Page 118 - Clinical Manual of Small Animal Endosurgery

106 Clinical Manual of Small Animal Endosurgery

in ventilation-perfusion mismatch. Respiratory and metabolic acidosis
and hypoxaemia may result, especially in obese patients, or in those with
pre-existent cardiopulmonary disease.
Assisted ventilation may be therefore necessary in obese or older
patients, and is recommended in all procedures over 30 min duration
to provide pulmonary support and reduce development of atelectasis.
Positive-pressure ventilation minimises the effects of pneumoperitoneum,
and normocapnia can be maintained by increasing the tidal volume
and keeping the respiratory rate low (Quandt, 1999). The minimal
amount of intra-abdominal pressure necessary to perform the procedure
should also be used, and values in excess of 15 mmHg have to be
avoided. A pressure of 8–10 mmHg allows excellent visualisation in
most cases.
As carbon dioxide is the insufflation gas of choice, end-tidal carbon
dioxide monitoring and pulse oximetry are important to detect hypox-
aemia and hypercarbia. Diffusion of carbon dioxide across the peritoneal
membrane will lead to significant increase in PaCO2 and decrease in
PaO2, even in patients receiving mechanical ventilation (Duke et al.,
1996). End-tidal carbon dioxide should be between 35 and 45 mmHg
and SpO2 greater than 95%. Hypercarbia contributes to myocardial
depression and increase in sympathetic tone, which may produce cardiac
arrhythmias. Sinus tachycardia, ventricular arrhythmias and asystole are
most commonly seen, especially in conditions of light anaesthesia. Brady-
cardia can instead develop as a consequence of peritoneal distension
(Quandt, 1999). Another effect of hypercarbia is increase in intracranial
pressure, mediated by increased cerebral blood flow. Thus, laparoscopy
should be used with caution in patients with head trauma.
Body position of the patient during the procedure may also adversely
affect the cardiovascular and respiratory systems. Inhalant anaesthetics
depress the baroreflex, with consequent diminished reflex control of
circulation following changes in body posture (Bailey and Pablo, 1999;
Joris et al., 1993). The head-down tilt (Trendelenburg position), espe-
cially accompanied by abdominal insufflation, decreases ventilation and
cardiac output. The risk of passive gastric reflux is also increased. The
reverse Trendelenburg position, or head-up tilt, leads instead to reflex
vasoconstriction, with increased heart rate and blood pressure (Abel et
al., 1963). To prevent complications, more than 15° of tilt of the patient
in a cranial or caudal direction should be avoided. Patients at higher risk
of gastric reflux, such as those with gastric outflow obstruction, hiatal
haernia or obesity may benefit from preoperative administration of meto-
clopramide, antacids and H2 blockers. An orogastric tube may be placed
if the stomach appears enlarged (Quandt, 1999).
Early detection of complications (haemorrhage, pneumothorax, punc-
ture of an organ and carbon dioxide embolism) is important to improve
the outcome. Serial measurements or estimations of blood loss rather
than serial determinations of packed cell volume or total protein levels
are useful for selecting the type of fluid for resuscitation. The former will