The surgical patient (second part)

Surgeons in theatreA few considerations regarding the control of surgical stress and peri-operative pain (i.e. pain during and after surgery) may be of some interest, even though their diffusion in the veterinary world still seems to be limited. Surgical trauma is an injury that may range from complications following minor elective surgery to a massive insult following major procedures complicated by sepsis. Inflammation constitutes the local response, and endocrine metabolic activation, leading to hypermetabolism with substrate mobilisation, constitutes the general response. The general response is highly dependent on the severity of injury, with procedures involving the thorax and the abdominal cavity eliciting a response that may last up to several days. The stress response has usually been considered an homeostatic defence mechanism extremely important for tissue healing and adaptation to the noxious insult, without considering the potentially harmful effects of surgically induced endocrine response, hypermetabolism, and the resulting increased demands on physiologic reserve. Nowadays, however, the biochemical changes after injury need not to be considered as an homeostatic response important for survival and recovery, since physiologic disturbances may be prevented or treated, and substrates, blood, and other fluids are readily available. During the last few decades the astonishing increase in knowledge within surgery and anaesthesia has allowed even major procedure to be performed in patient with severe complicating disease, previously contraindicating surgery. This explains the growing interest in mechanisms involved in surgical stress response and in techniques which may limit it.

Many studies have investigated methods to blunt the stress response to surgery. Though general anaesthesia may limit the perception of the injury, this has been proved not to be necessarily followed by a limitation of stimuli directed to hypothalamus, which means stress response may remain unaltered. Most injectable and inhalational anaesthetics have poor or no effect on endocrine changes induced by surgical trauma if administered at clinical doses, and this is true both for the intra-operative and (above all) for the post-operative period. Also high dose opioid anaesthesia, usually administered for cardiac surgery, has only a transient inhibitory effect on the stress response that is related to high plasma and tissue concentrations, and it has no prolonged postoperative effects on metabolism. Scientific evidence suggest that opioids, which are widely used to control peri-operative pain, do not produce a significant reduction in stress response to surgery, unless high doses are used. Since metabolites of the arachidonic cascade are involved in several steps of the response to injury, pre-operative administration of NSAIDs (non-steroid anti-inflammatory drugs) may only result in a slight modification of stress response and immunosuppression. However clinical implications of this modification are unknown. The peripheral application of local anaesthetics in the wound may reduce both pain and endocrine response, but not other systemic reactions (leukocytosis, temperature, etc.). Conversely, as far as epidural administration of local anaesthetics is concerned, many studies in human beings have proved its effectiveness, at T4 level, in effectively reducing endocrine response during surgery of the pelvis and lower limbs. Analgesia is more effective if upper dermatomes are blocked and the duration of sensitive blockade is prolonged. An epidural anaesthetic block lasting less than 4 hours has a transient inhibitory effect, with a 24 hours epidural analgesia demonstrated an inhibitory effect lasting for not less than 4 days. When surgery is performed above the umbilicus or in the thorax, epidural administration of local anaesthetics can only partly reduce the stress response. It has been assumed that in this case, stress response is activated by vagal, sympathetic and frenic afferent fibres, which are not blocked by epidural anaesthesia. The epidural administration of opioids produces effective analgesia, but the mitigation of stress response is less intense and less prolonged, compared to epidural local anaesthetics. However, at the same doses, the epidural administration is comparatively more effective than the intravenous administration, therefore requiring smaller doses. Duration of effect in the post-operative period does not exceed 8 hours, or slightly longer. Moreover, analgesia and reduction of stress response are not necessarily correlated. Peripheral nerve blocks have proved to produce no effects on stress response during abdominal or thoracic surgery, despite a good degree of analgesia. It is possible that peripheral blocks are more suitable as surgical stress inhibitors as far as lower or caudal parts of the body are involved, though this aspect has not been deeply investigated so far. The association of general anaesthesia with nerve blockade has no additional inhibiting or stimulating effect on neuroendocrine response, and the results are perfectly comparable with what can be obtained with epidural anaesthesia by itself.

In the light of these considerations, it seems evident that regional techniques are particularly appropriate even in a traumatized patient, though the presence of multiple lesions and respiratory or cardiovascular instability will require a  general anaesthetic to prevent aspiration, to assist ventilation, and to provide cardiovascular stability.