Heart Rate Variability in Intensive Care Unit Patients With Delirium
Publication: The Journal of Neuropsychiatry and Clinical Neurosciences
Abstract
Sympathovagal balance, assessed with heart rate variability (HRV), may be altered in intensive care unit (ICU) delirium. HRV was measured in the frequency domain [low frequencies (LF)=0.04–0.15 Hz and high frequencies (HF)=0.15–0.40 Hz] with HF in normalized units (HFnu) to evaluate parasympathetic tone and LF:HF ratio for sympathovagal balance. The authors assessed 726 ICU patients and excluded patients with conditions affecting HRV. No difference could be found between patients with (N=13) and without (N=12) delirium by comparing the mean (±standard deviation) of the HFnu (75±7 versus 68±23) and the LF:HF ratio (−0.7±1.0 versus −0.1±1.1). This study suggests that autonomic function is not altered in ICU delirium.
Delirium is a common disorder in patients in the intensive care unit (ICU) and is described to be an independent risk factor for death.1,2 The underlying mechanism for this increased mortality remains unclear. The phenomenology of delirium suggests an altered sympathovagal balance. Symptoms of hypertension and tachycardia suggest increased sympathetic activity, whereas lethargy may be associated with increased parasympathetic activity. However, until now, autonomic function has never been investigated in patients with delirium.
The activity of the autonomic nervous system can be assessed indirectly by measurement of heart rate variability (HRV).3 With HRV analysis, changes in the time interval between two consecutive heart beats are computed. Healthy physiologic states contain some degree of random variability in heart rate intervals.4 Because a decrease in HRV was found to be an independent risk factor for death in ICU patients4 and delirium is associated with increased mortality,1,2 we hypothesized that delirium is related to decreased HRV. The aim of this study was to investigate HRV in ICU patients with and without delirium.
Patients and Methods
Design and Patients
This prospective, observational study was performed in the 32-bed mixed ICU of the University Medical Centre Utrecht (The Netherlands) in a convenient sample. A waiver for informed consent was obtained from the local Medical Ethics Committee (Institutional Review Board number 10-181). All patients were informed of the purpose and measurements of the study, the confidentiality of the data being collected, and about their right to refuse to participate.
Patients from 30 to 80 years of age, admitted for more than 24 hours, were eligible for inclusion. Exclusion criteria were those conditions with expected or profound autonomic dysfunction: a history of any neurological disease, diabetes mellitus, end-stage renal failure, chronic obstructive pulmonary disease Gold stage IV, depression or anxiety disorder, coronary artery disease, heart transplantation, and severe sepsis. Additionally, patients with preexisting or new-onset cardiac arrhythmias or those receiving any adrenergic blocking agents or agonists were excluded.
Data Collection
Demographic data were collected at inclusion. Comorbidity at hospital admission was registered with the Charlson Comorbidity Index.5 Severity of illness at ICU admission was assessed using the Acute Physiology and Chronic Health Evaluation version IV score.6 The Sequential Organ Failure score was used to estimate the severity of illness.7
Eligible patients were screened for delirium using the Dutch version of the confusion assessment method for the ICU by a trained physician.8 In case of doubt, a neurologist (A.J.C.S.) was consulted who applied the DSM-IV criteria for delirium, based on clinical assessment for cognitive dysfunction and a review of the medical charts.9 Additionally, the Richmond Agitation and Sedation Score10 was used to classify different delirium subtypes.11 Hypoactive delirium was defined in persistent neutral or negative Richmond Agitation and Sedation Score scores (ranging from 0 to −3).11 Hyperactive delirium was present in patients with all positive Richmond Agitation and Sedation Score scores (ranging from +1 to +4).11 When both positive and negative Richmond Agitation and Sedation Score values were scored, patients were classified as having mixed-type delirium.11
Bipolar chest lead ECG was recorded in each patient for 15 minutes. In 10 patients, data were acquired for 2 days at four time points (8:00 a.m., 11:00 a.m., 2:00 p.m., and 5:00 p.m.) during the day to assess circadian influence. As no circadian rhythm could be shown, data from all time points for a particular patient could be pooled. Therefore, subsequently included patients could be measured less frequently with a minimum of one measurement to increase feasibility. During HRV measurements, every effort was made to keep the environmental conditions stable throughout data acquisition. All patients were in a temperature-controlled room in supine position. During HRV assessment, there were no interventions or changes in ventilator settings. As delirium is characterized by its fluctuating nature, only those measurements with patients scored delirious were included in the analysis.
