Chapter 36 - Anaesthesia for Intracranial Surgery

Question 1

How is cerebral perfusion pressure calculated?

Answer 1

CPP = MABP - ICP (or CVP whichever is higher)

Question 2

What is the normal range for intracranial pressure?

Answer 2

5-12mmHg

Question 3

How is cerebral blood flow calculated?

Answer 3

CBF = cerebral perfusion pressure/cerebral vascular resistance

Normal = 75.9 +/-10.4 ml/m

Question 4

At what arterial blood pressure does cerebral auto regulation of blood flow begin to fail?

Answer 4

<60mmHg
>180mmHg

Question 5

What variables can lead to the impairment of cerebral blood flow autoregulation?

Answer 5

Autoregulation can become impaired or abolished by a variety of insults, including trauma, hypoxemia, hypercapnia, and large-dose volatile anesthetics

Question 6

What effect does CO2 have on cerebral blood flow?

Answer 6

Carbon dioxide is a potent cerebral vasodilator, and a linear relationship exists between cerebral blood flow and arterial carbon dioxide concentrations (PaCO2) within the ranges of 20 and 60 mm Hg (2.7 and 8.0kPa)

Question 7

Hyperventilation to reduce ICP can further compromise damaged brain tissues. Physiologically whats occurring in this scenario?

Answer 7

Moderate hyperventilation (as short as 30 minutes) can significantly increase extracellular concentrations of mediators of secondary brain injury (i.e., glutamate, pyruvate, and lactate), potentially as a result of reduction in cerebral blood flow.

Question 8

What effect does PaO2 have on cerebral vascular tone?

Answer 8

Arterial oxygen concentration (PaO2) has a minimal effect on cerebral vascular tone until it decreases below 50 mm Hg (6.7 kPa), at which point cerebral blood flow increases significantly. Only severe hypoxemia will result in vasodilation and an increase in intracranial pressure.

Question 9

What methods can be used to reduced cerebral metabolic rate for oxygen?

Answer 9

Pharmacologic (e.g., benzodiazepines, barbiturates, propofol) and nonpharmacologic maneuvers (e.g., hypothermia) can be used to decrease CMRO2

Question 10

Briefly outline what is occurring during the Cushings reflex

Answer 10

Reduced delivery of oxygen to the brain stimulates release of catecholamines, which results in an increase in mean arterial pressure. This increase in mean arterial pressure stimulates pressure receptors in the aortic arch and carotid bodies, causing a reflexive decrease in the heart rate, giving rise to the Cushing response. The response therefore is characterized by systemic hypertension and bradycardia, which occur secondary to increased intracranial pressure.

Question 11

Hyperglycaemia is commonly encountered in stressed/injured animals. What are the possible negative effects of this glycemic derangement?

Answer 11

Hyperglycemia may lead to inflammation, infection, and multiorgan dysfunction.

Question 12

Outline the physiological effects of acepromazine

Answer 12

it is believed to depress portions of the reticular activating system, which assists in the control of alertness, body temperature, basal metabolic rate, emesis, vasomotor tone, and hormonal balance.

In addition, acepromazine has anticholinergic, antihistaminic, antispasmodic, and alpha-adrenergic blocking effects; the latter often results in vasodilatation and arterial hypotension

Question 13

Outline the pharmacology of remifentanil

Answer 13

Remifentanil is an ultrashort-acting opioid. It is unique among opioids in that it is hydrolyzed by nonspecific plasma and tissue esterases; its pharmacokinetics are independent of hepatic and renal functions. Remifentanil half-life in the dog is approximately 7 minutes.

Question 14

What is the proposed benefit of using remifentanil rather than fentanyl in cranial surgery?

Answer 14

Remifentanil may better suppress the stress response to surgery than fentanyl, possibly attenuating the development of subsequent hyperglycemia and inflammation.

Question 15

Outline the pharmacology of benzodiazepines

Answer 15

Benzodiazepines are nonselective central nervous system depressants that augment the action of gamma-aminobutyric acid (GABA) at GABAA receptors, causing increased conductance of chloride ions.

Question 16

What are the proposed benefits of midazolam CRI during cranial surgery?

Answer 16

Midazolam CRI in the dog: it resulted in a decrease in the CMRO2 and cerebral blood flow until a plateau was reached at approximately 75% of control values. In addition, there was a decrease in mean arterial pressure, but intracranial pressure was unaffected.

Question 17

What are the reported benefits of propofol anaesthesia during cranial surgery?

Answer 17

Propofol reduces cerebral blood flow and intracranial pressure, preserving cerebral autoregulation and vascular reactivity.

Question 18

Propofol is reported to be neuroprotective during cranial surgery. How is this potentially mediated?

Answer 18

The potential neuroprotective effect of propofol could be mediated by its antioxidant properties, which can play a role in apoptosis, ischemia-reperfusion injury, and inflammation-induced neuronal damage.

Question 19

Which inhalant agent has the greatest effect on cerebral blood flow?

Answer 19

In a study comparing isoflurane, sevoflurane, and desflurane in a porcine model of increased intracranial pressure at equipotent doses (1.0 minimum alveolar concentration) and normocapnia, cerebral blood flow and intracranial pressure were greatest with desflurane and least with sevoflurane.

Desflurane produced the greatest amount of cerebral vasodilation, followed by isoflurane and sevoflurane

Question 20

What is the recommended MAP for cranial surgery? How is this number derived?

Answer 20

Assuming a normal intracranial pressure of 5 to 12 mm Hg (0.7 to 1.6 kPa) and a target cerebral perfusion pressure ≥70 mm Hg (9.3 kPa), a mean arterial pressure of 80 mm Hg (10.7 kPa) would be recommended