IV determines if cardiovagal reflexes are intact. Decreases of greater than 20 mmHg during early phase II combined with absent phase IV or late phase II are indicative of blunted vasoconstrictive response. Equal inspirations and expirations at rates of 6 breaths per minute timed by a metronome are used. Cardiovagal inhibition and activation are tested. The difference between maximum and minimum heart rate for each cycle can be obtained and averaged over the sampling period as an index of the vagally mediated respiratory arrhythmia.
Blood pressure has a similar low frequency peak but much attenuated high frequency power.
Sympathetic blood pressure modulation is transduced into changes in heart rate through the actions of the vagal arm of the sinoaortic baroreflex. Thus the low frequency component relates both to parasympathetic and sympathetic factors. From the relation between variation in blood pressure and variation in heart rate we can compute a relation that reflects the action of the baroreflex based on transfer function analysis. Such a relation yields the change in heart rate with change in blood pressure, or equivalently, the vagal arm of the baroreflex as shown on the left. These findings are consistent of a state of vagal withdrawal as may be seen in dehydration and related states and can be reliably reproduced through classic vagolytic experiments with agents such as atropine.