All data were sampled at 500 Hz and monitored online (software Poly 5; Physiological Analysis Package, Inspector Research Systems, Amsterdam, The Netherlands). During off-line analyses, we verified all heartbeats detected by the software and manually corrected the software algorithm when necessary. From each 900 seconds of recording, a maximum of three 300-second time series were obtained that were devoid of artifacts and of ectopic heartbeats. When no 300-second period was available, we used at least 180-second periods, which has been described as sufficient.3,12 The power spectrum of the tachogram was computed using fast-Fourier transformation. For analysis of the power spectrum, fixed frequency bands were used, according to the guidelines of the Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology.3 Very LFs (VLFs) were defined as frequencies <0.04 Hz, low frequencies (LFs) as frequencies between 0.04 and 0.15 Hz, and high frequencies (HFs) as frequencies between 0.15 and 0.40 Hz.3 The area under the power spectral curve is considered the total power.3 Because of differences in spectral power between patients, HF power was converted to normalized units (nu) calculated as HFnu=HF/(total power – very LF) × 100. LFnu was not calculated as it is reciprocal to HFnu and would not provide additional information.3 It has been shown that the LF is a measure for activity of both the parasympathetic and the sympathetic system, whereas the HF and thus HFnu are solely measures of parasympathetic activity.3 The LF:HF ratio reflects sympathovagal balance and is therefore a measure for increased or decreased HRV.3
Statistical Analysis
Because the HRV frequency domain parameters are known to be skewed, we used log-transformed data of the LF power, HF power, and LF:HF ratio for statistical analysis. Differences between groups were assessed using Fisher’s exact test, Student t test, Kruskal-Wallis test, or Mann-Whitney U test where appropriate. Multivariable linear regression was used to evaluate the effect of delirium on HRV independent from age, gender, and severity of disease. Statistical tests were performed against two-sided alternatives, and p<0.05 was defined as being significant. Statistical analysis was performed using R version 3.0.1 and SPSS (SPSS 20; IBM, New York, NY).
Results
A total of 726 patients were screened, of whom 694 were excluded for analysis. In total, 32 patients were enrolled for HRV recording. In two patients, no 180-second intervals could be obtained due to frequent ectopic heartbeats. After careful examination of the medical charts, we had to exclude five patients from our study population due to a history of myocardial infarction (N=1) or coronary artery disease (N=3) and because one patient was about to develop a new septic episode (N=1). Therefore, our final study population included 13 patients with delirium and 12 patients without delirium. Patients with delirium were slightly older, had slightly higher Sequential Organ Failure scores, and included more women than patients without delirium, although this did not reach statistical significance. Other clinical characteristics of the study population did not differ between the two groups, as shown in Table 1. Within the 13 delirious patients, hypoactive delirium was the most common motoric subtype, present in seven patients (54%). Hyperactive delirium was present in two patients (15%), and in four patients (31%), mixed-typed delirium was observed.
| Clinical Characteristics | No Delirium (N=12) | Delirium (N=13) | Statistical Test (df) | p | ||
|---|---|---|---|---|---|---|
| Age (years), mean (SD) | 57 | (16) | 67 | (12) | t(23)=–1.76 | 0.09 |
| Male gender, N (%) | 11 | (92%) | 7 | (54%) | b | 0.07 |
| BMI, kg/m2, median (IQR) | 25 | (24–26) | 25 | (23–26) | MWU(25)=78.5 | 0.98 |
| APACHE IV, median (IQR) | 74 | (37–82) | 74 | (62–85) | MWU(25)=96.0 | 0.35 |
| CCI, median (IQR) | 1 | (0–3) | 1 | (0–3) | MWU(25)=76.0 | 0.94 |
| Admitting discipline, N (%) | ||||||
| General medicine | 7 | (58%) | 5 | (38%) | b | 0.43 |
| Surgery | 5 | (42%) | 8 | (62%) | ||
| ICU LOS at study inclusion, median (IQR) | 4 | (3–8) | 9 | (4–22) | MWU(25)=103.5 | 0.17 |
| Mean SOFA during measurements, mean (SD) | 3 | (2) | 5 | (2) | t(23)=–1.68 | 0.11 |
| Mechanical ventilation during measurements, N (%) | 6 | (50%) | 9 | (69%) | b | 0.43 |
| Agitation during measurements, N (%) | 3 | (25%) | 6 | (46%) | b | 0.41 |
| Sedation during measurements, N (%) | 2 | (16.6%) | 3 | (23%) | b | 1.00 |
| Median RASS during measurements, median (IQR) | 0 | (0–0) | –0.5 | (–1.5–1.0) | MWU(25)=77.0 | 0.98 |
| Delirium subtype, N (%) | ||||||
| Hypoactive delirium | — | 7 | (54%) | |||
| Hyperactive delirium | — | 4 | (31%) | |||
| Mixed-type delirium | — | 2 | (15%) | |||
| Admitting diagnosis, N (%) | ||||||
| Primary respiratory failure | 2 | (16.7%) | 2 | (15%) | ||
| Pneumonia | 2 | (16.7%) | 3 | (23%) | ||
| Hemorrhage | 3 | (25%) | 3 | (23%) | ||
| Trauma | 2 | (16.7%) | 1 | (8%) | ||
| Postoperative GI surgery | 1 | (8.2%) | 1 | (8%) | ||
| Infection, other | 2 | (16.7%) | 3 | (23%) | ||
a
APACHE: Acute Physiology and Chronic Health Evaluation; BMI: body mass index; CCI: Charlson Comorbidity Index; df: degree(s) of freedom; GI: gastrointestinal; ICU: intensive care unit; IQR: interquartile range; LOS: length of stay; MWU: Mann-Whitney U test statistic; RASS: Richmond Agitation and Sedation Score; SD: standard deviation; SOFA: Sequential Organ Failure Assessment.
