Autonomic dysfunction in the ICU patient

Autonomic Nervous System and Cardiovascular Disease

Because anesthesia affects the integrity of the autonomic nervous system, anesthesiologists use vital signs to maintain respiratory and circulatory homeostasis. However, patients with genetic predispositions or with autonomic dysfunctions are at risk of severe complications from anesthesia. For these patients, the monitoring of vital signs may not give sufficient warning to avoid complications. The development of methods to measure autonomic tone could be of interest to anesthesiologists because they could warn of changes in autonomic tone before vital signs are affected. New noninvasive methods are being developed to obtain measurements of parasympathetic and sympathetic output allowing for the monitoring of perioperative autonomic tone. These measurements are based on analysis of heart rate and blood pressure variability. In this report, the principals of the analysis of heart rate and blood pressure variability will be explained and the usefulness of these methods to anesthesiologists will be discussed.

Autonomic nervous system function and depth of sedation in adults receiving mechanical ventilation

BACKGROUND

The effect of the depth of sedation on the function of the autonomic nervous system is not well known.

OBJECTIVES

To describe the effect of level of sedation on heart rate variability as a marker of the function of the autonomic nervous system in patients receiving mechanical ventilation.

METHODS

This pilot study was part of a larger study in which sedation level was measured continuously for up to 24 hours. The sample consisted of 14 patients receiving mechanical ventilation. The R-R interval was measured continuously via electrocardiography. Sedation level was determined by using the Patient State Index and was categorized as deep (<60) or light (=60). Continuous heart rate data of 5 to 10 minutes for each sedation level for each patient were analyzed.

RESULTS

Parasympathetic activity as indicated by root mean square of successive difference of the R-R interval, the high-frequency component, and the percentage of differences of successive N-N intervals (intervals due to normal sinus depolarization) that differed more than 50 milliseconds was significantly lower for deep sedation than for light sedation. The markers indicating sympathetic activity, including the low-frequency component and the ratio of the low-frequency component to the high-frequency component, did not differ significantly between the 2 levels of sedation. Most patients were receiving benzodiazepines.

CONCLUSIONS

Deep sedation may be associated with depression of parasympathetic function in patients receiving mechanical ventilation. Use of benzodiazepines most likely contributed to this finding.

The vasopressin and copeptin response in patients with vasodilatory shock after cardiac surgery: a prospective, controlled study

OBJECTIVE

To evaluate arginine vasopressin (AVP) and copeptin plasma concentrations in patients with vasodilatory shock after cardiac surgery.

DESIGN

Prospective, controlled, clinical study.

SETTING

Surgical intensive care unit and cardiac surgery ward in a tertiary university teaching hospital.

PATIENTS AND PARTICIPANTS

Thirty-three critically ill patients with vasodilatory shock after cardiac surgery and ten control patients undergoing uncomplicated aorto-coronary bypass surgery.

MEASUREMENTS AND RESULTS

Hemodynamic, laboratory and clinical data were recorded daily in all patients during the first 7 days after cardiac surgery. At the same time, points blood was withdrawn to determine plasma concentrations of AVP (radioimmunoassay) and copeptin (immunoluminometric assy). Standard tests, a mixed effects model and regression analyses were used for statistical analysis. The course of AVP was significantly different between groups (P < 0.001). While AVP concentrations were lower in the study group on the first postoperative day, they were higher than that in the control group from postoperative day 3 on. There was no difference in the postoperative AVP response between study patients with or without chronic angiotensin-converting enzyme inhibitor therapy. Except during continuous veno-venous hemofiltration, AVP and copeptin correlated significantly with each other (P < 0.001; r = 0.749).

CONCLUSIONS

The AVP response to cardiac surgery is significantly different between patients with vasodilatory shock and patients undergoing uncomplicated aorto-coronary bypass surgery. Although no causative relationship between AVP concentrations and cardiovascular instability can be drawn from these results, our data support the hypothesis that inadequately low AVP plasma levels contribute to the failure to restore vascular tone in vasodilatory shock after cardiac surgery.

