39 Notes on sedation and general anaesthesia
General Considerations
Clinical Tip
• Where the patient’s condition allows, every effort should be made to delay anaesthesia until the patient is stabilized.
Potential risks of sedation and anaesthesia
Clinical Tip
• Before commencing sedation or anaesthesia, the patient should be assessed in the context of how the major body systems are likely to cope with the adverse effects of the drugs employed, and also in turn how the course of anaesthesia is likely to be influenced by the patient’s condition. Potential problems should be anticipated and prepared for in advance.
See Tables 39.1 and 39.2.
Table 39.1 Potential adverse effects of anaesthesia on body systems
| Body system | Adverse effects |
|---|---|
| Cardiovascular | Reversal of peripheral vasoconstriction which the patient may be relying on to maintain perfusion of vital organs |
| Respiratory | |
| Nervous | |
| Renal | Reduced renal perfusion |
| Hepatic | Reduced hepatic perfusion |
| Gastrointestinal | Increased risk of regurgitation and subsequent aspiration |
| Temperature | Hypothermia |
Table 39.2 Potential effects of patient condition on the course of anaesthesia
| Patient condition | Effect on anaesthesia |
|---|---|
| Cardiovascular compromise (e.g. hypovolaemia) | Risk of overdose (e.g. due to reduced volume of drug distribution, delayed response to intravenous injection) Reduced cardiac output increases rate of inhalation anaesthetic uptake resulting in more rapid inhalation induction |
| Respiratory compromise | Reduced ventilation results in slower onset of inhalation anaesthesia and a slowed response to altered inhalation anaesthetic concentration |
| Central nervous system disease | Reduced level of consciousness increases sensitivity to all sedatives and anaesthetic agents |
| Renal disease | Reduced renal elimination of drugs, prolonging duration of effect |
| Hepatic disease | Reduced hepatic metabolism of anaesthetic drugs, prolonging duration of effect |
| Hypoalbuminaemia | Increases nonprotein-bound, active concentration of many drugs, increasing sensitivity to these drugs |
Maximizing patient safety prior to sedation and anaesthesia
The fundamental goal is to ensure adequate oxygen delivery to the vital organs.
Maximizing patient safety during general anaesthesia
3 Do anything you can do without anaesthesia beforehand to minimize anaesthetic time (e.g. pre-clip surgical site).
9 Intravenous fluids. Unless the patient’s condition contraindicates it, run isotonic crystalloid fluids at surgical rates (typically 10 ml/kg/hr) as induction commences to offset any potential drop in blood pressure. The type and rate of fluids employed subsequently under anaesthesia in general will be based on that already instituted for stabilization (see Ch. 4). To what extent these rates are modified for anaesthesia depends on:
(c) Expected on-going pathological fluid losses (bleeding, body cavity effusions or gut sequestration)
10 Calculate the approximate expected dose of anaesthetic agent required – be conservative. Consider using an intravenous sedative pre-induction to minimize the intravenous anaesthetic dose (e.g. diazepam). Dose in increments until the desired effect is reached (usually to allow smooth intubation), allowing plenty of time between doses if the patient is cardiovascularly unstable. More rapid administration may be justified if patent airway or intermittent positive pressure ventilation (IPPV) is a priority.
11 Ensure smooth intubation. Use local anaesthetic spray in cats in a timely fashion and allow it time to work.
12 Perform an airway, breathing, circulation (ABC) check immediately on intubation and be prepared to implement IPPV:
(b) Breathing = confirm breathing through tube (chest movement produces bag movement) or give a breath if no breath observed (bag movement produces chest movement)
13 Use a balanced anaesthetic technique for the maintenance of anaesthesia: use nitrous oxide in the gas mixture if it is safe to do so, timely opioid analgesia bolus or constant rate infusion (local anaesthetic techniques, muscle relaxants if familiar with their use). Minimize anaesthetic concentrations at all times.
Table 39.3 Being prepared for anaesthesia of critical patients
ECG, electrocardiograph; ET, endotracheal; IPPV, intermittent positive pressure ventilation.
Clinical Tip
• Because anaesthetic drugs cause cardiorespiratory depression in a dose-dependent manner, it is important to try to minimize the dose used. Anaesthetic administration can be minimized by the rational use of additional drugs which contribute to the anaesthetic state, without producing significant cardiorespiratory effects of their own.
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