What causes sudden but limited changes in heart rate variability (HRV)?

From the Guidelines

Sudden but limited changes in heart rate variability (HRV) are most commonly caused by autonomic influences, including shifts in the balance between sympathetic and vagal tone, as evidenced by studies such as 1 and 2. These changes can be triggered by various factors, including:

• Physical exertion
• Emotional stress
• Breathing patterns
• Postural changes
• Consumption of substances like caffeine or alcohol Normal daily activities such as standing up quickly, experiencing a sudden emotional reaction, or taking a deep breath can temporarily alter HRV. Environmental factors like temperature changes or loud noises may also trigger brief HRV fluctuations. These changes are typically part of the body's normal autonomic nervous system response, where the sympathetic ("fight-or-flight") and parasympathetic ("rest-and-digest") systems work to maintain homeostasis. The vagus nerve plays a crucial role in this regulation, with increased vagal tone generally associated with higher HRV, as noted in 1 and 2. While these limited changes are usually benign and represent normal physiological adaptation, persistent abnormal patterns in HRV might warrant medical attention as they could indicate underlying cardiovascular or autonomic nervous system issues. It's also important to consider that high-intensity exercise can trigger malignant ventricular arrhythmias, leading to cardiac arrest and sudden cardiac death among at-risk individuals, as discussed in 3. However, the most recent and highest quality study 3 suggests that exercise-induced changes in HRV are a complex phenomenon, influenced by various factors, including the nature of the underlying predisposition, such as genetic heart diseases, and the effect of exercise on the heart. Therefore, it is essential to consider the individual's overall health and medical history when interpreting changes in HRV, and to prioritize the single most recent and highest quality study, such as 3, when making definitive recommendations.

From the FDA Drug Label

CLINICAL PHARMACOLOGY Mechanism of Action Metoprolol is a beta 1-selective (cardioselective) adrenergic receptor blocker. Animal and human experiments indicate that metoprolol slows the sinus rate and decreases AV nodal conduction In several studies of patients with acute myocardial infarction, intravenous followed by oral administration of metoprolol caused a reduction in heart rate, systolic blood pressure and cardiac output

The cause of sudden but limited changes in heart rate variability is not directly addressed in the provided drug label. Beta blockers like metoprolol can affect heart rate, but the label does not provide information on sudden changes in heart rate variability. 4

From the Research

Causes of Sudden but Limited Changes in Heart Rate Variability

• Sudden but limited changes in heart rate variability can be caused by various factors, including: + Autonomic nervous system function, which can be affected by sympathetic and parasympathetic nervous system activity 5, 6 + Pathological conditions, such as coronary artery disease, essential hypertension, and diabetic neuropathy 5, 6 + Physiological factors, such as age and physical exercise 5, 6, 7 + Psychological factors, such as mental performance tasks and stress 6, 7 + Environmental factors, such as lifestyle and genetic factors 6 + Medications, such as beta-blockers, which can affect heart rate variability by reducing sympathetic tone and increasing parasympathetic tone 8, 9, 7

Effects of Beta-Blockers on Heart Rate Variability

• Beta-blockers, such as metoprolol and atenolol, can improve heart rate variability by reducing sympathetic tone and increasing parasympathetic tone 8, 9, 7
• The effects of beta-blockers on heart rate variability can vary depending on the specific medication and dosage used 8, 9
• Beta-blockers can also reduce the negative effects of stress on heart rate variability, such as during mental performance tasks and physical exercise 7

Clinical Implications

• Heart rate variability analysis can be used as a monitoring tool in clinical conditions with altered autonomic nervous system function, such as postinfarction and diabetic patients 5
• Individual therapy adjustments to achieve the most favorable sympathetic-parasympathetic balance may be possible using heart rate variability analysis 5
• The use of beta-blockers, such as metoprolol and atenolol, can be beneficial in improving heart rate variability and reducing the risk of sudden cardiac death in certain patient populations 8, 9, 7