Molecular events in the cardiomyopathy of sepsis

Septic cardiomyopathy is a well-described complication of severe sepsis and septic shock. However, the interplay of its underlying mechanisms remains enigmatic. Consequently, we constantly add to our pathophysiological understanding of septic cardiomyopathy. Various cardiosuppressive mediators have been discovered, as have multiple molecular mechanisms (alterations of myocardial calcium homeostasis, mitochondrial dysfunction, and myocardial apoptosis) that may be involved in myocardial dysfunction during sepsis. Finally, the detrimental roles of nitric oxide and peroxynitrite have been unraveled. Here, we describe our present understanding of systemic, supracellular, and cellular molecular mechanisms involved in sepsis-induced myocardial suppression.

The alteration of autonomic function in multiple organ dysfunction syndrome

Autonomic dysfunction is associated with the severity of illness and mortality in patients with multiple organ dysfunction syndrome (MODS) and may contribute significantly to the pathogenesis of this syndrome. Several treatment approaches may possibly restore autonomic function in MODS and thus cause the survival benefit.

Diminished functional association between proximal and distal gastric motility in critically ill patients

OBJECTIVE

This study examined the effects of critical illness on the relationship between proximal and distal gastric motor activity during fasting and duodenal nutrient stimulation.

DESIGN

Prospective, case-controlled study.

PATIENTS AND PARTICIPANTS

Ten critically ill patients and ten healthy volunteers.

INTERVENTIONS

Concurrent proximal gastric (barostat) and antro-pyloro-duodenal (manometry) motility were recorded during fasting and during two 60-min duodenal nutrient infusions (Ensure at 1 kcal/min and 2 kcal/min) in random order, separated by a 2-h wash-out period.

RESULTS

Baseline proximal gastric volumes were similar between the two groups. At 10 min nutrient-induced fundic relaxation was lower in patients than healthy subjects (45 +/- 26 vs. 196 +/- 29 ml). In patients the frequency and volume amplitude of fundic waves were also lower. There were fewer propagated antral waves in patients than in healthy subjects during both fasting and nutrient infusion. These were more retrograde, shorter in length and associated with a pyloric contraction. The proportion of fundic waves followed by a distally propagated antral wave was significantly less in patients (0%, 0-8%) than controls 36% (11-44%).

CONCLUSIONS

In critical illness, in addition to impairment of proximal and distal gastric motor activity, the association between the two gastric regions is abnormal. This disturbance may interfere with meal distribution and further contribute to slow gastric emptying in these patients.

Mechanisms of sepsis-induced cardiac dysfunction

OBJECTIVES

To review mechanisms underlying sepsis-induced cardiac dysfunction in general and intrinsic myocardial depression in particular.

DATA SOURCE

MEDLINE database.

DATA SYNTHESIS

Myocardial depression is a well-recognized manifestation of organ dysfunction in sepsis. Due to the lack of a generally accepted definition and the absence of large epidemiologic studies, its frequency is uncertain. Echocardiographic studies suggest that 40% to 50% of patients with prolonged septic shock develop myocardial depression, as defined by a reduced ejection fraction. Sepsis-related changes in circulating volume and vessel tone inevitably affect cardiac performance. Although the coronary circulation during sepsis is maintained or even increased, alterations in the microcirculation are likely. Mitochondrial dysfunction, another feature of sepsis-induced organ dysfunction, will also place the cardiomyocytes at risk of adenosine triphosphate depletion. However, clinical studies have demonstrated that myocardial cell death is rare and that cardiac function is fully reversible in survivors. Hence, functional rather than structural changes seem to be responsible for intrinsic myocardial depression during sepsis. The underlying mechanisms include down-regulation of beta-adrenergic receptors, depressed postreceptor signaling pathways, impaired calcium liberation from the sarcoplasmic reticulum, and impaired electromechanical coupling at the myofibrillar level. Most, if not all, of these changes are regulated by cytokines and nitric oxide.

CONCLUSIONS

Integrative studies are needed to distinguish the hierarchy of the various mechanisms underlying septic cardiac dysfunction. As many of these changes are related to severe inflammation and not to infection per se, a better understanding of septic myocardial dysfunction may be usefully extended to other systemic inflammatory conditions encountered in the critically ill. Myocardial depression may be arguably viewed as an adaptive event by reducing energy expenditure in a situation when energy generation is limited, thereby preventing activation of cell death pathways and allowing the potential for full functional recovery.

Vasopressin in vasodilatory shock: ensure organ blood flow, but take care of the heart!

