Heart Rate Variability Following Ischemic Stroke
To the Editor:
We have read with interest the article by Korpelainen and colleagues1 regarding heart rate variability (HRV) in patients with ischemic stroke. The authors report impaired beat-to-beat HRV as measured by traditional spectral components and impaired long-term continuous HRV as measured by SD2 PoinCare plots in patients with hemispheric and medullary brain stem but not pontine brain stem stroke compared with control subjects. These findings raise a number of important questions regarding cardiovascular autonomic control after acute ischemic stroke.
First, previous work from our department has not identified impairment of HRV, whether assessed by time domain2 or frequency domain3 techniques, in keeping with the authors’ own findings with regard to SD1 Poincare, complexity, and fractal measures of HRV.1 Furthermore, absolute levels and variability of blood pressure (BP) are an important consequence of abnormalities of cardiovascular autonomic control after acute ischemic stroke4 and an important variable in the interpretation of other cardiovascular data. It is therefore unfortunate that the article does not present the data in respect of BP variability and BP matching between study and control populations.
Second, if impaired HRV is a reproducible finding after acute ischemic stroke, it would be important to speculate on the underlying mechanism. One presumes that it reflects sympathetic hyperactivity, though the low-frequency power is reported as significantly lower in those groups with impaired HRV.1 Of course, this measure has considerable variability, and it would be more important to compare either the normalized low-frequency power or the low-frequency/high-frequency ratio.
Third, it is interesting to note the authors’ findings with respect to stroke site. No significant difference in HRV was identified between right- and left-hemisphere strokes. This is an important observation if sympathetic hyperactivity is the only explanation for impaired HRV, given the reported right hemisphere dominance for sympathetic effects.4 Furthermore, the authors report differences between medullary and pontine brain stem strokes, in contrast to their own previous observation that patients with both medullary and pontine infarcts had a more pronounced suppression of HR responses to parasympathetic stimuli than hemisphere strokes.5
Finally, frequency domain analysis of cardiovascular parameters can be difficult in poststroke patients, particularly over prolonged recording periods because of the well-recognized increased ventricular ectopy rate.4 It is unclear from the article how the authors accepted the data for subsequent analysis on the basis of ectopy and spike rates, which is clearly an important factor in the subsequent interpretation of the findings.
It is clear that cardiovascular autonomic dysfunction is an important aspect of acute stroke. However, further studies should address its clinical relevance in terms of prognosis and in the dilemma surrounding the therapeutic manipulation of BP in acute stroke.
- Copyright © 1999 by American Heart Association
Korpelainen J, Sotaniemi K, Mäkikallio A, Huikuri H, Myllylä V. Dynamic behavior of heart rate in ischemic stroke. Stroke.. 1999;30:1008–1013.
Robinson T, James M, Youde J, Panerai R, Potter J. Cardiac baroreceptor sensitivity is impaired after acute stroke. Stroke.. 1997;28:1671–1676.
Klingelhofer J, Sander D. Cardiovascular consequences of clinical stroke. Clin Neurol.. 1997;6:309–335.
Korpelainen J, Sotaniemi K, Suominen K, Tolonen U, Myllylä V. Cardiovascular autonomic reflexes in brain infarction. Stroke.. 1994;25:787–792.
We thank Drs Robinson, Panerai, and Potter for their interest in our recent article concerning abnormalities of HR regulation following ischemic stroke.R1 Comments of this research group with known interest in cardiovascular autonomic regulation are valuable.
First, they ask about absolute levels and variability of BP of our patients. We agree that stroke-related abnormalities of BP are interesting and clinically important, reflecting dysfunction of the cardiovascular autonomic regulatory system. During the study, however, no suitable equipment was available for ambulatory BP measurements in our laboratory. Therefore, data about BP variability could not be presented.
The pathomechanisms of impaired HR variability are still unclear and speculative. Our view is that its etiology is multifactorial, including at least neuronal as well as hormonal components. We have also calculated the low-frequency/high-frequency power ratio and presented the data in one of our previous works.R2 This ratio is not generally accepted, however, and we decided to exclude it from the last study.
A lot of clinical and experimental data are available for cortical asymmetry in the regulation of HR and other cardiovascular functions. Although our research group has failed to show such an asymmetry using measurements of HR and BP, there has been a trend toward more pronounced abnormalities in patients with right-sided strokes.R1 R2 R3 R4 R5 Further experimental and human studies with larger numbers of stroke patients that we have presented may solve this problem.
We are all well aware of the technical problems related to long-term ambulatory ECG recordings. All the artifacts and ectopic beats were first deleted automatically and then manually with our customized software. All the segments (8000 R-R intervals) with less than 15% sinus beats were excluded from the final analysis.
We agree that further studies should focus on the clinical relevance of HR and BP variability in order to create new therapeutic strategies.
Korpelainen JT, Sotaniemi KA, Mäkikallio A, Huikuri HV, Myllylä VV. Dynamic behavior of heart rate in ischemic stroke. Stroke.. 1999;30:1008–1013.
Korpelainen JT, Sotaniemi KA, Huikuri HV, Myllylä VV. Abnormal heart rate variability as a manifestation of autonomic dysfunction in hemispheric brain infarction. Stroke.. 1996;27:2059–2063.
Korpelainen JT, Sotaniemi KA, Suominen K, Tolonen U, Myllylä VV. Cardiovascular autonomic reflexes in brain infarction. Stroke.. 1994;25:787–792.
Korpelainen JT, Sotaniemi KA, Huikuri HV, Myllylä VV. Circadian rhythm of heart rate variability is reversibly abolished in ischemic stroke. Stroke.. 1997;28:2150–2154.