Page 360 - Clinical Small Animal Internal Medicine

328 Section 4 Respiratory Disease

Arterial Blood Gas Analysis circumstances may necessitate maintenance beyond
VetBooks.ir Arterial blood gas analysis is paramount in patients with this timeframe due to continual effusion, residual
pneumothorax, and for therapeutic lavage or delivery
pulmonary dysfunction, as it will provide rapid identifi-
cation of hypoxemia, hypoventilation, and respiratory of chemotherapeutics. Pleural drainage may be passive
or active, but active drainage is more commonly imple-
failure. When arterial access is unobtainable, central mented in veterinary patients. The normal transpul-
venous (S cv O 2 ) or mixed venous (S v O 2 ) oxygen satura- monary pressure is 4–8 mmHg, which is equivocal to
tion as well as venous CO 2 (PvCO 2 ) can be extrapolated 5–10 mL of suction in a syringe. Intrapleural infusion,
to evaluate respiratory function. Normally, S cv O 2 is via the thoracostomy tube, of lidocaine and bupiv-
10–15% higher than S v O 2 but in states of circulatory acaine (1.0 mg/kg each) provides local analgesia fol-
shock or failure, these two measurements closely parallel lowing intermittent suction and may be performed
one another. The normal arterial‐venous gradient for every 4–6 hours. The rationale behind using both lido-
PCO 2 is 4–6 mmHg, but it may approach 10 mmHg in caine and bupivacaine is that the lidocaine provides
some patients. This allows for serial ventilatory monitor- immediate relief and allows for a more comfortable
ing via venous blood sampling.
infusion of the longer‐acting bupivacaine. This method
of analgesia delivery has been investigated and deemed
safe in patients following pericardectomy.
Postoperative Management Intermittent active suction is applied often postopera-
tively, but continuous active suction may be necessary
Intense postoperative management following thoracic for larger volumes of air or effusion which compromise
surgery comprises adequate analgesia, appropriate ventilation. The pleural space in small animals can
thoracostomy tube care, intravenous fluid therapy, and accommodate approximately 100–140 mL/kg of fluid or
intensive nursing care. Supplemental oxygen therapy is air. Beyond this volume, continuous pneumothorax or
often implemented in the immediate postoperative hydrothorax (usually hemothorax) should be suspected.
period and occasionally continued mechanical ventila- Obstruction of the thoracostomy tube may rarely occur,
tion is required. depending on tube diameter and viscosity of the thoracic
effusion. The Mac technique consists of turning the
Analgesia thoracostomy tube 180° in each direction and observing
whether the tube returns to the prior position. This
Optimal analgesic protocols are critical for the post- method can be used to evaluate kinking of the tube when
thoracotomy patient to prevent secondary hypoxemia, persistent negative pressure is observed despite evidence
hypoventilation, and cardiac arrhythmias. Multimodal of significant pleural effusion or pneumothorax. If the
analgesia is preferred to avoid noxious effects of single‐ tube consistently spins back into the initial position, tube
agent protocols. Combination therapy of systemic anal- kinking should be suspected.
gesics allows for synergistic analgesia and lower dosing The thoracostomy tube itself will induce approxi-
of each chosen drug. Local analgesia including intercos- mately 2.0 mL/kg/day of pleural effusion. Once the vol-
tal nerve blocks, incisional diffusion catheters, and ume of pleural effusion drops below this value, removal
intrapleural infusion of local anesthetics via the thora- of the thoracostomy tube may be considered. Ideally,
costomy tube provides effective adjunctive analgesia. consistent negative pressure should be observed for 24
Intercostal nerve blocks can provide very effective anal- hours following pneumothorax prior to thoracostomy
gesia when utilized for lateral thoracotomy. It is recom- tube removal.
mended to perform the procedure at the intercostal
space of the incision, one space cranial and one space
caudal to the incision. As discussed later, much of the Fluid Therapy
postoperative pain seems to be associated with large‐ Fluid therapy is a mainstay of postthoracotomy therapy.
bore thoracostomy tubes. Most postoperative patients will require, at minimum,
maintenance fluid therapy, as they may be unwilling to
drink following surgery. Of further importance, continual
Thoracostomy Tube Management and Pleural fluid losses via the thoracostomy tube should be
Drainage
accounted for, and at least a percentage of this loss should
Thoracostomy tubes generally remain in place be replaced with IV fluids to avoid hypovolemia due to
for 12–24 hours post thoracotomy in patients third spacing. Postthoracotomy patients may experience
without ongoing pleural space disease, but certain significant protein loss with pleural effusion. The ideal