| Cardiovascular monitoring and vasopressor therapies are important adjuncts to clinical practice especially in the fields of anaesthesia and critical care. However, the developments of safe and effective methods need rigorous testing that cannot be done solely in the clinical arena because of ethical limitations. In this thesis I developed an anaesthetized canine model in which cardiac output and contractility were measured using a high-performance flow probe placed on the aorta via a left thoracotomy and renal blood flow was measured by a flow probe placed on the left renal artery. This model also provided gold standard measurements of cardiac contractility (dF/dt (max)) from the aortic flow waveform. Arterial and central venous blood pressure was also measured. The circulation of the canine model was manipulated using vasopressor infusions and halothane.;The model enabled me to evaluate: (i) the performance of a new impedance cardiograph, the RheoCardioMonitor; (ii) the performance of a new ultrasonic Doppler cardiac monitor, the USCOM, and (iii) the circulatory effects of five commonly used vasopressor agents on the systemic and renal circulation under normal and septic conditions.;My main findings were that: (i) impedance cardiography cardiac output measurements were affected by changes in peripheral resistance, so that it underestimated at low peripheral resistance and overestimated at high peripheral resistance; (ii) impedance cardiac contractility was better assessed using indices based on the slope of the impedance waveform, rather than the timing intervals of cardiac cycle; (iii) the USCOM reliably detected changes in cardiac output over a wide range of values; and (iv) noradrenaline, which showed a powerful vasopressor effect without decreasing of cardiac output and renal blood flow, was the best choice of vasopressors in septic shock, but the effects of all catecholamine vasopressors were attenuated in septic shock, and with the exception of vasopressin which probably was the alternative choice in refractory septic shock.;In conclusion, the canine flow probe model proved a valuable asset for testing cardiac monitors and evaluating vasopressor therapies, particularly in sepsis. It enabled me to answer a number of clinically relevant questions relating to these devices and therapies, and set up the foundation for future studies. |
