Lidocaine
Lidocaine can be administered to dogs to diminish ventricular dysrhythmias and improve blood pressure when those dysrhythmias
are decreasing cardiac output. It is also possible that lidocaine administered during surgery will prevent plasticity of
spinal cord neurons in response to nociceptive stimuli and, even though discontinued at the end of anesthesia, improve comfort
of the patient in the next 2-3 days. The loading dose of lidocaine is 1-2 mg/kg and can be given before or after induction
of anesthesia. Blood levels of lidocaine are maintained by a constant infusion of lidocaine, 0.025-0.05 mg/kg/min. Lidocaine
can be administered direct from a syringe using a syringe pump or in diluted form, 1 mg/ml, by adding 500 mg lidocaine to
500 ml 0.9% saline and using a fluid administration set. Continuous IV infusion of lidocaine provides some sedation and may
decrease the requirement for isoflurane or sevoflurane by 20-30%. Lidocaine infusion is not usually recommended for cats.
Lateral Chest Thoracotomy
Anesthesia for thoracotomy involves more than just choice of anesthetic agents. Controlled ventilation must be maintained
in the face of decreased lung volume from surgical manipulation of lung lobes or intrathoracic space occupying mass or impaired
gas exchange from pneumonia. Multimodal analgesia is recommended to ensure patient comfort. Two or more of the following
techniques can be combined in the same patient: IM or IV opioid, IV lidocaine, opioid epidural nerve block, intercostal nerve
blocks, and interpleural bupivacaine block. Anesthetic considerations of the specific disease or surgical problem may impose
further limits on management.
Intercostal nerve blocks are achieved by injection of a bleb of bupivacaine caudal to the heads of the two ribs cranial to
the surgical incision site and two ribs caudal to the site. The nerve runs adjacent to blood vessels and therefore aspiration
should always precede injection of local anesthetic. The total injected volume should not exceed 2.5 mg/kg in dogs and 1.5-2.0
mg/kg in cats. This volume can be split between the intercostal blocks and interpleural instillation.
Interpleural nerve block is performed at the end of surgery either immediately before closure of the incision or by administration
through the chest tube. Bupivacaine, 1.5 mg/kg, is instilled into the pleural cavity. Analgesia of the thoracic wall is
most complete when the animal is in dorsal recumbency because the bupivacaine flows by gravity towards the spine and blocks
the intercostal nerves near their origins. Instillation of bupivacaine down a chest tube with the surgical side uppermost
results in local pleural analgesia and reduces discomfort from friction of the chest tube during breathing.
Tips for anesthesia for thoracotomy
1. Negative pressure in the pleural cavity is abolished when the thorax is surgically incised. Consequently the rib cage
enlarges such that the lungs may not reach the surgical excision during inspiration even when inflation is adequate and ventilation
is satisfactory.
2. Pressure required for adequate lung inflation will be lower when the thorax is open than before thoracotomy.
3. Lung lobes that have been packed off during the surgical procedure may be gently re-expanded when replaced into the normal
position. The duration of lung collapse is short and reinflation does not induce the same problem as encountered with diaphragmatic
hernia repair.
4. Trapped air in the thorax during closure of the surgical incision can cause a tension pneumothorax, hypoxia and hypotension.
Leaving the chest tube uncapped during closure provides insufficient room for air flow during lung inflation. The chest tube
should be plugged and air aspirated before closure of subcutaneous tissue and skin.
5. Circumferential bandages applied to anesthetized patients frequently are too tight and cause hypoxemia during recovery.
6. Lobectomy may result in bleeding into the bronchus and blood clots can occlude the endotracheal tube.
Patent Ductus Arteriosus (PDA)
Specific points about PDA physiology that influence anesthetic management are
1. Mean arterial pressure is low because the diastolic pressure is low.
Figure 1
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2. Ligation induces pressure changes that acutely increase diastolic pressure (Fig. 1).
3. Mature dogs with PDA are likely to have left atrial and left ventricular enlargement, pulmonary hypertension and edema.
Anesthesia in animals with a PDA can be expected to be accompanied by hypotension. Consequently, the best combination of agents
chosen is one causing least depression of contractility and least impact on peripheral vascular tone and the drugs should
be administered incrementally and in small doses. Fluid restriction is practiced to avoid pulmonary edema after ligation.
Intraoperative fluid administration is limited to 6 ml/kg/h and hypotension is not treated by fluid boluses. Dobutamine or
dopamine, 5-7 micrograms/kg/h, is infused to maintain mean arterial pressure above 60 mm Hg. Patients ≤ 3 months of age should
receive 5% dextrose during anesthesia at 3 ml/kg/h with balanced electrolyte solution at 3 ml/kg/h.
Big dog-Little dog Syndrome
Skin wounds or bruising over the thorax may be external signs of thoracic wall penetration. The potential for pneumothorax
should be incorporated into the anesthetic management plan until the wounds have been thoroughly investigated.
Dogs that have been picked up by their necks and shaken may have spinal cord damage and swelling. This not infrequently is
accompanied by decreased respiratory muscle function, hypoventilation, and hypoxia when breathing air. The addition of anesthetic
agents may result in life-threatening respiratory depression.
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