Page 199 - Ultrasonography in Prehospital and Emergency Medicine - Rein Ketelaars
P. 199

                 Increase in intracranial pressure by application of a rigid cervical collar 197
reported to be as small as 0.2 (range 0.1–0.5) mm for experienced sonographers.11 Measuring structures this small might introduce a standard variation because of pixel density or soft- ware limitation of the ultrasound machines. Although Sonosite machines do not have to be calibrated periodically, a small difference in firmware might have introduced a systematic measurement error (M-Turbo; Fujifilm SonoSite Inc.). In future research the sonographers should regularly switch sides to prevent this type of possible bias.
As we described before, the optical nerve sheath’s response to ICP depends on its elastic- ity. The sheath contains the fewest trabeculae 3 mm behind the retina. This explains the hyper elasticity at this part of the sheath.12 The cut-off point for ONSD for an increased ICP (> 20 mmHg) is still under debate.9,10,13,15,17,18 Goeres suggests a difference in ONSD be- tween sexes and advocate a different cut-off for men and women.19 Maude suggests possible differences between ethnicities.20 In our previous study we found a cut-off point of 5.0 mm representing increased ICP (> 20 mmHg) in sedated and intubated head-injured Dutch pa- tients (67% men). As sheath elasticity varies between individuals ONSD measurement is a qualitative than a quantitative assessment of ICP.9,18 Because of this we can state that the increased ONSD during collar application does represent an increase in ICP, but it is not possible to calculate the exact increase without knowing the elasticity coefficient of the sheath of that individual. The main question that remains, is whether or not this increase in ICP impairs CBF. If the ICP compensation mechanisms described by Kellie Monroe are exhausted, the slightest increase in venous volume in the head might result in an increase in ICP and a compromised CBF.
In daily clinical practice, let alone in a prehospital setting, CBF cannot be measured easily and reliably. CBF is directly related to cerebral perfusion pressure (CPP). CPP can be calcu- lated as the mean arterial pressure (MAP) minus the ICP.1 When autoregulation is disturbed after trauma, CPP should be maintained between 60 and 70 mmHg to prevent ischemia of the brain and cardiorespiratory complications of induced hypertension.14 The slightest com- promise of venous drainage from the head after application of a rigid cervical collar might impair CBF in TBI patients and may be counterproductive whenever ICP-lowering strate- gies are indicated.21 In a healthy brain, cerebral autoregulation maintains CBF when systolic blood pressure fluctuates or venous blood temporarily pools in the head. In an injured brain, autoregulation might be altered or entirely dysfunctional, which makes the brain vulnerable to arterial pressure fluctuations and venous stasis.21 This possible harmful effect of the collar and local pressure pain might explain the exacerbation of discomfort and agitation that we sometimes observe after application.22
Since 2016, Dutch prehospital trauma protocols differ from international advanced trauma life support and prehospital trauma life support protocols on the subject of cervical spine
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