Cardiovagal Reflexes

Cardiovagal reflexes assess the parasympathetic control of heart rate. Vagal nerve traffic’s final effect on heart rate is accessible by electrocardiography (ECG). Cardiovagal physiology has a complex basis; therefore, testing cardiovagal reflexes should be performed under controlled conditions, analyzed by a standardized method, and interpreted in light of potentially confounding variables, such as medications, sympathetic tone, and respiratory and heart rates that can alter the response. The most sensitive test is the magnitude of respiratory sinus arrhythmia in response to cyclical deep breathing. Another test of cardiovagal function is the Valsalva ratio, which is the ratio of the greatest to the least heart rate in response to respiratory straining. This ratio should always be measured in conjunction with beat-to-beat blood pressure recordings. Power spectral analysis is useful in assessing spontaneous heart rate variability. A number of less frequently utilized tests of cardiovagal function are also discussed.