Supplementary arginine vasopressin infusion in advanced vasodilatory shock may be accompanied by a decrease in cardiac index and systemic oxygen transport capacity in approximately 40% of patients. While a reduction of cardiac output most frequently occurs in patients with hyperdynamic circulation, it is less often observed in patients with low cardiac index. Infusion of inotropes, such as dobutamine, may be an effective strategy to restore systemic blood flow. However, when administering inotropic drugs, systemic blood flow should be increased to adequately meet systemic demands (assessed by central or mixed venous oxygen saturation) without putting an excessive beta-adrenergic stress on the heart. Overcorrection of cardiac index to hyperdynamic values with inotropes places myocardial oxygen supply at significant risk.

Heart rate and blood pressure variability as markers of sensory blockade with labour epidural analgesia

PURPOSE

To evaluate the correlation between the progression of somatosensory blockade and changes in autonomic outflow following the onset of labour epidural analgesia.

METHODS

Twelve labouring parturients consented to participate in the study. Baseline electrocardiogram, blood pressure (BP) and respiratory rate were recorded for ten minutes. The epidural consisted of 0.125% bupivacaine with 50 microg of fentanyl (total volume 20 mL). Measurements were repeated for ten minutes after initiation of the block. The level of sensory block was measured bilaterally with loss of sensation to ice at two-minute intervals. Wavelet transform was used to obtain heart rate (HR) and BP variability every two minutes following the loading dose of epidural medication. High frequency power of HR variability was used to assess changes in parasympathetic activity. The total power of BP variability was used to assess changes in sympathetic activity. A nonparametric repeated measures ANOVA was used for the variability data, and a Spearman rank correlation test was used to evaluate the relationship between the sensory block and HR and BP variability.

RESULTS

The sensory block progressed to T9 at ten minutes post-epidural and was the mirror image of the decrease in total power of BP variability. High frequency power of HR variability increased to a plateau at six minutes post-epidural. A significant correlation was found between the increase in sensory block and the observed decrease in BP variability (r = -1.000, P = 0.0028).

CONCLUSION

In this study of labouring parturients, BP variability correlated with the progression of both sympathetic and somatosensory block following epidural anesthesia, while HR variability was shown to be a surrogate marker of increased parasympathetic activity.

Physiology and immunology of the cholinergic antiinflammatory pathway

Cytokine production by the immune system contributes importantly to both health and disease. The nervous system, via an inflammatory reflex of the vagus nerve, can inhibit cytokine release and thereby prevent tissue injury and death. The efferent neural signaling pathway is termed the cholinergic antiinflammatory pathway. Cholinergic agonists inhibit cytokine synthesis and protect against cytokine-mediated diseases. Stimulation of the vagus nerve prevents the damaging effects of cytokine release in experimental sepsis, endotoxemia, ischemia/reperfusion injury, hemorrhagic shock, arthritis, and other inflammatory syndromes. Herein is a review of this physiological, functional anatomical mechanism for neurological regulation of cytokine-dependent disease that begins to define an immunological homunculus.

Proximal gastric motility in critically ill patients with type 2 diabetes mellitus

AIM: To investigate the proximal gastric motor response to duodenal nutrients in critically ill patients with long-standing type 2 diabetes mellitus.

METHODS: Proximal gastric motility was assessed (using a barostat) in 10 critically ill patients with type 2 diabetes mellitus (59 ± 3 years) during two 60-min duodenal infusions of Ensure^® (1 and 2 kcal/min), in random order, separated by 2 h fasting. Data were compared with 15 non-diabetic critically ill patients (48 ± 5 years) and 10 healthy volunteers (28 ± 3 years).

RESULTS: Baseline proximal gastric volumes were similar between the three groups. In diabetic patients, proximal gastric relaxation during 1 kcal/min nutrient infusion was similar to non-diabetic patients and healthy controls. In contrast, relaxation during 2 kcal/min infusion was initially reduced in diabetic patients (p < 0.05) but increased to a level similar to healthy humans, unlike non-diabetic patients where relaxation was impaired throughout the infusion. Duodenal nutrient stimulation reduced the fundic wave frequency in a dose-dependent fashion in both the critically ill diabetic patients and healthy subjects, but not in critically ill patients without diabetes. Fundic wave frequency in diabetic patients and healthy subjects was greater than in non-diabetic patients.

CONCLUSION: In patients with diabetes mellitus, proximal gastric motility is less disturbed than non-diabetic patients during critical illness, suggesting that these patients may not be at greater risk of delayed gastric emptying.

