 10 to 50 mm Hg. This necessitated a modification in the normal method of calculating total peripheral resistance to account for the true slope of the pressure-flow plot (herein called 'slope' resistance). It was found that the mean chord resistance value in a 5 to 6 lb. rabbit was approximately twice the magnitude of the mean slope resistance value (0.33 to 0.17 mm Hg/ml/min). Furthermore, these did not always vary in the same direction when a variety of different stresses were imposed on the circulation. Compliance value calculations demonstrated a hysteresis effect. The mean compliance value of the systemic arterial circulation in a 5 - 6 lb. rabbit measured during inflation was 0.037 ml/mm Hg: measured during deflation, it was 0.054 ml/mm Hg. There was good correlation between the compliance values measured by the square wave infusion method and by a method using excised lengths of aorta. An inertial component was found to be present which had an approximate mean value of 30 gm cm-4 in 5 - 6 lb. rabbits. The general response of the systemic arterial circulation to square wave infusion showed that it could primarily be considered to act as a linear first order system. The transient pressure response to a step function in flow was a single exponential curve with a mean of 509 msecs in inflation and 743 msecs in deflation. More exact modelling required consideration of the systemic arterial circulation as a third order system to account for the small inertial component. An electrical model was built whose response to a simulated square wave infusion of fluid, gave good correspondence to that obtained in the living rabbit. The correlation coefficient between cardiac output values measured by the square wave infusion technique and by the Fick method was +0.80. The effects of various CO2 concentrations, asphyxia, haemorrhage, nor-adrenalin infusion, and long-term cholesterol feeding on the rabbit systemic arterial circulation were investigated by this method.