Pressure Half Time (PHT) in Aortic Regurgitation

Pressure half-time (PHT) is a core echocardiographic parameter for assessing the severity of aortic regurgitation (AR). It measures the time it takes for the peak diastolic pressure gradient between the aorta and the left ventricle (LV) to drop to half of its initial value.

Because Doppler ultrasound measures velocity rather than pressure directly, we rely on the simplified Bernoulli equation (ΔP = 4V²). Therefore, PHT is measured on the Continuous Wave (CW) Doppler trace as the time it takes for the peak velocity (V_max) to fall to V_max/√2.

The Hemodynamic Mechanism

The slope of the AR jet’s deceleration profile reflects the rate at which aortic and LV diastolic pressures equalize.

• Mild AR: A small regurgitant volume slowly leaks into the LV. The pressure gradient falls gradually throughout diastole, creating a flat CW deceleration slope and a long PHT.
• Severe AR: A large volume of blood rapidly empties from the aorta into the LV. Aortic diastolic pressure drops quickly, while LV end-diastolic pressure (LVEDP) rises sharply. This rapid pressure equalization creates a steep deceleration slope and a short PHT.

Echocardiographic Grading Guidelines

According to the guidelines, PHT values correlate with severity as follows:

AR Severity	Pressure Half-Time (PHT)
Mild	> 500 ms
Moderate	200 – 500 ms
Severe	< 200 ms

Clinical Nuances and Caveats

While highly useful, PHT is completely dependent on chamber compliance and systemic hemodynamics, not just the regurgitant orifice area. Relying on PHT in isolation can lead to misdiagnosis in specific clinical scenarios:

• Acute Severe AR: The LV has not had time to adapt and dilate. It is small and non-compliant. The sudden volume load causes LVEDP to skyrocket instantly, resulting in an exceptionally steep slope and a very short PHT (often < 150 ms), alongside early closure of the mitral valve.
• Chronic Severe AR with a Dilated LV: If the LV has undergone massive eccentric hypertrophy and is highly compliant, it can absorb a massive regurgitant volume without a rapid rise in diastolic pressure. This slow pressure equalization can falsely prolong the PHT, making severe AR appear moderate on the CW trace.
• Vasodilator Therapy: Reducing systemic vascular resistance (SVR) lowers aortic diastolic pressure, which can flatten the slope and artificially lengthen the PHT, masking the true severity of the regurgitation.
• Elevated LVEDP from other causes: Conditions like severe diastolic dysfunction or ischemia that elevate LVEDP independently of the AR can artificially shorten the PHT.

Because of these variables, PHT should always be corroborated with quantitative measures like the Vena Contracta width, Effective Regurgitant Orifice Area (EROA), and the presence of holodiastolic flow reversal in the descending aorta.