Within the circulating renin-angiotensin system (RAS), angiotensin-converting enzyme (ACE) cleaves angiotensin I to yield angiotensin II (Ang II), whose role as a vasoconstrictor is mediated by agonist action at the AT₁ receptor. ACE also degrades the vasodilator bradykinin. Meanwhile, the tissue RAS performs a variety of functions including regulation of local inflammatory, vascular, metabolic and growth responses.

A common functional polymorphism exists in the ACE gene with the absence (deletion, D) rather than the presence (insertion, I) of a 287 base pair fragment being associated with raised circulating and tissue ACE activity.1 This “high activity” D allele is associated with increased mortality in some critical illnesses such as adult acute respiratory distress syndrome (ARDS)2 and paediatric meningococcal infection,3 and worse functional outcome following traumatic brain injury.4 We thus postulated that a similar association might exist between the D allele and poorer outcome after major trauma and have tested this hypothesis.

METHODS

Subjects were consecutive trauma victims brought to the Royal London Hospital (RLH) by the Helicopter Emergency Medical Service (HEMS) between 1 June 2001 and 31 May 2003 when one of the investigators (MH) was on duty, and whose admission required trauma team activation. The Injury Severity Score (ISS), Revised Trauma Score (RTS), age, sex and outcome data for each subject were recorded from the RLH trauma database. The primary outcome measure was survival to discharge.

ACE genotype was determined from leucocyte genomic DNA using polymerase chain reaction amplification by experienced staff blind to all clinical data, using internationally accepted methodologies.

Statistical analysis

Statistical analysis was conducted using Minitab Version 14 (Minitab, Pennsylvania, USA). Deviation from Hardy-Weinberg equilibrium was examined using a χ² test. ACE genotype was considered as II versus D carriers, in keeping with earlier studies.4 ISS is not a true continuous variable and was therefore categorised into quartiles. Age and RTS were not normally distributed and are presented as median and interquartile range (IQR). Categorical data were assessed using a χ² test and continuous data by one-way ANOVA or a Kruskal-Wallis test as appropriate. Throughout, p values <0.05 were considered statistically significant.

RESULTS

Over the study period, 348 adult patients were admitted to the RLH via HEMS with major trauma. The median age, ISS and RTS and mortality were 37 years, 17 and 7.1, respectively; 76% were men and the overall mortality was 22%. Of these, 41 patients (14 women) were included in the study. Their median age, RTS and ISS are shown in table 1 and their overall mortality was 24.4% (n = 10). RTS was unavailable in two subjects who had been intubated before arrival of the HEMS.

ACE genotype could not be determined in five subjects, including one who later died, a failure rate consistent with other studies. In the remainder the genotype distribution for II, ID and DD was 14 (38.9%), 13 (36.1%) and 9 (25.0%), respectively, which was consistent with Hardy-Weinberg equilibrium (p = 0.11).

Neither age (p = 0.4), ISS (p = 0.9), RTS (p = 0.6), ACE genotype (0.6) nor survival (p = 0.7) varied significantly by gender.

As expected, increasing age (p = 0.044) was associated with mortality, as was the presence of one or more D alleles (p = 0.048). There was also a tendency for an association between RTS and mortality (p = 0.08), but ISS showed no association (p = 0.46, table 2).

The D allele was significantly associated with a lower RTS (p = 0.002) but not with increased age or ISS (p = 0.15 and 0.08, respectively; table 1).

DISCUSSION

These findings suggest that the ACE D allele may be associated with an increased risk of dying after major trauma and are the first data to suggest a genetic locus of influence upon this outcome. Such data are in keeping with similar detrimental associations noted in other critical illness groups.2 3

A number of mechanisms may underlie this association. Increased endocrine RAS activation in subjects carrying a D allele may cause greater systemic vasoconstriction in response to a given fall in mean arterial pressure, leading to loss of clot integrity and provoking further haemorrhage. Such a mechanism might account for the previously reported association of the D allele with increased bleeding in a study of postoperative patients.5

Meanwhile, local RAS in individual organs modify injury responses: ACE inhibition protects against acute lung injury in rats with sepsis and chemical injury whereas ACE and Ang II promote disease pathogenesis, induce lung oedema and impair lung function.6 In the brain, Ang II contributes to cerebrovascular dysregulation after traumatic brain injury, while AT₁ blockade protects against such secondary brain injury.7

Furthermore, RAS activity is pro-inflammatory8 and outcome in critical illness seems causally associated with the magnitude of the inflammatory response.9 This may underlie the association of the D allele with the genesis of and poorer outcome from ARDS.2

However, the gut is now considered by many to be the “engine driving multiple organ failure”.10 The disproportionate mesenteric vasoconstriction response to haemorrhagic shock permits gut translocation of bacteria and endotoxin release, provoking systemic inflammatory response syndrome and, ultimately, multiple organ failure. Such vasoconstriction is mediated by Ang II action at the AT₁ receptor,10 and several studies have demonstrated beneficial effects of AT₁ blockade in animal models of haemorrhagic shock, including improved survival.11 Through such mechanisms ACE genotype might modulate the physiological response to major trauma, accounting for its association with RTS and not ISS.

This study has several limitations. First, numbers are small and our findings should therefore be interpreted with caution. A meaningful comparison of outcomes between men and women, blunt versus penetrating trauma, single versus multisystem trauma or different ethnic groups is thus also impossible. We have no data relating to the impact of ACE genotype on prehospital death or morbidity in survivors and, further, subjects with the II genotype tended to be younger with a lower ISS. Although this association was not statistically significant, it may have contributed towards the lower mortality observed in this group.

Further large prospective studies are thus required. If this association is confirmed, new therapeutic targets for ACE inhibitors and angiotensin-receptor blocking drugs might well be suggested. In addition, genetic analysis might help contribute to risk stratification.

CONCLUSION

The results of this study suggest that common variation within the ACE gene may modify the risk of death following major trauma with a significantly excess mortality in subjects having one or more D allele. Definition of genetic markers of poor outcome such as this may help to define high-risk subgroups that would benefit from early intensive management, improve the predictive modelling of outcome and identify novel applications for existing drugs for the benefit of all major trauma patients.