Postcraniotomy Management

Chapter 145 Postcraniotomy Management






POSTOPERATIVE MANAGEMENT


Immediately following surgery, while the patient is still under anesthesia, is the ideal time to perform an anatomic imaging study such as magnetic resonance imaging. The degree of lesion resection is assessed, as well as any intracranial complications associated with the surgical procedure such as cerebral edema, parenchymal damage, or hemorrhage (hematoma). Immediate surgical decompression is indicated if there is an expanding hematoma or if brain compression is significant.


After recovery from anesthesia, animals usually are monitored in a critical care or intensive care area for signs of neurologic deterioration for at least 48 hours. Animals should be kept in a comfortable, well-padded, and quiet environment with minimal light stimulation. Cerebral edema may evolve for up to 48 hours after injury and persist for a week or more.2 Physiologic monitoring during the hours to days following intracranial surgery should be performed on a frequent, if not continual, basis. This type of monitoring commonly includes assessment of heart rate and rhythm, respiratory rate and character, blood pressure, blood gases, oxygenation, urine production and, in some instances, ICP through objective means. The goal of such monitoring is to maintain adequate cerebral blood flow without compromising other organs.


Cerebral blood flow is maintained primarily through support of systemic blood pressure using fluid therapy and vasopressive drugs if needed, at the same time preventing increases in ICP. Blood gas measurements or capnometer use aids in controlling respiration to prevent increases in PaCO2 and subsequent cerebral vasodilation. Capnometer measurements, however, are usually less accurate than direct blood gas measurements.


ICP is monitored objectively in some situations; however, this type of measurement is not performed routinely in animals. ICP monitoring has been described in dogs and cats.3-11 An advantage with this measure is that trends toward increasing ICP can be recognized early and treated before life-threatening increases occur.12 An objective measure of ICP and blood pressure also allows for calculation of cerebral perfusion pressure (CPP) as a reflection of cerebral blood flow.


Disadvantages to invasive ICP monitoring include added surgery time for implantation of the monitoring system, expense, and the potential for iatrogenic brain damage from the monitoring system. Until some of these disadvantages are overcome, ICP monitoring will probably not become routine for animals undergoing intracranial surgery.4,10 Newer, noninvasive techniques for measurement of the cerebral blood flow such as transcranial Doppler ultrasonography may provide an indirect measure of ICP.13 Similar procedures and information have been investigated in dogs and cats.14,15


Neurologic parameters monitored include pupil size and responsiveness to light, level of consciousness, behavior, and the ability to move and walk. Cranial nerve abnormalities may provide important clues to underlying intracranial injury or deterioration.


Animals that have recovered from anesthesia are placed in a neutral or head-elevated position. Head elevation to 30 degrees above cardiac level decreases ICP primarily by facilitating venous drainage.16-18 In humans it has been shown that CPP and cerebral blood flow are maintained in the 30-degree head elevation position and ICP is concurrently decreased.16 There continues, however, to be debate about the degree of benefit of head elevation in animals following intracranial surgery.


Oral food and water are withheld until the animal is fully alert. With the combination of intracranial depressant effects of surgical manipulation and anesthetic or anticonvulsant drug therapies, appropriate swallowing, which prevents aspiration of oral contents, may take a number of days to return, putting the patient at risk for aspiration of oral contents (see Nonneurologic Complications later in this chapter). Stools are monitored regularly for melena that might indicate gastrointestinal (GI) ulceration. If noted, an antiulcer medication (e.g., ranitidine) is administered concurrently because of the apparent increase in GI ulcers in patients with neurologic impairment.




Ventilation


Cerebral vessels are directly responsive to PaCO2 concentrations, with cerebral blood flow coupled to cerebral metabolic rate. The cerebral vessels have the ability to change diameter in response to PaCO2 (chemical autoregulation) and blood pressure (pressure autoregulation) in order to maintain a relatively constant cerebral blood flow. Cerebral vessels change diameter through perivascular changes in pH, occurring as a direct result of PaCO2 concentrations.


As PaCO2 concentrations increase, cerebral vessels dilate to increase blood flow to the brain. Poor ventilation and increasing PaCO2, such as from obstruction or crimping of the endotracheal tube, can lead to disastrous increases in brain volume, terminal brain swelling, and subsequent herniation.21 If autoregulation is intact, hyperventilation to decrease PaCO2 will cause cerebral vasoconstriction, decreased cerebral blood volume, and subsequently decreased ICP.


Unfortunately, cerebrovascular autoregulatory capability is negatively affected by a variety of intracranial pathologies including local acidosis, which is common in many hypoxic and ischemic brain areas.21 Animals often are hyperventilated during intracranial surgical procedures to maintain PaCO2 in the range between 28 and 32 mm Hg, to prevent cerebral hypoxia from poor ventilation. During the postoperative period, ventilatory support may be indicated. Endotracheal intubation and ventilator support can be performed under the influence of barbiturate anesthesia or neuromuscular blockade. Appropriate ventilator management is imperative.


When ventilatory support is not immediately necessary or is not feasible, it is still important to recognize that decreased ventilatory effort or capacity may result in increases in ICP. Preventing atelectasis by frequent (hourly) movement of the animal from a lateral recumbent position may also be necessary.

Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Postcraniotomy Management
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