The Head-Up Tilt Test (HUTT) is a diagnostic procedure used primarily to investigate recurrent, unexplained fainting (syncope) or severe near-syncope episodes. Physicians typically perform this test when they suspect symptoms are due to orthostatic intolerance or Neurally Mediated Syncope (NMS), also known as vasovagal syncope. The goal of the HUTT is to provoke a temporary failure in the body’s autonomic control system, which regulates heart rate and blood pressure, to pinpoint the mechanism behind the patient’s symptoms.
Defining a Positive Tilt Test
A positive result is defined by the reproduction of the patient’s typical fainting symptoms, accompanied by specific, measurable changes in heart rate or blood pressure. The test moves the patient from a lying position to a near-vertical tilt (typically 60 to 80 degrees), simulating the stress of prolonged standing. The test is considered successful only if the patient reports symptoms like lightheadedness, nausea, or loss of consciousness that match their real-life events.
A positive outcome requires this symptomatic response to align with a significant, sudden drop in blood pressure (hypotension) or heart rate (bradycardia), or both. This correlation confirms the patient’s episodes are likely due to a malfunction in the reflex that normally maintains blood flow to the brain when upright.
Specific Numerical Criteria for Positive Results
The criteria for a positive result are based on precise numerical thresholds for drops in heart rate and blood pressure, though exact numbers vary slightly depending on the protocol used. A definitive drop in systolic blood pressure (SBP) is a common criterion, often defined as a fall below 80 millimeters of mercury (mmHg). Some protocols may trigger test termination if the SBP falls below 70 mmHg, even if the patient has not yet lost consciousness.
For heart rate, a positive result for a bradycardic response is typically marked by the heart rate dropping below 40 beats per minute (bpm) for ten seconds or longer. A more pronounced finding is complete asystole, where the heart stops beating for a period exceeding three seconds. These pronounced drops reflect the failure of the autonomic nervous system to maintain adequate cerebral perfusion.
Types of Vasovagal Responses Identified
A positive tilt test is classified into three primary types of Neurally Mediated Syncope (NMS), based on the dominant physiological mechanism of the failure. This classification guides the selection of the most appropriate management strategy. These types are categorized as cardioinhibitory, vasodepressor, or mixed.
Cardioinhibitory Response
This response is characterized by a significant slowing of the heart rate, often reaching the criteria for severe bradycardia or asystole. The primary problem is a sudden, excessive increase in vagal nerve activity, causing the heart to slow down or briefly stop. Pacemaker implantation may be considered for patients with recurrent syncope and a pronounced cardioinhibitory response, especially one involving asystole lasting several seconds.
Vasodepressor Response
The vasodepressor response is defined by a profound drop in blood pressure with little to no change in heart rate. This indicates a failure of the blood vessels to constrict, leading to excessive pooling of blood in the lower limbs and subsequent systemic hypotension.
Mixed Response
The mixed response is the most common type. It involves significant drops in both heart rate and blood pressure, sharing features of both the cardioinhibitory and vasodepressor types.
Next Steps Following a Positive Result
A positive tilt test confirms a diagnosis of vasovagal syncope or orthostatic intolerance, providing a clear path for management. Initial treatment involves non-pharmacological lifestyle adjustments, which are the most conservative and effective first steps.
Patients are typically advised on two main strategies:
- Increase daily fluid intake to two to three liters.
- Increase salt consumption, provided they do not have high blood pressure or heart failure.
- Learn physical counter-pressure maneuvers to use when feeling warning signs of fainting.
- Perform actions like crossing the legs and tensing muscles, or gripping hands and pulling them apart, to increase central blood volume and blood pressure.
If these conservative measures are insufficient to control recurrent episodes, pharmacological options may be considered. These include midodrine, which constricts blood vessels, or fludrocortisone, which helps the body retain salt and water.