b
Proportions were compared with Fisher’s exact test.
Patients with delirium and those without delirium had a similar respiratory rate during the HRV measurements (t(23)=−0.23, p=0.82). There was no difference in mean (±standard deviation) HFnu between patients with delirium (67±20) and without (57±21; t(23)=−1.18, p=0.25; Figure 1A). Further, no significant differences in log-transformed HRV parameters were found between patients with and without delirium: mean (±standard deviation) LF (2.8±1.9 and 3.1±1.9, respectively; t(23)=0.31, p=0.76), HF (3.7±1.7 and 3.4±2.1, respectively; t(23)=−0.34, p=0.73), and LF:HF ratio (after log transformation: −0.7±1.0 and −0.1±1.1, respectively; t(23)=1.46, p=0.16; before log transformation, see Figure 1B). Multivariable regression analysis showed no influence of delirium on HFnu or LF:HF ratio with coefficients (95% confidence interval) of 11.9 (−7.2 to 30.9) and −0.7 (−1.6 to 0.3), respectively. No differences in HFnu and LF:HF ratio were observed between delirium subtypes (h(2)=2.44, p=0.30 and h(2)=1.97, p=0.37; Figure 2).
FIGURE 1. Heart Rate Variability (in HFnu [A] and LF:HF ratio [B]) in Intensive Care Unit Patients With and Without Deliriuma
a HF: high frequency; ICU: intensive care unit; LF: low frequency; nu: normalized unit.3
FIGURE 2. Heart Rate Variability (in HFnu [A] and LF:HF ratio [B]) in Intensive Care Unit Patients With Different Subtypes of Deliriuma
a HF: high frequency; ICU: intensive care unit; LF: low frequency; nu: normalized unit.3
Discussion
This study suggests that HRV is not different between ICU patients with and without delirium. These findings contrast with our hypothesis of a decrease in HRV. Our hypothesis was based on the phenomenology of delirium, which suggests altered sympathovagal balance. Moreover, a decreased HRV is associated with increased vagus nerve activity, which limits the innate immune response.13,14 Excessive or persistent activation of this cholinergic anti-inflammatory pathway might lead to an immune-suppressed state that renders the body vulnerable to infection and therefore increased mortality.13,14
To our knowledge, this is the first study assessing HRV in delirium. As our main focus was on exploring altered sympathovagal balance in delirium, we excluded all patients with known disturbed autonomic function. Because of this homogeneity, we could be sure that any difference in HRV could only be explained by the presence of delirium. However, these exclusions, such as sepsis and the use of vasopressors, impairs the generalizability to a general ICU population. We cannot exclude the possibility that our study is underpowered due to the small number of included patients. The sample size was based on previous studies on HRV in other diseases.15,16 The power of our study with an HFnu effect size of 10±20, α=0.05, and sample size of N=15 is, however, 0.49. Due to our exclusion criteria, we expected much less variance between individuals and thus lower standard deviations. For example, with an HFnu effect size of 10±13.5, instead of the found 10±20 and α=0.05, 15 patients in both groups would have been sufficient to reach a power of 0.80. However, our study is important for generating hypotheses. In our study, patients with delirium were on average slightly older, included more women, and had higher Sequential Organ Failure scores. Increased age has been linked to higher LF:HF ratios,17 whereas female gender and higher severity of disease have been associated with lower LF:HF ratios.4,17 Importantly, when we adjusted for age, gender, and severity of disease, our findings did not change.
This first explorative study on HRV and delirium suggests that sympathovagal balance seems not to be altered in delirium.
Acknowledgments
The authors thank S. Verhoeven, M.Sc., Department of Intensive Care Medicine, University Medical Centre Utrecht, Utrecht, The Netherlands, for contributing to the screening and inclusion of patients.
References
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Information & Authors
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Received: 18 September 2013
Revision received: 6 December 2013
Revision received: 25 February 2014
Accepted: 7 March 2014
Published online: 26 December 2014
Published in print: Spring 2015
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