Oxygen utilisation and prevention of secondary brain injury quiz

The overall oxygen extraction ratio of the body in adults is normally < 0.3. It is increased when tissue oxygen supply is low and is measured by SvO2 (mixed venous O2 saturation), which is normally > 70% in adults. (link)

The heart has the highest oxygen extraction ratio of the major tissues at 60%. The O2 extraction ratios of the brain is 33%, the liver is 15% and the kidney is 7.5%. (link)

The SvO2 is usually higher in the IVC than the SVC due to the low O2 extraction ratio of the kidneys. The SvO2 can also normally be lower in children. (link)

Blood flow to organs (mL/100g tissue/min) at rest is: kidney 400; heart 80; brain 50; liver 20; skeletal muscle 5. Tissues with the highest blood flows are the most prone to the adverse effects of hypovolaemia. (link)

The target MAP is > 80mmHg and should be achieved with pressors if euvolaemia present. (link)

Hyperventilation has a more rapid onset of effect on ICP than hyperosmolar therapies, but usually a shorter duration of effect. (link)

Hyperosmolar therapies do not have a substantial effect on brain oedema surrounding a cerebral mass lesion. (link)

The effects of hyperosmolar therapies are generally considered to be more consistent than the effects of hyperventilation in brain injury. (link)

Whilst various recommendations exist, and previously lower thresholds were thought to be beneficial, 10mmol/L is the appropriate threshold level. Hypoglycaemia in brain injury is associated with an increased mortality, so must be avoided at all costs. (link)

Head elevation of 20-30 degrees is considered the best position as it improves venous return to a greater extent than it reduces cerebral blood flow, hence improving cerebral perfusion pressure. Higher degrees of head elevation are associated with decreases in cerebral blood flow greater than the reduction in venous pressure. (link)

Lignocaine prior to (or post) induction is of unproven benefit. The thiopentone dose required to reduce ICP usually also reduce BP and cerebral perfusion pressure. Suxamethonium may transiently increase ICP but is probably of little significance compared to the effect of laryngeal manipulation / irritation on ICP. Maintenance of sedation to avoid gagging on the ETT is most likely to provide the greatest benefit of the options provided. (link)

Whilst it could be argued that a PCO2 of 40mmHg should be the target for ventilation, most people err on the side of low normal PCO2. However a PCO2 < 35 is associated with a decrease in cerebral blood flow, hence should be avoided. A PO2 of 100mmHg is sufficient as hyperoxia may worsen brain injury. (link)

In patients with brain injury, avoid hypo-osmolar therapies whenever possible (such as 5% dextrose, N/2 Saline, Hartmann’s solution / Ringer’s lactate). 3% saline is used as hyperosmolar therapy to transiently reduce ICP when a neurosurgical drainage procedure is planned to reduce ICP, but is not a maintenance fluid. (link)

Rocuronium and ketamine are probably the best combination as they are the least likely to cause hypotension, or increase ICP, although the increase in ICP with suxamethonium is probably of little significance.

Therapeutic hypothermia in patients with bacterial meningitis is associated with an increased mortality, so should be avoided. The strongest evidence to support its use is for out of hospital VF/VT arrest, with weaker evidence for PEA. There is no evidence that it is of benefit in head injury. (link)