Attenuated autonomic function in multiple organ dysfunction syndrome across three age groups

Multiple organ dysfunction syndrome (MODS) is the failure of several organs after a trigger event. The mortality is high, at up to 70%. We hypothesize that autonomic dysfunction may substantially contribute to the development of MODS and speculate that there is an age dependence of autonomic dysfunction in MODS. A total of 90 consecutively admitted MODS patients were assigned to this study. Three variables of autonomic function were analyzed: heart rate variability (HRV), baroreflex sensitivity (BRS) and chemoreflex sensitivity (CRS). The patient cohort was divided into three age groups. The main finding was that BRS, CRS and almost all indices of HRV were attenuated in comparison to normal range data and there was no age dependence for HRV indices or CRS, but there was for BRS. In conclusion, autonomic function in MODS is attenuated. The influence of MODS on autonomic function overwhelms the age dependence of autonomic function observed in healthy subjects.

Septicemia and Cardiovascular Infections in Horses

This article first reviews cardiovascular infections, including endocarditis, myocarditis, vasculitis, and pericarditis. It then addresses what is known at this stage about the effects of sepsis on the cardiovascular system. Some information is provided from current human literature to familiarize the reader with the diagnostics and therapeutics that may eventually be used in equine practice as well.

Proximal gastric response to small intestinal nutrients is abnormal in mechanically ventilated critically ill patients

AIM: To determine the response of the proximal stomach to small intestinal nutrients in critically ill patients.

METHODS: Proximal gastric motility was measured in 13 critically ill patients (49.3 ± 4.7 years) and 12 healthy volunteers (27.7 ± 2.9 years) using a barostat technique. Recordings were performed at baseline, during a 60-min intra-duodenal infusion of Ensure^® (2 kcal/min), and for 2 h following the infusion. Minimum distending pressure (MDP), intra-bag volume and fundic wave activity were determined.

RESULTS: The MDP was higher in patients (11.7 ± 1.1 vs 7.8 ± 0.7 mmHg; P < 0.01). Baseline intra-bag volumes were similar in the 2 groups. In healthy subjects, a ‘bimodal’ proximal gastric volume response was observed. In patients, the initial increase in proximal gastric volume was small and delayed, but eventually reached a maximal volume similar to that of healthy subjects. In healthy subjects, the proximal gastric volume rapidly returned to baseline level after nutrient infusion (median 18 min). In contrast, the recovery of volume to baseline was delayed in critically ill patients (median 106 min). In 6 patients, the volume had not returned to baseline level 2 hours after nutrient infusion. In patients, fundic volume waves were less frequent (P < 0.05) and had lower amplitude (P < 0.001), compared to healthy subjects.

CONCLUSION: In critical illness, proximal gastric motor responses to small intestinal nutrient stimulation are abnormal.

Prognostic impact of autonomic information flow in multiple organ dysfunction syndrome patients

BACKGROUND

Multiple organ dysfunction syndrome (MODS) is the sequential failure of several organ systems after a trigger event, like cardiogenic shock or decompensated heart failure. Mortality is high, up to 70%. Autonomic dysfunction (AD) may substantially contribute to the development of MODS. In cardiology, it has recently been shown that nonlinear parameters could predict mortality. Our study aimed at 1. characterising the complex characteristics of AD of critically ill MODS patients by the nonlinear parameters of autonomic information flow (AIF), 2. comparing AIF with autonomic function of healthy controls, and 3. characterising the accuracy of this parameter in predicting mortality in MODS.

METHODS

We enrolled 43 score-defined MODS patients who were consecutively admitted to a twelve-bed medical intensive care unit in a university centre into this prospective outcome study. Additionally, we assigned 50 healthy controls to the study. AIF was assessed as a complexity function of AD using 24-h ECG. Measures of AIF were introduced according to the standard HRV concept. The patients were followed up for 28-day mortality.

RESULTS

MODS causes a disorganisation of short term AIF in favour of an enhanced (rigid) long term AIF. Concerning prognosis increased short term AIF was associated with survival. Short term AIF discriminated between MODS survivors and non-survivors at the level of APACHE II score.

CONCLUSIONS

This is the first study providing evidence that complex AD of MODS patients is specifically assessed by AIF time scales and that AIF has significant prognostic impact.

