Wed3November0919PM 5

A 57-year-old female is admitted to the Emergency department with a suspected subarachnoid haemorrhage.

The Glasgow coma score on admission was 7. The patient is now intubated, sedated and ventilated and awaiting a CT scan.

The patient's blood pressure is 140/70 mmHg.

Baseline arterial blood-gas analysis is as follows:

pH 7.2 (7.35 - 7.45)

PaO2 70 mmHg (80-100)

9.2 kPa (10.5-13.1)

PaCO2 78 mmHg (35-45)

10.2 kPa (4.6-6)

BE -3 mEq/L (-3 +/-3)

Standard bic 27 mmol/L (21-27)

SaO2 94% -

What is the single most likely cause of an increase in global cerebral blood flow (CBF) in this patient?

(Please select 1 option)

Hypoxia

Hyperthermia

Hypercapnia This is the correct answer

Acidosis

Failure of autoregulation Incorrect answer selected

Explanation

One of the most important factors that regulate cerebral vascular tone is PaCO2. Carbon dioxide induces cerebral vasodilatation and therefore increases CBF. There is a linear increase between a PaCO2 between 20 mmHg (2.7 kPa) and 80 mmHg (8.6 kPa).

There are likely to be areas where autoregulation may have failed locally but not globally. Similarly local versus systemic acidosis will have similar effects. The CBF does not significantly change until the PaO2 falls below 50 mmHg (6.5 kPa) below which the CBF progressively increases.

Hyperthermia causes an increase in the cerebral metabolic rate for oxygen (CMRO2) and therefore CBF. (CMRO2 and CBF are linked). Hyperthermia secondary to hypothalamic injury can be a late feature of cerebral injury and is therefore not the most likely cause of an increased CBF in this scenario.

Answer Statistics

1

4%

2

1%

3

88%

4

5%

5

5%

Times answered: 257