Head-up tilt test — indication, procedure and interpretation

Principle. Head-up tilt testing reproduces the haemodynamic stress of standing under controlled, monitored conditions. Passive upright tilt causes venous pooling of about 500–800 mL of blood in the lower limbs and splanchnic bed, reducing venous return and cardiac output. In a susceptible individual this triggers the Bezold– Jarisch reflex — vigorous contraction of an underfilled ventricle activates mechanoreceptors, producing paradoxical sympathetic withdrawal and vagal activation with vasodilatation and/or bradycardia, and hence syncope. Increasing use has been made of tilt testing to obtain evidence supporting a suspected diagnosis of vasovagal syncope and to investigate the underlying mechanisms.

Indications. (i) Unexplained recurrent syncope after cardiac causes have been excluded, particularly where vasovagal syncope is suspected but the history is not diagnostic. (ii) To distinguish reflex syncope from orthostatic hypotension and to demonstrate delayed orthostatic hypotension that standing measurements miss. (iii) Unexplained falls or drop attacks in the elderly, where there is no recall of loss of consciousness. (iv) To differentiate syncope from epilepsy or psychogenic pseudosyncope — the semiology of tilt-induced psychogenic pseudosyncope is characteristic, and testing may be combined with video-EEG. (v) To demonstrate susceptibility for the purpose of patient education and counter-manoeuvre training. (vi) In suspected postural orthostatic tachycardia syndrome or autonomic failure. Contraindications: severe aortic stenosis, critical coronary or cerebrovascular stenosis, hypertrophic obstructive cardiomyopathy, pregnancy and inability to stand safely.

Procedure. The patient is fasted for 2–4 hours, and vasoactive drugs are withheld where safe. They lie on an electrically driven tilt table with a footboard, secured with straps, with continuous ECG and beat-to-beat non-invasive blood pressure monitoring and resuscitation facilities available. There is a supine stabilisation phase of 5–20 minutes, then passive tilt to 60–70° for 20–45 minutes. If nothing occurs, a provoked (drug) phase follows: the proportion of positive tilt tests is increased following the administration of glyceryl trinitrate (400 µg sublingual, tilt maintained for a further 15–20 minutes), or isoprenaline may be used. Carotid sinus massage is often performed as part of the same assessment, supine and erect, since carotid sinus syndrome is a common cause of syncope in the elderly and is not confined to those with a history of fainting on neck movement; it carries a very low complication risk despite underlying atheroma, but must not be performed if there is a carotid bruit unless ultrasound has excluded severe stenosis. The table is returned to horizontal as soon as syncope or presyncope occurs. The test has been shown to be safe in the elderly, including with glyceryl trinitrate provocation, over 10 years of experience.

Interpretation — a positive test. The test is positive when symptoms are reproduced together with hypotension and/or bradycardia. Provocation of the patient's habitual attack is diagnostically very suggestive. Classify the response (VASIS classification):

Limitations. The major difficulty in interpreting the results is the occurrence of positive tests in a significant minority of control subjects, which limits the positive predictive value. Sensitivity is around 50–70% and specificity around 80–90% for passive tilt, with sensitivity improving but specificity falling after nitrate provocation. Reproducibility is imperfect — a negative test does not exclude reflex syncope. Crucially, the type of response on tilt does not reliably predict the mechanism of the patient's spontaneous attacks, which is why an implantable loop recorder is often preferred where the diagnosis will change management. The test is also time-consuming and labour-intensive, and in frail older patients prolonged standing may be poorly tolerated. It is also worth noting that a postural fall in blood pressure of more than 20 mmHg is present in about 30% of a large elderly population, so an abnormal result must always be interpreted alongside the clinical history.

Clinical use of the result. A positive test confirms susceptibility, allows patient education, and justifies conservative treatment — recognition of prodromal symptoms, avoidance of triggers (heat, prolonged standing, dehydration, alcohol, large meals), physical counter-pressure manoeuvres (leg crossing, calf tensing, abdominal compression), adequate salt and fluid intake, and compression stockings. Drug options where these fail include fludrocortisone, midodrine and, in some patients, beta-blockers. Pacing is reserved for documented cardioinhibitory responses with prolonged asystole in older patients with recurrent injurious syncope — but note that pacing does not improve hypotension in patients with severe orthostatic hypotension, so it is not a treatment for the vasodepressor form. In carotid sinus syndrome with a cardioinhibitory response, pacing reduces falls (SAFE PACE). Finally, the risks of serious injury from falls in an elderly population are very high, so the patient must be counselled to take precautions to avoid attacks.

The diagnostic role of tilt testing has narrowed as prolonged rhythm monitoring has become easier and cheaper. Current practice, consistent with the 2018 ESC syncope guideline and the Canadian Cardiovascular Society clinical practice update, is that in recurrent unexplained syncope where a cardiac mechanism is suspected, an implantable loop recorder gives a far higher diagnostic yield than tilt testing and should be used early rather than as a last resort, because — unlike tilt — it documents the mechanism of the spontaneous attack. Tilt testing has correspondingly been repositioned as a test principally used to demonstrate susceptibility to reflex syncope for patient education, to unmask delayed orthostatic hypotension, and to diagnose psychogenic pseudosyncope — the last being an indication where it remains uniquely valuable, especially when combined with video and EEG recording. Alongside this, smartphone and smartwatch single-lead ECG recordings are increasingly used to capture symptom–rhythm correlation before more invasive testing is arranged.