Cardiovascular Disease and Anaesthesia
Introduction
Cardiovascular disease comprises of any disease process that affects the heart, blood vessels and blood flow. It can affeect patients of any age as congenital defects are not uncommon, and the cardiovascular function reduces with age. It is this change and deterioration in function that can lead to anaesthetic issues and so consideration needs to be taken into planning an anaesthetic protocol in these patients.
Pharmacokinetic Effects of Cardiovascular Disease
Anaesthetic agents are dependent on a number of different physiological systems functioning at their optimum. The cardiovascular system plays an important role and any disease affecting this system will have a number of effects.
- Volume of distribution reduced - Patient is more sensitive to injectable agents meaning lower doses required.
- Reduced circulation time - Delayed response after intravenous administration.
- Poor peripheral perfusion - Reduced bioavailablity of agents via any route other then intravenous.
- Decreased cardiac output - Increased induction rates and uptake of gaseous agents meaning more sensitivity to vapouriser settings.
- Decreased renal perfusion - Affects renal clearance of agents and concurrent hypoalbuminaemia increases albumin-bound compound sensitivity.
- Decreased hepatic perfusion - Prolonged affect of drugs metabolised via the hepatic system.
Preanaesthetic Considerations
Preanaesthetic Examination
One of the most important objectives to the preoperative examination in cardiac patients is to determine whether the patient will be able to withstand the anaesthetic and surgery, and to aid with appropriate selection of agents used for the protocol. The easiest and most common indicator used to assess cardiopulmonary function in small animal patients, particularly dogs, is exercise tolerance. On any cardiac patient, an electrocardiogram (ECG) should be performed and in addition thoracic radiographs, ultrasound, echocardiogram and arterial blood gas may be required further tests to assess cardiac function and the severity of the disease. However, car should be taken when undergoing any of these tests as it may stress the patient further, exacerbating the underlying condition and worsening the patients status.
Preanaesthetic Preparation
The aim of the preanaesthetic preparation is to reduce the risks of anaesthesia as much as possible before any procedure is performed. For example, uncontrolled heart failure is a complete contraindication to general anaesthesia and so needs to be treated before any procedure. This in turn should correct any secondary conditions that have developed, such as arrhythmias, pulmonary oedema and other organ dysfunction, such as liver and kidney. These secondary complications need to be controlled as they themselves can add risk to any anaesthetic procedure. However, care should be taken when selecting treatments for cardiac disease as these drugs can have undesired complications further adding to the risks of anaesthesia.
Selection of Agents
As with any anaesthetic protocol there are a number of aims to ensure a safe anaesthetic. In cardiac patients these include:-
- Adequate analgesia and muscle relaxation
- Optimal oxygenation of tissue and myocardium via optimum blood oxygenation.
- Optimal cardiac output and systemic blood pressures to allow for adequate perfusion of major organs such as liver, kidney and brain.
- Agents used should have no contraindications for use with other therapeutic agents used preoperatively to stabilise the patient.
Induction
As in any patient, induction should be stress free. This is especially important in cardiac patients as it may exacerbate their condition further making it unsafe for them to become anaesthetised. The minimum amount of anaesthetic agent should be used to allow for intubation.
Anaesthetic Maintenance
As mentioned previously, anaesthetic agents used in cardiac patients should preserve cardiac function as well as ideally being non cumulative. This can be performed by using multimodal techniques thereby reducing the doses of other drugs used which may have adverse effects on the cardiovascular system. Body position throughout the procedure should also be thought about to allow for adequate blood flow around the body and back to the heart, as well as optimising oxygenation by preventing impairment of breathing. Depth of anaesthesia should be closely monitored as in any patient. Inadequate anaesthesia can cause catecholamine release which in itself can have detrimental affects on the patient, while overdosing can compromise cardiac function. Close monitoring is required throughout the procedure. This should include such things as pulse rate and quality, mucous membrane colour, capillary refill time, blood pressure, respiratory rate and effort and temperature among other parameters. Fluid loss should also be closely monitored and replaced to maintain adequate cardiac output.
Postanaesthetic Considerations
Close monitoring of cardiac patients should continue into the recovery period to ensure that any post operative complications are noted and treated rapidly. Oxygen should be delivered to these patients throughout recovery to maintain oxygenation. Pain should be closely managed to help control stress levels, with multimodal analgesia being most effective.