Heart rate variability analysis: a tool to assess cardiac autonomic function

Heart rate monitoring is commonly used to provide an acute indicator of an individual's cardiovascular status and responsiveness. An increasingly popular technique involves quantifying the very small amounts by which the heart rate changes from one cardiac cycle to the next. This "heart rate variability (HRV) analysis" provides a substantial amount of additional information about the cardiovascular system and enables quantification of cardiac regulatory influences on the autonomic nervous system. The autonomic nervous system consists of two main components: the sympathetic system and the parasympathetic system. The relative influence of these two components on the sino-atrial node of the heart determines the heart rate. A number of physiological factors, including blood pressure and respiratory rate, can have a profound effect on this autonomic "balance." HRV analysis therefore provides a noninvasive method for investigating the dynamic influence of changing physiological parameters on cardiac regulation.

Autonomic dysfunction predicts mortality in patients with multiple organ dysfunction syndrome of different age groups

OBJECTIVE

Multiple organ dysfunction syndrome (MODS) is the sequential failure of several organ systems after a trigger event, like sepsis or cardiogenic shock. Mortality rate is high, up to 70%. Autonomic dysfunction may substantially contribute to the development of MODS. Our study aimed to characterize a) the spectrum of autonomic dysfunction of critically ill MODS patients; b) whether autonomic dysfunction is different in patients receiving sedation, mechanical ventilation, or catecholamines; c) the age dependency of autonomic dysfunction in MODS; and d) whether autonomic dysfunction predicts mortality in MODS.

DESIGN

Prospective cohort study.

SETTING

Twelve-bed medical intensive care unit in a university center.

PATIENTS

Ninety consecutively admitted score-defined MODS patients.

INTERVENTIONS

Assessment of heart rate variability, baroreflex sensitivity, and chemoreflex sensitivity as markers of autonomic dysfunction. The patients were followed for 28-day mortality.

MEASUREMENTS AND MAIN RESULTS

Baroreflex sensitivity, chemoreflex sensitivity, and almost all indexes of heart rate variability were attenuated in comparison to normal range data. There was no association between the assessed heart rate variability variables, baroreflex sensitivity or chemoreflex sensitivity, and the presence of sedation or catecholamine therapy. Except for frequency-domain variables, pNN50 (percentage of differences of successive RR intervals differing >50 msecs) and rMSSD (root mean square of successive difference of N-N intervals), none of the measured variables were related to the presence of mechanical ventilation. Age dependency was detected for baroreflex sensitivity but not for heart rate variability indexes or chemoreflex sensitivity (across ages 24-96 yrs). lnVLF predicted 28-day mortality best in the entire cohort of patients and in a subgroup of patients with cardiogenic-triggered MODS.

CONCLUSIONS

Autonomic function of MODS patients is blunted, and this attenuation has prognostic implications. The extensive influence of MODS on autonomic function overwhelms and masks the well-known age dependency of autonomic function seen in healthy persons.

Chemo- and ergoreflexes in health, disease and ageing

The chemo- and ergoreflexes (muscle receptors) are among the major reflex arches, which adapt the respiratory and the cardiovascular system to the needs of the body and contribute to its homeostasis. The present paper reviews the interplay of these reflexes with other major cardiovascular reflex arches; the methods used for their calculation and their normal range data. The clinical implications of chemoreflex sensitivities and ergoreflexes in chronic heart failure (CHF) as well as the application of chemoreflexes in coronary artery disease, sudden cardiac death and multiple organ dysfunction syndrome are discussed.

Complex systems and the technology of variability analysis

Characteristic patterns of variation over time, namely rhythms, represent a defining feature of complex systems, one that is synonymous with life. Despite the intrinsic dynamic, interdependent and nonlinear relationships of their parts, complex biological systems exhibit robust systemic stability. Applied to critical care, it is the systemic properties of the host response to a physiological insult that manifest as health or illness and determine outcome in our patients. Variability analysis provides a novel technology with which to evaluate the overall properties of a complex system. This review highlights the means by which we scientifically measure variation, including analyses of overall variation (time domain analysis, frequency distribution, spectral power), frequency contribution (spectral analysis), scale invariant (fractal) behaviour (detrended fluctuation and power law analysis) and regularity (approximate and multiscale entropy). Each technique is presented with a definition, interpretation, clinical application, advantages, limitations and summary of its calculation. The ubiquitous association between altered variability and illness is highlighted, followed by an analysis of how variability analysis may significantly improve prognostication of severity of illness and guide therapeutic intervention in critically ill patients.

