Anaesthesia and Hypothermic Cardiopulmonary Bypass: Haemodynamic and Metabolic Variables

Alston, Robin Peter (1990) Anaesthesia and Hypothermic Cardiopulmonary Bypass: Haemodynamic and Metabolic Variables. MD thesis, University of Glasgow.

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Abstract

Despite extensive investigation, the effects of some aspects of cardiopulmonary bypass upon haemodynamic and metabolic variables remain unresolved. Also, there have been great changes in the practice of cardiopulmonary bypass over the years and the findings of early research may no longer be applicable to present day techniques. Three aspects of cardiopulmonary bypass were identified as requiring investigation regarding their haemodynamic and metabolic effects: low flow rates; flow character ie nonpulsatile or pulsatile perfusion; and acid-base management te pH or alpha-stat control. Anaesthesia during cardiopulmonary bypass has been found in the past to have important metabolic effects which could be used to improve patient wellbeing and hence, outcome. However, these agents have been largely superseded by modern drugs that are metabolically untested. Although the effects on haemodynamic variables of most modern anaesthetics have been extensively studied before and after cardiopulmonary bypass, their actions during the abnormal conditions of cardiopulmonary bypass have not been rigorously examined. It was hypothesised that cardiopulmonary bypass and anaesthetic techniques have important haemodynamic and metabolic effects. This thesis was undertaken to test this hypothesis. In all the studies, arterial and mixed venous blood samples were analysed for oxygen content, saturation and tension, pH, carbon dioxide tension, base excess and lactate concentration. Systemic oxygen uptake and delivery were calculated. Initially, a computerised system was developed to act as a data logger for haemodynamic, arterial pH and temperature measurements as well as to enhance thermostatic and acid-base control. This system was tested and found to function well both as a recording device and as a means of obtaining good thermostatic control. However, the system performed poorly with regard to arterial pH control. The haemodynamic and metabolic effects of flow rate, flow character and acid-base management during hypothermic cardiopulmonary bypass were studied in a factorial experiment. Of the three factors, only alternation of flow rate between 1.5 and 2.0 L. min-1.m-2 was found to have a significant effect on systemic oxygen uptake. Flow rate was also found to have a significant effect on mean arterial pressure and peripheral vascular resistance. Arterial pH and stage during cardiopulmonary bypass were found to significantly interact to influence mean arterial pressure but not peripheral vascular resistance. Alternation of flow character between pulsatile and nonpulsatile perfusion had no significant effect on haemodynamic variables. Over and above these effects, a progressive vasoconstriction throughout cardiopulmonary bypass was noted. Isoflurane's effects on haemodynamic and metabolic variables were examined during hypothermic cardiopulmonary bypass. Isoflurane was found to be a vasodilator during these abnormal haemodynamic conditions although no systemic metabolic effects were identified. Next, the haemodynamic and metabolic effects of atracurium during hypothermic cardiopulmonary bypass were studied. Neither haemodynamic nor metabolic effects were found from the use of atracurium. Finally, the haemodynamic and metabolic effects of alfentanil and it's antagonism with naloxone during hypothermic cardiopulmonary bypass were investigated. Neither alfentanil nor its antagonism with naloxone had any significant metabolic effect. However, administration of alfentanil prevented the expected increases in mean arterial pressure and peripheral vascular resistance that occur during the course of cardiopulmonary bypass. In contrast, antagonism of alfentanil with naloxone produced greater increases in mean arterial pressure and peripheral vascular resistance than would be predicted to occur simply as a result of stage. Flow rate proved to be an important determinant of haemodynamics and metabolism during hypothermic cardiopulmonary bypass. This finding makes questionable the practice of reducing the pump flow rate to low levels after induction of hypothermia. The lack of difference in haemodynamic and metabolic effects between nonpulsatile and pulsatile perfusion would add weight to the body of opinion which holds that flow character has no important actions during clinical cardiopulmonary bypass. Arterial pH interacted with stage during cardiopulmonary bypass to influence mean arterial pressure. However, the clinical importance of this finding is uncertain as the size of effect was small. This haemodynamic finding and the lack of any difference in metabolic effect add no weight to the use of either alpha or pH-stat acid-base management. The progressive vasoconstriction, found throughout these studies, is an important and well recognised phenomenon of cardiopulmonary bypass. (Abstract shortened by ProQuest.).

Item Type: Thesis (MD)
Qualification Level: Doctoral
Keywords: Medicine
Date of Award: 1990
Depositing User: Enlighten Team
Unique ID: glathesis:1990-78173
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:38
Last Modified: 30 Jan 2020 15:38
URI: http://theses.gla.ac.uk/id/eprint/78173

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