# Heart Rate Variability > Heart rate variability in MBBS Physiology: respiratory sinus arrhythmia, SDNN and frequency domains, Ewing's tests in diabetes. - Canonical URL: https://prepelephant.com/topics/mbbs/physiology/heart-rate-variability - Exam / course: MBBS · Subject: Physiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Heart Rate Variability", PrepElephant, https://prepelephant.com/topics/mbbs/physiology/heart-rate-variability ## Direct answer Breathe in and your heart rate rises; breathe out and it falls — respiratory sinus arrhythmia, the visible face of heart rate variability (HRV), the beat-to-beat fluctuation in R-R interval produced by continuous vagal and sympathetic modulation of the sinoatrial node. Quantified on ECG, time-domain measures include SDNN (standard deviation of all N-N intervals over 24 hours; roughly 141 ms in health, with values below 50 ms markedly abnormal) and RMSSD (a pure vagal index), while frequency-domain analysis separates high-frequency power (0.15-0.4 Hz, parasympathetic) from low-frequency power (0.04-0.15 Hz, mixed baroreflex) — the LF/HF ratio being a rough sympathovagal balance. Reduced HRV predicts mortality after myocardial infarction and is the earliest subclinical marker of diabetic cardiac autonomic neuropathy, detected at the bedside by Ewing's battery: heart rate variation with deep breathing, the Valsalva ratio, and the 30:15 ratio on standing. ## What you must remember - **Physiological basis:** inspiration accelerates the heart via vagal withdrawal (and slight sympathetic gain); expiration restores vagal tone — the rhythm is vagally mediated, abolished by atropine and blunted by age. - **Time-domain indices:** SDNN (overall variability, needs 24-hour recording; about 141 plus or minus 39 ms in normal adults, below 50 ms markedly abnormal), RMSSD and pNN50 (both vagal, short recordings acceptable). - **Frequency-domain indices:** HF power 0.15-0.4 Hz equals vagal modulation; LF power 0.04-0.15 Hz reflects baroreflex-mediated sympathetic and vagal mix; the LF/HF ratio is a crude balance index — quote it as commonly used but simplistic. - **Ewing's battery cutoffs:** expiration-to-inspiration ratio on deep breathing abnormal below about 1.10-1.20 (age-dependent); Valsalva ratio abnormal below about 1.20; 30:15 ratio on standing abnormal below about 1.03; orthostatic systolic fall above 30 mmHg abnormal; diastolic rise on sustained handgrip below 10 mmHg abnormal. - **Prognostic punchline:** post-infarction SDNN below 50-70 ms carries several-fold higher mortality — HRV is among the strongest Holter-derived predictors, ahead of ventricular ectopy in several cohorts. - **Diabetes cascade:** heart rate variation with deep breathing is the first test to fail in type 1 diabetes — cardiac autonomic neuropathy detected sometimes within a year or two of diagnosis, long before symptoms such as resting tachycardia or postural hypotension. - **Clinical therapeutics angle:** cardiac rehabilitation, yoga and beta-blockade raise HRV — measurable autonomic benefit, a favourite integrative answer in Indian vivas. ## A worked case in diabetes A 34-year-old with twelve years of type 1 diabetes is screened annually. The nurse counts the ECG R-R change while he breathes deeply at six breaths per minute: the ratio has drifted from 1.25 three years ago to 1.08 now — early parasympathetic damage. The Valsalva ratio is 1.15 (abnormal), the 30:15 ratio 1.02 (borderline), and lying-to-standing systolic fall is 15 mmHg (normal). Two abnormal heart-rate tests plus one borderline: early cardiac autonomic neuropathy, at a stage when the patient has no symptoms whatsoever. The physiology explains the silent progression: the longest vagal fibres to the SA node are damaged first, so resting heart rate creeps up (loss of vagal brake) before orthostatic sympathetic failure produces dizziness. Management is preventive — rigorous glycaemic control, foot and cardiovascular risk care, and avoidance of drugs that worsen autonomic failure. The same annual numbers also guide anaesthetic risk: blunted HRV predicts intraoperative hypotension, which is why the physician examination (not just the cardiologist's) asks for these ratios. ## Where students slip The first error is treating HRV as a heart property rather than an autonomic readout — the numbers are neural signals displayed on ECG; atropine abolishes respiratory arrhythmia while beta-blockers leave the vagal component largely intact, and knowing which drug removes which component is the standard probe. The second is cutoff confusion: the Ewing thresholds are age-adjusted, so a ratio of 1.15 may be normal at 70 and abnormal at 30 — say "age-adjusted" and the viva stays friendly. Third, students equate a high LF/HF ratio with sympathetic overactivity; current consensus treats it as a rough, contested index — quoting it with that caveat reads as current rather than parroted. ## Frequently asked questions ### What physiological mechanism underlies respiratory sinus arrhythmia? Inspiratory vagal withdrawal accelerates the sinoatrial node and expiratory vagal restoration slows it — a vagally mediated oscillation abolished by atropine and dampened by ageing. ### Which HRV index best reflects vagal activity? RMSSD and high-frequency power (0.15-0.4 Hz) are the cleanest parasympathetic markers, usable even on short recordings, unlike the 24-hour SDNN that quantifies overall variability. ### What are Ewing's tests and their use? A five-test bedside battery for cardiac autonomic neuropathy — deep-breathing heart rate variation, Valsalva ratio, 30:15 standing ratio, orthostatic blood pressure and sustained handgrip response — standard in diabetes follow-up. ### Why does HRV predict post-infarction mortality? Depressed variability signals reduced vagal and overall autonomic regulation of the heart, correlating with electrical instability and ventricular arrhythmia risk; SDNN below 50 ms marks a high-risk group. ### How does diabetes affect heart rate variability earliest? The longest vagal efferents fail first, so deep-breathing R-R variation falls years before resting tachycardia or postural hypotension appear — making it the earliest subclinical marker of cardiac autonomic neuropathy.