Mask or chamber inductions are very risky and poor choices for the respiratory-compromised patient.
To decrease doses of inhalant anesthetic necessary for maintenance and the hypotension associated with higher inhalant doses, a concurrent opioid CRI is recommended during surgery. Fentanyl, remifentanyl, or hydromorphone may be used. Alternatively, the MLK (morphine, lidocaine, ketamine developed by Dr. William Muir at Ohio State University) infusion may be used in dogs, although more control of opioid dosing may be desired in painful thoracotomy surgeries (Table 22.1). If that is the case, a separate opioid CRI in a syringe pump with small bolus capabilities may be a better choice.
The “LK” portion of the MLK drip may be given without the morphine. A ketamine CRI is also a good option, especially for cats, at CRI of 0.1–0.6 mg/kg/hr. Ketamine CRIs should be used as part of a multimodal analgesic approach (Table 22.2).
During intrathoracic surgery the negative pressure that normally exists in the thoracic cavity and is necessary for lung expansion is disrupted by surgery. Therefore, these patients need to be provided with positive pressure ventilation. This can be done manually, which can be very labor intensive for the anesthetist, or by the use of a mechanical ventilator, which is recommended. Usually normal tidal volumes of 10–15 mL/kg are acceptable unless there is significant pathology in the lung parenchema that renders it friable and easily damaged. Peak inspiratory pressure (PIP) is normally limited to 15–20 cm H2O (Muir et al. 2000). In some cases where there is significant pressure on the diaphragm from enlarged abdominal organs or in the case of a diaphragmatic hernia, higher pressures may be needed to deliver adequate tidal volumes. Respiratory rates of 8–12 breaths per minute are usually adequate. It is important to maintain adequate tidal volumes to help prevent atelectasis. The use of capnography to monitor the adequacy of ventilation is ideal. ETCO2 levels should be maintained at 35–45 mm Hg.
The brachycephalic breeds typically have anatomical abnormalities that cause some degree of upper airway obstruction. These include hypoplastic tracheas, stenotic nares, elongated soft palates, laryngeal saccule eversion, laryngeal collapse, and very large, thick tongues (Fig. 22.1).
These abnormalities are often the reason these pets are presented to the veterinarian. They can cause snoring, respiratory stridor, exercise intolerance, cyanosis, and collapse (Cuvelliez and Rondenay 2002). Obesity often exacerbates these abnormalities. Surgical intervention can improve some of these conditions (soft palate reduction, everted saccules), but even after surgery these patients can be difficult and challenging anesthetic candidates (Fig. 22.2). The brachycephalic breeds have, for the most part, learned to live with their functional limitations and have found ways to compensate for their respiratory insufficiencies. They learn to maintain their own airways. Sedating these patients removes these protective mechanisms and mannerisms and so the anesthetist must take over and become the keeper of the airway. These patients should never be left alone once they are sedated. They may need assistance in positioning their heads in a way that maintains a patent airway. Bulldogs in particular have extremely small tracheas in proportion to their body size. For instance, a 35 kg bulldog may be able to take only a 5 or 6 mm endotracheal tube. A nonbrachycephalic breed dog of that body weight would require twice that size. When preparing to anesthetize a brachycephalic patient, it is wise to have many different-sized endotracheal tubes available. In general, only mild sedatives are needed and should be chosen for premedication of these patients. They tend to be somewhat sensitive to the sedative effects of most drugs. Often these patients are exuberant and excitable when they are feeling well, so choosing larger doses seems necessary. Overall, it is better to dose too low and add more if needed than to overdose. Low doses of acepromazine (0.01–0.02 mg/kg) work very well in taking the edge off of healthy brachycephalics presented for routine surgery. Ace can also help ease anxiety (and therefore upper airway obstruction) without respiratory depression in postop airway surgery patients. Some of these patients may need ongoing oxygen therapy postoperatively. The brachycephalic breeds are famous for being unwilling to be extubated at the end of a procedure. Very often these patients are quite content with their tubes and may be very much awake, yet still intubated.
In general, it’s best not to rush extubation. Be prepared to reintubate in recovery if an upper airway obstruction occurs after extubation. These patients should be monitored long after extubation because reobstruction can occur. The oral cavity should be cleared of blood and debris after airway surgery and prior to extubation to eliminate the possibility of aspiration. If opioids are indicated postoperatively, doses should be titrated to effect and lower dose ranges used. For airway surgeries, the partial agonist, buprenorphine often provides adequate analgesia with minimal respiratory depression.