Impact of high volume hemofiltration on hemodynamic disturbance and outcome during septic shock

The purpose of this study was to evaluate the effect of high volume continuous venovenous hemofiltration (HVCVVH) on hemodynamic and outcome in patients with septic shock. The primary end point was mortality at 28 days. Study design was a prospective case series, and study setting was a 12 bed intensive care unit at a university hospital. A total of 24 consecutive patients with septic shock were included, with dysfunction of more than two organs. All patients were treated by HVCVVH with ultrafiltration rate between 40 ml x kg(-1) x hr(-1) and 60 ml x kg(-1) x hr(-1) for 96 hours. In all patients, the increase in hemodynamic parameters was statistically significant (p < 0.05), with a significant linear decrease in norepinephrine doses (p < 0.05). The predicted 28 day mortality by three different severity scores was more than 70%, and the mortality in the hemofiltration group was 46% (p < 0.075). In the present study of septic shock patients with organ dysfunction, the hemodynamic parameters increased regularly during treatment by HVCVVH. This study suggests a beneficial effect of HVCVVH on 28 day mortality (46% vs. 70%), and further studies with larger cohorts are required.

[End-organ damage in inflammation and sepsis]

Inflammatory components form part of the pathogenesis of infectious, but also non-infectious diseases. One of the most important revelations of the last decades was the finding of an activation of the natural immune response in numerous physiological and pathophysiological processes despite the absence of an infectious agent. Even physical exercise or psychical stress may evoke an inflammatory response. Substantial insight has been gained in the recent decades into the molecular mechanisms of cellular and organ dysfunction by the impact of mediators with inflammatory potential. Validated score systems allow a precise detection and quantification of organ dysfunctions in inflammation and sepsis. In contrast therapy of organ damage in severe septic and non-septic inflammatory processes presently is mainly still based on supportive therapeutic measures.

Influence of thoracic epidural analgesia on cardiovascular autonomic control after thoracic surgery

BACKGROUND

Thoracic epidural analgesia (TEA) is effective in alleviating pain after major thoracoabdominal surgery and may also reduce postoperative mortality and morbidity. This study investigated cardiovascular autonomic control in patients undergoing elective thoracic surgery and its modulation by continuous TEA.

METHODS

Thirty-eight patients were randomly assigned to receive patient-controlled analgesia (PCA group) or thoracic epidural analgesia (TEA group) with doses of bupivacaine (0.25% during operation, 0.125% after operation) and fentanyl (2 microg ml(-1)). Heart rate variability (HRV), baroreflex function and pressure response to nitroglycerine and phenylephrine were assessed before operation, 4 h after the end of surgery (POD 0) and on the first and second postoperative days (POD 1 and POD 2).

RESULTS

Early after surgery, all HRV variables and baroreflex sensitivities were markedly decreased in both groups. In the TEA group, total HRV and its high-frequency components (HF) increased towards preoperative values at POD 1 and POD 2, whereas the ratio of low to high frequencies (LF/HF) was significantly reduced (mean (SD), -44 (15)% at POD 0, -38 (17)% at POD 1, -37 (18%) at POD 2) and associated with blunting of the postoperative increase in heart rate and blood pressure. In the PCA group, the ratio of LF/HF remained unchanged and the decrements in HRV variables persisted until POD 2. In the two groups, baroreflex sensitivities and pressure responses recovered preoperative values at POD 2.

CONCLUSIONS

In contrast with PCA management, TEA using low concentrations of bupivacaine and fentanyl blunted cardiac sympathetic neural drive, resulting in vagal predominance, while HRV variables were better restored after surgery.

Heart rate variability in critical care medicine

The autonomic nervous system plays an integral role in homeostasis. Autonomic modulation can frequently be altered in critically ill patients. Assessment of heart rate variability (HRV) is based on analysis of consecutive normal R-R intervals and may provide quantitative information on the modulation of cardiac vagal and sympathetic nerve input. The hypothesis that depressed HRV may occur over a broad range of critical illness and injury and may be inversely correlated with disease severity and outcome has been tested in the last decade. In this article, we review recent literature concerning assessment of HRV in patients with critical illness or injury, as well as the potential clinical implications and limitations of HRV assessment in this area.