The article by George E. Billman critically examines the use of the LF/HF ratio to measure cardiac sympatho-vagal balance. The LF/HF ratio, which compares the power at low frequencies (LF) to high frequencies (HF) in heart rate variability (HRV), is widely used to assess the balance between sympathetic and parasympathetic nervous system activities. However, this method has been challenged due to several limitations and assumptions that are not always met in physiological and pathological conditions. Key points include: 1. **Assumptions and Limitations**: The LF peak is often assumed to reflect sympathetic activity, while the HF peak reflects parasympathetic activity. However, these assumptions are overly simplistic and do not account for complex interactions between sympathetic and parasympathetic systems. 2. **Non-linear Interactions**: The relationship between sympathetic and parasympathetic activities is often non-linear and reciprocal, with interventions that increase one activity not necessarily decreasing the other. 3. **Mechanical and Respiratory Influences**: Mechanical factors and respiratory parameters can significantly affect HRV, independent of changes in cardiac autonomic nerve activity. 4. **Mathematical Considerations**: The LF/HF ratio can be influenced by mathematical manipulations, leading to distorted values, especially when both sympathetic and parasympathetic activities are minimal. 5. **Clinical and Experimental Evidence**: Studies have shown that the LF/HF ratio does not accurately reflect sympatho-vagal balance, as it can be affected by various factors such as exercise, myocardial ischemia, and mechanical events. In conclusion, the LF/HF ratio is not a reliable measure of cardiac sympatho-vagal balance due to the complex and non-linear interactions between sympathetic and parasympathetic systems, as well as the influence of mechanical and respiratory factors.The article by George E. Billman critically examines the use of the LF/HF ratio to measure cardiac sympatho-vagal balance. The LF/HF ratio, which compares the power at low frequencies (LF) to high frequencies (HF) in heart rate variability (HRV), is widely used to assess the balance between sympathetic and parasympathetic nervous system activities. However, this method has been challenged due to several limitations and assumptions that are not always met in physiological and pathological conditions. Key points include: 1. **Assumptions and Limitations**: The LF peak is often assumed to reflect sympathetic activity, while the HF peak reflects parasympathetic activity. However, these assumptions are overly simplistic and do not account for complex interactions between sympathetic and parasympathetic systems. 2. **Non-linear Interactions**: The relationship between sympathetic and parasympathetic activities is often non-linear and reciprocal, with interventions that increase one activity not necessarily decreasing the other. 3. **Mechanical and Respiratory Influences**: Mechanical factors and respiratory parameters can significantly affect HRV, independent of changes in cardiac autonomic nerve activity. 4. **Mathematical Considerations**: The LF/HF ratio can be influenced by mathematical manipulations, leading to distorted values, especially when both sympathetic and parasympathetic activities are minimal. 5. **Clinical and Experimental Evidence**: Studies have shown that the LF/HF ratio does not accurately reflect sympatho-vagal balance, as it can be affected by various factors such as exercise, myocardial ischemia, and mechanical events. In conclusion, the LF/HF ratio is not a reliable measure of cardiac sympatho-vagal balance due to the complex and non-linear interactions between sympathetic and parasympathetic systems, as well as the influence of mechanical and respiratory factors.