Ch 35 - Cranial Surgery Flashcards
What is the normal intracranial pressure of dogs and cats?
5-12mmHg
Affected by arterial inflow, venous outflow and resistance of cerebral vasculature
How do you calculate cerebral perfusion pressure (CPP)?
CPP = MAP - ICP
ICP closely approximately venous outflow pressure
What factors can alter cerebral blood volume without altering blood flow?
- Low head position
- Venous outflow obstruction
- Increases intrathoracic pressure
What factors alter cerebral blood volume by altering the blood flow?
- CO2
- O2
- Lactic acid
- Drugs
What three primary homeostatic mechanisms maintain intracranial pressure?
Volume buffering
- Increase in one parameter (blood volume) causing a decrease in another (CSF volume)
Autoregulation of blood flow
- Reflex vascular changes regulated by the pial arterioles at pressures between 50 - 150mmHg
Chemical autoregulation
- Cerebral vasodilation with increases PaCO2
- Cerebral vasoconstriction with decreased PaCO2
- Decreased PaO2 will eventually cause cerebral vasodilation (less then 60mmHg)
What is the Cushing’s reflex?
- Uncontrolled ICP leads to reduced cerebral blood flow, ischaemia and hypercarbia
- Stimulates catecholamine release leading to systemic vasoconstriction and increased cardiac output (thereby increased CPP)
- Baroreceptors sense hypertensive stare and cause a vagally mediated bradycardia
Indicates imminent brain herniation
What is the ideal PaCO2 which is aimed for during intracranial surgery?
30 - 35mmHg
Below 30, neuronal ischaemia can occur and exacerbate intracranial hypertension
What monitoring systems are available for monitoring ICP?
- Fiber-optic based systems
- miniaturised strain gauge devices
- Transcranial Doppler ultrasound (indirect)
What are the anaesthetic goals for managing intracranial hypertension
- Prevent hypercapnia and hypoxia
- Prevent systemic hypotension (maintain at or above 80mmHg to maintain a CPP of 50 - 90mmHg)
- Reduce cerebral oedema (mannitol/hypertonic saline)
- Corticosteroids (neoplasm associated oedema and primary inflammatory disease)
- Control cerebral venous blood volume (head elevation etc)
- Control cerebral oxygen demand (Hyperthermia, seizures, pain and ketamine all increased oxygen demand)
What is unique about burring for cranial surgery?
- Smallest burr size allows a secure fit if planning to replace bone
- Inner cortical bone has highly irregular undulations with marked differences in bone thickness
- Dorsal calvaria is considerably thicker
- Cutting at angle of 30 degree allows for almost complete apposition
- Cancellous bone is called diploe
What electrosurgical device is preferred when working on dural and parenchymal tissues?
Irrigation-coupled bipolar device
Standard bipolar on minimum effective setting with constant slow, steady saline drip sufficient
What can be used to make the initial dura cut?
If wanting to close the durs, what needs to be done intraop?
- # 11 Bard Parker blade, the tip of a needle or microscissors
- It must be kept stretched during the procedure to prevent shrinkage - attach to surrounding tissues using suture, skin staples or mosquito forceps
What can be used to close a dural defect?
- Fascia of temporalis muscle (surface of fascia facing towards brain)
- Porcine SISM
Sutured with 4-0 PDS or tissue glue
What are some options for cranioplasty?
- Replacement of excised bone
- Acrylic cranioplasty (PMMA)
- Metallic mesh with low profle self-tapping screws
Avoid placing foreign material where it may become contaminated by air from paranasal sinuses
What fluorescent dyes can be used intra-op to aid in lesion localisation?
- 5-aminolevulinic acid
- Indocyanine green