We have demonstrated that the use of vasopressin or norepinephrine is associated with serum epinephrine undetectability in patients over 60 years of age. We have also demonstrated that, when serum levels of epinephrine is undetectable, a serum norepinephrine level ≥ 2006.5 pg/mL is associated with a greater risk of death. Therefore, we speculate that patients with a lower intrinsic sympathetic response would be at a higher risk for poor outcomes and require more vasopressors in general.
In trauma patients, hemodynamic stability is achieved through adequate volume resuscitation and the use of vasoactive drugs. Various types of stimuli (trauma, hypovolemia, pain, anxiety, inflammation, stress, and infections) increase activity in the sympathetic nervous system, resulting in epinephrine release by the adrenal medulla as an adaptive response [9–11]. Emergency surgery, whether in the setting of trauma or not, increases sympathetic nervous system activity.
Catecholamines are among the first mediators to be released in a stress response. Once released into the blood, these stress hormones exert multiple effects, especially on the cardiovascular system, leading to appropriate adjustments of blood pressure and heart rate, as well as of the energy metabolism, enabling the organism to respond to threats to its survival [12]. To ensure appropriate catecholamine secretion, the needs of which differ considerably from basal to stressful conditions, there is continuous adaptation of the stimulus-secretion coupling of chromaffin cells in the adrenal medulla. That adaptation occurs through transient or persistent remodeling of crucial molecular, cellular, and tissue elements of the medulla [13]. The mechanisms of such adaptation include neurotransmission between the splanchnic nerve and chromaffin cells at the cholinergic splanchnic–adrenal synapse, intercellular communication among chromaffin cells at the intercellular gap junction, and activation of voltage-gated calcium channels on the chromaffin cell membranes [13]. A number of situations commonly seen in critically ill patients are related to the poor functioning of one or more of those mechanisms, such as the hypothalamic–pituitary–adrenal axis dysfunction observed in head trauma patients [14–16]; cellular-level damage to the adrenal gland due to hypoxia or to local or systemic pH changes; and a progressive decline in epinephrine release due to prolonged circulatory shock, which leads to chromaffin cell depletion [17].
It is estimated that dysfunction of the hypothalamic–pituitary–adrenal axis occurs in up to 20% of critically ill patients and in 60% of those with sepsis, abolishing or reducing the intensity of cortisol activity [18, 19]. The reported incidence of such dysfunction is higher in patients over 60 years of age [20, 21], which is in keeping with our findings in the present study. The higher odds ratio for epinephrine undetectability in our > 60-year age group also corroborates the established concept of the frequency of hypothalamic–pituitary–adrenal axis dysfunction being higher in critically ill patients over 60 years of age, as described by Rushworth et al. [22, 23]. Similarly, Johansson et al. [6] reported that epinephrine levels were lower in older trauma patients.
By measuring the variability in heart rate and the sensitivity of chemoreceptors and baroreceptors, Schmidt et al. [24] demonstrated a link between attenuation of the autonomic response and higher mortality in critically ill patients. We hypothesized that the undetectability of epinephrine would be due to attenuation of the sympathetic response. Abboud et al. [25] published data suggesting that exogenous epinephrine alters the endogenous norepinephrine metabolism in patients with sepsis. However, that phenomenon has not been described in trauma patients.
Our study has some limitations. First, it was an observational study, with no control group. In addition, we did not have access to the data collected before ICU admission and before the administration of vasoactive drugs, which could have helped us better analyze the autonomic mechanism involved. Another limitation was the heterogeneity of the sample, in terms of the severity of the trauma and the inclusion of non-trauma patients. Furthermore, the fact that we used single rather than serial measurements made it difficult to estimate the intensity and duration of the hypothalamic–pituitary–adrenal axis dysfunction and reduced the accuracy of its correlation with the variables evaluated. Nonetheless, our study raises an important question: In older critically ill patients, should we start administering higher doses of epinephrine when serum norepinephrine levels are already high?
In conclusion, there is an evident association between age and mortality in surgical critical care patients. An undetectable level of epinephrine, which appears to be more common in older patients, could contribute to poor outcomes in such patients.