Intrathoracic Surgery (Thoracotomy)
Depending on the lesion or reason for the thoracotomy, these cases can be especially challenging. Some of the potential reasons for a thoracotomy include diaphragmatic hernia, cardiac surgery (heart-based tumor, pericardial window), space-occupying lesions in the thorax, impalement, lung lobe torsion, spontaneous pneumothorax, chylothorax, or any condition that restricts chest wall excursions. These lesions are referred to as “extrapulmonary lesions” because they affect lung expansion. Other lesions involving the pulmonary system, such as pneumonia, pulmonary edema, atelectasis, and interstitial disease affect perfusion and diffusion at the site of oxygen and CO2 transfer in the lungs. These are called “intrapulmonary lesions,” and most are not the primary reason for surgery (Paddleford and Greene 2007). These intrapulmonary conditions should be stabilized prior to surgery when possible. In any case, thoracotomy patients are considered critical patients, and the presurgical prep should include placement of at least two large-bore IV catheters for the administration of maintenance fluids, rapid fluid administration in cases with moderate to severe blood loss, constant rate infusions, drug administration, and possible transfusions if indicated. Depending on the nature of the surgery (cardiac surgery, mass removal, impalement), it may be wise to make sure the patient is blood-typed prior to surgery and that blood/plasma is available. Ideally, these patients will have an arterial catheter placed during prep to allow for blood gas and direct blood pressure monitoring. Pulse oximetry monitoring is instrumental in these patients for quick detection of desaturation. Monitoring of the electrocardiogram for potential arrhythmias is also highly recommended. Manipulation of the heart during thoracotomy surgery usually results in at least a few ventricular premature contractions (VPCs). Aggressive manipulation of the heart during surgery may cause runs of VPCs or worse. The negative effects of the manipulation and/or arrhythmia on cardiovascular stability (including effects on blood pressure) should be communicated to the surgeon. Usually, simply stopping the manipulation treats the arrhythmia and allows hypotension to resolve. Occasional “breaks” are needed between manipulations to allow the patient to stabilize if possible. Sometimes the surgeon requires that one set of the lungs be “packed off” to prevent inflation and improve visibility and access to certain parts of the heart or thorax. This is usually accomplished by gently stuffing damp lap sponges over the lung lobes so that they can’t expand. Careful monitoring of ventilator pressures, ETCO2, and pulse oximetry during this time is mandatory. Blood gas analysis is ideal. Normally, patients do fine with one lung ventilation. The advantage of having the chest open is that either the anesthetist or the surgeon can see how well the other lung is inflating and whether adequate pressures are being used. Atelectasis can be visible and an indicator that higher tidal volumes are needed. When it becomes time to release the packed-off lung, be aware that increasing peak inspiratory pressures in an attempt to “blow open” the deflated lung usually backfires. In most instances, applying excessive pressure (by increasing tidal volume) results in volutrauma (damage) to the functioning lung lobes long before atelectasis in the packed-off lung is reversed. The healthy packed-off lung will reinflate with time. It is best not to force it.
Assume that the patient will desaturate at some point during the procedure, so a plan for dealing with it should exist. Desaturation can occur at any time with a respiratory-compromised patient. It often seems to occur just after induction during the surgical prep. If it is a case of pneumo/chylo/hemothorax, a thoracocentesis or aspiration of existing chest tubes usually helps saturation return to acceptable levels. If possible, the affected side of the chest should be put down. This is not always practical, though, especially in the case of a lateral thoracotomy. Ideally, respiratory-compromised patients should be placed in sternal recumbency, but this is also not practical for surgery. Very often, especially in severe cases of diaphragmatic hernia (DH), turning a patient onto its back for surgical prep can have catastrophic results. In the case of a DH, some of the abdominal organs can push into the thoracic cavity, leaving no room at all for lung expansion. Patients can actually arrest when they are flipped into dorsal recumbency. If at all possible, prep these patients “side to side” and build an incline on the surgery table so that the head remains higher than the tail; in cases of severe DH it is advantageous to attempt to gently “shake” the abdominal organs out of the chest cavity. This is done, either before or immediately after induction and intubation, by setting the patient on its haunches and literally shaking the upper body in an effort to have gravity help remove the abdominal organs from the chest cavity. This obviously needs to be done gently, depending more on gravity than motion, because organs can be injured as they move through the vent in the diaphragm.
Flushing the thoracic cavity with saline prior to closing or while searching for a spontaneous pneumothorax will cause the patient to desaturate. Fluid in the chest cavity, whether it is supplied by the surgeon or the patient (as in pneumonia, hemothorax), disrupts perfusion and oxygen delivery. Usually, suctioning the cavity quickly restores saturation. Communication with the surgeon about the status of the patient throughout surgery is imperative to a positive outcome.
