Intracranial Pressure As a Therapeutic Target
The BTF has declared that "The optimal ICP monitoring device is one that is accurate, reliable, cost effective, and causes minimal patient morbidity." None of the currently available devices fir that description. The utility of ICP monitoring is therefore weighed against the risk of destructively introducing a potentially infected object into the very delicate organ which you are trying to protect. The advantages and disadvantages of ICP as a therapeutic target have been interrogated in Question 27 from the first paper of 2014, as well as Question 4 from the first paper of 2005.
What kind of ICP monitor to use
The advantages and disadvantages of various ICP monitoring techniques are debated elsewhere.
In short, the gold standard is still the EVD. Its cheap and effective.
The EVD complies with all of the BTF's criteria for the "ideal" ICP monitor, with the exception perhaps of the morbidity part. It is still a slightly thicker object to insert than the internal strain transducer, so in that regard it tends to damage more parenchyma.
It can also be recalibrated, so that when its accuracy drifts, one can reset the zero reference value. This cannot be done with the internal strain gauges- they are zeroed before they are inserted, and they can never be rezeroed. This, with the new gauges, is not such a major issue - a recent study has discovered that they drift by no more than 5mmHg from atmospheric zero.
Risks of ICP monitoring
Shoving something into a person's brain parenchyma cannot be without risk.
What are the risks?
- Risks of anaesthesia
- Risks of craniotomy
- Risks of haemorrhage
- Risk of infection
- Malposition and poor monitoring quality
- Incorrect readings may stimulate incorrect management
- EVDs may clog with debris; parenchymal monitors may "drift" from their zero calibration value
- ICP-guided and CPP-guided therapy may not improve outcomes.
Benefits of ICP-guided therapy
In their model answer to Question 16 of the first paper of 2009, the college refeer to a study by O.L Cremer, whose 2005 retrospective cohort study associated ICP-guided therapy with increased intensity of therapy and more prolonged mechanical ventilation, without a benefit to either survival or functional outcome. In the same answer the examiners also mention Shahid Shafi's 2008 analysis of The National Trauma Data Bank (1994–2001) which found that ICP monitoring was associated with a 45% reduction in survival. A 2012 randomised controlled trial of ICP monitoring versus imaging and clinical examination did not demonstrate superiority of one over the other in terms of mortality or functional outcome. In 2014, a meta-analysis scraped together 11,038 patients worth of trial and cohort data, also concluding "no benefit" in the face of severe heterogeneity.
However, an earlier (2000) analysis by Lane et al concluded that ICP monitoring was associated with increased survival. Similarly, a 2013 prospective observational study of ICP monitoring in TBI revealed that the overall in-hospital mortality was significantly higher in patients who did not undergo ICP monitoring (53.9% vs 32.7%), possibly due to missed episodes of brain herniation. In 2015 another supportive retrospective study was published, again finding a substantial decrease in mortality (30.7% vs. 45.7%).
At this point, the BTF Guidelines recommend ICP monitoring be carried out in those patients which meet their indications. These guidelines were put together in 2007, and do not reflect more recent data; however it is unlikely that ICP monitoring will be abandoned entirely in future versions. "Multimodal" monitoring is the new catchphrase, and recent position statements from various eminent Societies suggest that there is a move to integrate several different bedside monitoring techniques for simultaneous observation of multiple important parameters, with fewer "hard" universal physiological targets.
Furthermore, patient selection probably plays a major role. Recent observational data suggest that certain groups might benefit from ICP monitoring more than others.
Specifically, groups which benefit in terms of mortality are as follows:
- GCS of 3-5 at admission.
- GCS of 9-12 which drops to 3-8 within 24 hours.
- Those who had a probability of death greater than 60% at 6 months.