During the prehospital treatment of major traumatic brain injury, hypotension and hypoxia occur frequently. The presence of either is associated with decreased survival, but little is known about the effect on patient outcomes when they occur together. Spaite et al (2016) evaluate how hypotension and hypoxia, occurring in combination or separately, affect survival.
The authors debate what effect combining the two factors would have on mortality. Reducing oxygen delivery to neurons will have similar physiological consequences—whether the mechanism of that is hypotension or hypoxia—and some authors have suggested that their combination may add little to the risk of mortality. However, little previous evidence exists to monitor these circumstances, and even less is known about the prehospital setting. These are key reasons for evaluating the effect of blood pressure and oxygenation in the prehospital environment—particularly when the highly sensitive injured brain can be damaged even by brief periods of low oxygenation, which initiates the secondary brain injury response.
This study recruited 17 105 patients, of which 13 151 met the inclusion criteria of moderate or severe traumatic brain injury diagnosed within the trauma centre. This was defined by either CDC Barell matrix type 1, International Classification of Diseases ≥3 or Abbreviated Injury Score-head region ≥3. The study excluded:
The authors adjusted for confounders in their data analysis such as age, sex, ethnicity, blunt or penetrating trauma type, Abbreviated Injury Score, Injury Severity Score and interfacility transfer. The treating trauma centre was another significant confounder, which the authors adjusted for in their research model.
Patient age of 10 years was used as the lower age limit to simplify data analysis, as it becomes difficult to adjust the results to compensate for paediatric blood-pressure differences. Also, it is unknown how this would affect the results if children have a differing neurological response to hypotensive or hypoxic insults. It is also unclear why the authors chose to exclude patients with blood pressures at the extremes of high and low.
It is arguable that these patients would be more likely to have coexisting major injuries, causing measurements that are more likely to result in death, and therefore limiting the application of the study to brain injury alone. The chosen limit of <10% for oxygen saturations appears an unusual choice. Anecdotally, oxygen saturation monitors fail to detect an accurate reading at approximately <60%.
Additionally, patients suffering a single abnormal reading were analysed the same as patients who suffered multiple or continuous abnormal readings. Despite these limitations, the authors explain that this is a comprehensive study with data retrieved directly from care records. Therefore, it is rare in its level of scrutiny and consistency for a prehospital study.
The results of this study show that among the cohort, 11 545 (87.8%) had neither hypotension nor hypoxia; 604 (4.6%) had hypotension only; 790 (6.0%) had hypoxia only; and 212 (1.6%) had both hypotension and hypoxia. Mortality rates were 5.6%; 20.7%; 28.1%; and 43.9%, respectively. Unlike all previous studies published to date, its large scale has allowed for the recruitment of reasonably-sized cohorts of patients. This is particularly important when evaluating the data from the much more infrequent combined hypotensive-hypoxic cohort.
Evaluating for an interaction between hypotensive and hypoxic occurrences showed that effects were additive. Combined hypotension and hypoxia were associated with an increased mortality far greater than for patients with only a single insult, and this important finding is significantly stronger than other studies have previously reported.
As this study is observational by design, the data cannot be used to identify how effective treatment of hypotension and hypoxia is at reducing mortality once an initial insult has occurred; although it is clearly logical to treat these deteriorations with the current evidence in mind. The study notes the need for further research in order to determine the influence of treatment of hypotension and hypoxia upon brain injury outcome. These findings highlight the importance for paramedics to continue doing the basics of trauma care well, with airway management, adequate oxygenation, ventilation, and haemorrhage control, to prevent the secondary brain injury from cascading. Aside from the benefits that prehospital anaesthesia for brain injury provide, it must be remembered that the intervention must be balanced with the risks that it carries in the non-fasted, multi-system trauma patient in an uncontrolled environment. Misplaced endotracheal tubes lead to hypoxia, and induction hypotension are two of the most common complications of this advanced intervention but also carry negative consequences for the patient according to this study's findings. The authors of this study emphasise their support for aggressive prevention strategies, and treatment of hypotension and hypoxia.