The use of positive end expiratory pressure (PEEP) can be extremely helpful in treating desaturation during surgery. PEEP can be applied manually or with PEEP valves, which are commercially available in various sizes. To apply PEEP manually the patient must be removed from the ventilator and hand-bagged. At the end of expiration, the bag remains squeezed so that 5–10 cm H20 pressure is shown on the pressure manometer and remains in the airways as the next breath is initiated. This can help reopen small airways and prevent atelectasis. Flow rates may need to be adjusted and some gas released through the APL valve periodically so that pressures do not continue to rise in the system. Some amount of PEEP can be applied by using very high oxygen flow rates as well. High oxygen flow rates will exacerbate hypothermia and dry out airways. PEEP, although useful, adds pressure to the thoracic cavity, which can impede venous return to the heart, especially in hypovolemic patients. Judicious cardiovascular monitoring is warranted during the use of PEEP.
Recovery and Postop Analgesia
Often, recovery is the most difficult period to manage in thoracic surgery patients. During anesthesia the patient is intubated, ventilated, and provided with 100% oxygen. All body systems are being supported under anesthesia. Attempting to wean the patient off of the ventilator and eventually off of 100% oxygen can be tricky. As the surgeon is closing, ventilator and inhalant settings can be reduced in an effort to raise CO2 levels and stimulate the respiratory center. At this time, it may be wise to discontinue any opioid CRIs as well. A plan for postoperative analgesia should be in place. Thoracotomies are considered highly painful surgeries and analgesics are essential to promote adequate ventilation and chest excursions in the postoperative period. Patients with inadequate pain relief are at risk of hypoventilation due to small tidal volumes (breathing hurts). Respiratory depression from opioid administration is always a concern, but they should not be withheld and the drugs can be titrated to effect and antagonized if necessary. Use of opioids as a constant rate infusion postoperatively often eliminates the peaks and valleys of intermittent dosing and the side effects that go along with spiking and diminishing blood levels. The partial agonist opioid buprenorphine may cause less respiratory depression but is likely not a potent enough analgesic for immediate postop relief of thoracotomy patients. The patient may be “stepped down” to buprenorphine several days postop. For these patients, a multimodal approach to pain relief is often the best option. Patients may benefit from an epidural of preservative-free morphine (Duramorph at 0.1 mg/kg) at the lumbosacral space prior to surgery. This provides good pain relief with fewer systemic side effects. Local analgesia techniques are a great adjunct to systemic analgesics in this patient group. Local and regional analgesia allow for lower doses of the systemic analgesics to be used and therefore also decrease the side effects. Intercostal blocks can be performed prior to surgery. Patients with a chest tube should have local anesthetic infused into the tube prior to recovery and at regular intervals thereafter. Human beings report chest tubes to be extremely painful. A very effective local anesthesia technique involves the use of a wound soaker or diffusion catheter placed into the wound/incision. These catheters have a closed end with several perforations around the catheter that allow seepage of local anesthetic to diffuse into the wound bed. Use of these catheters can significantly reduce the dose of systemic analgesics needed to control pain in postop thoracotomy patients (Fig. 22.3, Table 22.3).
After weaning from the ventilator and discontinuing anesthesia, the patient will need to be closely monitored to see whether it can be weaned from oxygen. The pulse oximeter should be watched, along with mucous membrane color, as an oxygen test is done. If the patient cannot tolerate being off of oxygen, desaturation will occur within about a minute. Sometimes these patients just need time to recover a bit more; sometimes they need ongoing oxygen support in the form of nasal O2 lines or an oxygen cage. These cases often require a great deal of the anesthetist’s time and patience in recovery. Extubating these patients should be done as late as possible, unless the patient is a cat and prone to laryngospasm. Although usually rare, be prepared to reanesthetize and reintubate if needed. Having a clean ET tube, a laryngoscope, and a little induction agent nearby is good planning. Patients should be recovered in sternal position to maximize lung inflation of both lung fields. The head may be supported with a pillow or rolled towels. Once the patient is extubated, pulse oximetry monitoring should continue. Once patients are alert and moving this can be difficult, but if a toe or ear or some other remote area doesn’t work, monitor by mucous membrane color, feel for air movement at the nostrils, listen for obstructive sounds, and monitor chest excursions. The experienced anesthetist will know right away whether the patient is going to tolerate being extubated and off of oxygen.