Cardiac ultrasound is one of the more common adjuncts to cardiology, so how should a clinician properly interpret an ultrasound report?
Understanding the basic principles of cardiac ultrasound and the anatomical structures shown in each ultrasound section can help us understand the ultrasound report more easily. (Click on the blue word to view directly)
In this article, we will talk about the key points of the report one by one (note: there is no uniform standard for the size of some diameters of the heart, so don’t get too hung up on them, so I won’t go into details in this article).
1
Aortic valve
Aortic valve: normal velocity ＜ 2 m/s, normal orifice area 3-4 cm².
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Image credit: Courtesy of the authors
In patients with aortic stenosis (AS), it is necessary to know its orifice velocity, pressure difference and area, according to which the degree of stenosis is determined (table below), which is important for whether to treat it surgically or not.
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Of course, there are cases in which the orifice area does not match the pressure difference, such as patients with reduced left ventricular systolic function, high peripheral blood pressure, small body size, etc.
In patients with excessive blood pressure, the afterload pressure is too high, which can lead to a reduced transvalvular pressure difference and may overestimate the orifice area. It is recommended that such patients be reviewed after strict blood pressure control to avoid delaying treatment.
In smaller patients, the normal orifice area is often less than 3 cm², and a combination of factors is needed to determine the degree of stenosis.
The absence of main valve stenosis can be seen in patients with greater than 2 m/s: anemia, hyperthyroidism, severe aortic valve insufficiency, and hypertrophic obstructive cardiomyopathy.
2
Pulmonary valve
Pulmonary valve: normal < 2 m/s.
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Image credit: Courtesy of the author
In pulmonary stenosis: < 3 m/s considered mild, 3 to 4 m/s considered moderate, > 4 m/s considered severe.
Pulmonary valve velocities are generally less than aortic valve velocities due to pressure differences.
Pulmonary valve velocities greater than the main valve velocity are commonly seen in: pulmonary stenosis, left-to-right shunt precordial disease (atrial defect, ventricular defect, pulmonary venous malformation drainage, etc.).
3
Mitral valve
Mitral valve: normal orifice area 4 to 6 cm².
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Image credit: Courtesy of the authors
Mitral valve orifice area is measured in two main ways, either by direct sketching of a 2D image showing a short-axis view of the ventricle or by calculation using the pressure difference halving Time.
The pressure halving time (PHT) is the time required for the maximum pressure difference between the left atrium and the left ventricle to decrease by half in early diastole. It has been found that in patients with mitral stenosis, the degree of stenosis is inversely proportional to PHT, which leads to the empirical formula, MVA (cm²) = 220/PHT.
This formula is only applicable to natural valve orifice area calculations and cannot be applied to the orifice area of prosthetic valves. It is also inaccurate in patients with rapid heart rate, irregular heart rate, and combined aortic regurgitation.
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4
Left ventricular ejection fraction
Left ventricular ejection fraction (LVEF) = (end-diastolic volume – end-systolic volume)/end-diastolic volume. The normal value is 50-60%, with <50% being considered reduced systolic function.
M-mode ultrasound was previously used to measure LVEF, but it is no longer recommended because of the many factors that affect it and the large measurement error. The preferred method is now biplane Simpson formula measurement.
The shape of the left ventricular cavity is sketched on the ultrasound 2D image, and then the left ventricular cavity is imagined as an irregular potato, which is cut into several slices, calculated in slices and then added together to give the left ventricular volume by a computer program.
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Image credit: Courtesy of the author
When a patient has severe mitral regurgitation, some of the ejected blood from the left ventricle flows back into the left atrium; or in severe aortic regurgitation, the blood ejected into the aorta returns to the left ventricle in diastole; or in patients with obstruction in the hypertrophic heart, the heart is working extra hard with each stroke, but the volume of ejected blood is not sufficient because of where the obstruction is located.
All of these conditions will result in a reduction in the amount of blood that can eventually enter the circulation, and the patient will experience the corresponding symptoms, even if the ultrasound indicates a normal LVEF, it does not mean that the patient is safe.
In the case of hyperthyroidism, anemia, and use of cardiac drugs, LVEF > 60% may occur.
5
Left ventricular diastolic function
EA peak: two peaks in the mitral diastolic flow spectrum, corresponding to the period of rapid ventricular filling and atrial systole, respectively.
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Image credit: Courtesy of the authors
This also makes it clear that patients with atrial fibrillation report no “A-peak”. Without regular and effective atrial contraction, there is naturally no A-peak, and the E-peak will vary in size and frequency.
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Image source: Courtesy of the authors
EDT: The time from the E peak to zero velocity, with a normal value of 199 ± 32 ms.
EDT can be prolonged or shortened with loss of diastolic function. edT is strongly influenced by heart rate, shortening as heart rate increases and lengthening vice versa.
e’: The mitral annulus moves with the ventricle during systole and diastole. The velocity of mitral annular motion is measured by tissue Doppler techniques, i.e., s’ (systole), e’ (diastole, corresponding to the E peak), and a’ (diastole, corresponding to the A peak).
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Image credit: Courtesy of the authors
In patients with normal LVEF, left ventricular diastolic function is currently evaluated in combination with several indicators.
① mean E/e’ > 14.
② ventricular septal e’ < 7 cm/s or lateral wall e’ < 10 cm/s.
(iii) Maximum tricuspid regurgitation velocity > 2.8 m/s.
④ Left atrial volume index > 34 mL/m2.
Three of these criteria are considered diastolic insufficiency, two of them are considered in combination with other indicators, and one or none of them are considered normal diastolic function.
When the mitral valve is severely diseased, E and e’ cannot accurately reflect the left ventricular diastolic function, and other indicators need to be combined to evaluate.
6
Pulmonary hypertension
Ultrasound does not directly measure pulmonary artery pressure; it is estimated by a series of calculations.
The first step is to understand that blood flow is from the side of high pressure to the side of low pressure. Then understand a formula, the simplified Bernoulli equation, which is ΔP ≈ 4 × Vmax². That is, for a pressure difference on either side of a point equal to the square of the maximum velocity at that point multiplied by four, read it three times to understand.
Assuming a pulmonary artery flow velocity of about 1 m/s, the pressure difference is 1² x 4 = 4 mmHg, i.e., right ventricular systolic pressure – 4 = pulmonary artery systolic pressure, which can be further simplified to mean right ventricular systolic pressure ≈ pulmonary artery systolic pressure.
If tricuspid regurgitation occurs during right ventricular systole, we have right ventricular systolic pressure – systolic right atrial pressure = tricuspid regurgitation pressure difference = tricuspid regurgitation velocity ² x 4.
A quick spin of the brain yields: pulmonary artery systolic pressure ≈ tricuspid regurgitation velocity² x 4 + right atrial pressure.
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Image credit: Courtesy of the author
The right atrial pressure can be derived using an estimation method as follows.
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Ultrasound estimation of pulmonary artery pressure is subject to some measurement error, and a right heart catheterization can be performed to accurately determine pulmonary artery pressure in patients who require it.
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