Aortic Valve

Aortic Valve Anatomy

The aortic valve exists within the greater context of the aortic root. The root defines the structures between where the aortic valve cusps attach to the LVOT and the sinotubular junction distally. The aortic root is typically defined by a series of circumferential rings - the diameter of which can be easily measured with TEE - that define the aortic root anatomy. From proximal to distal, these include:

1. The aortic "annulus" - this is the virtual ring formed where the AV cusps meet the LVOT.

2. Anatomic ventriculo-aortic ring - 

3. The sino-tubular junction - 

The aortic valve itself is (typically) made up of three cusps, each with an associated sinus. The cusps and sinuses are named after their association with the ostia of the coronary arteries. The right coronary artery comes off of the right coronary sinus and the left coronary artery comes off of the left coronary sinus. The remaining sinus and cusps are named the "non-coronary" since there is no associated coronary artery.

The cusps are best identified from the aortic valve short axis view. The easiest to identify is often the non-coronary cusp, or "the non", which is always next to the inter-atrial septum. This will be at the top left of the display from the aortic short axis view (see below). The right coronary cusp is also easily identified as it will be next to the RVOT at the bottom of the display in the AV SAX view. The final cusp is the left coronary cusp.

From the AV long axis view, there are typically two coronary cusps that can be seen (one at the top of the AV and one at the bottom). The cusp at the bottom can be easily identified due to its proximity next to the RVOT in this view; this cusp is the right coronary cusp. The cusp that will be seen at the top of the valve cannot be confidently identified in this view alone, as it could be either the left or the non depending on how the ultrasound plane transects the aortic valve.

AV/LVOT Measurements

There are multiple measurements that are typically made from the AV long axis view which can be seen below. The most relevant for hemodynamic monitoring in the OR or ICU is the LVOT diameter, which is used along with the LVOT VTI to calculate stroke volume. If making this measurement it is important to spend a little bit of time optimizing the probe rotation and omniplane to ensure the diameter of the LVOT is maximized on the display. The LVOT radius will be squared later, so any inaccuracy in the initial measurement will disproportionately affect the calculated stroke volume.

Of note, it is important to make these measurements at the appropriate time in the cardiac cycle to ensure their validity. The LVOT diameter and diameter of the aortic annulus should be measured during mid-systole, when the aortic valve is open. The diameter of the aortic sinuses, sinotubular junction, and ascending aorta should be measured in diastole with the aortic valve closed.

Aortic Insufficiency

Physiology

As systole ends, the aortic valve closes and the left ventricle begins isovolumetric relaxation. This will generate a significant pressure gradient between the proximal aorta and the left ventricle. Any incompetence in the aortic valve will lead to regurgitant flow back into the LV.

There are three broad mechanisms that have been proposed to classify aortic insufficiency. These categories differ in how the cusps of the aortic valve function, as defined below.

1. Type 1: normal cusp motion - this includes disorders that distort the normal anatomy of the aortic root such as dilation of the root or ascending aorta, as well as damage to the actual leaflets. This can result from endocarditis or trauma and results in a central regurgitant jet.

2. Type 2: excessive cusp motion - this includes disorders that cause either prolapse or flail of an aortic valve leaflet. These leaflets typically cause eccentric jets.

3. Type 3: restricted cusp motion - this includes all of the calcific diseases but can also include endocarditis or rheumatic diseaes. The jets from these lesions can either be central or eccentric.

Assessing Severity of AI

The following is not a complete list of methods to assess and grade aortic insufficiency, but is a "brief" list of the more commonly used methods. This is intended to provide the critical care physician or non-cardiac anesthesiologist a basic understanding of how severe (or not) their patient's aortic pathology is.

Color Flow Doppler: Jet/LVOT

The severity of aortic insufficiency can be quickly evaluated by assessing what % of the LVOT is taken up by the AI jet during diastole. This is done by first acquiring a long axis view of the aortic valve from the mid-esophageal location (~120 degrees omniplane) and placing the color flow doppler box so that it includes the aortic valve and LVOT. The ratio of jet width to LVOT diameter can be measured either by freezing the image in diastole, or using M-mode through the LVOT (see below). This works best for central jets, as eccentric jets can create a coanda effect which will lead to an underestimation of the severity.

The ratios are graded as below:

<25% - mild AI

25-65% - moderate AI

>65% severe AI

Vena Contracta

The vena contracta refers to the narrowest point of a regurgitant jet, found as the jet passes through the valve itself. It is easiest to measure with the same ME AV LAX view, zoomed to image just the valve itself. A small amount of time should be spent otimizing the color flow image to ensure that the vena contracta is imaged at its largest possible width (if off axis it will be underestimated). For the echo-savy, biplane imaging can be used to help with this. Grading is as follows:

<0.3 cm - mild AI

0.3-0.6 cm - moderate AI

>0.6 cm - severe AI

Pressure Half-Time

Pressure half-time (PHT) is a creative way of using the slope of doppler gradients to assess AI severity. The concept is as follows:

At the very beginning of diastole, the pressure in the proximal aorta is near systolic as the LV relaxes, generating a large pressure gradient between the aorta and the LV cavity. As diastole continues, the pressure in the aorta falls to the diastolic blood pressure and this gradient decreases. The rate at which this decrease occurs is inversely proportional to the severity of the regurgitation. If the lesion is small, very little blood will flow backwards into the LV and the pressure gradient between the aorta and LV cavity will remain high for longer. If the lesion is large however, the pressure gradient between the aorta and LV will equilibrate quickly.

Pressure half-time is assessed by obtaining a continuous wave doppler tracing through the aortic valve, capturing the AI jet with the CW cursor. With TEE, this requires either a deep transgastric view, or a transgastric long-axis view for proper doppler alignment. The baseline and scale of the CWD tracing should be adjusted to capture the full AI envelope. The "pressure half-time" refers to the amount of time it would take for the initial pressure gradient to decay to half of its initial value. This is typically calculated by software present on most echo machines and only requires that the operator trace the downward slope of the AI envelope (see below).

PHT > 500 ms - mild AI

PHT 200-500 ms - moderate AI

PHT <200 ms - severe AI

Regurgitant Fraction/Volume

The severity of a regurgitant lesion can be described by estimating the volume of blood that flows backwards through said lesion during one cardiac cycle. There are two methods of calculating this; one is by using standard echo hemodynamic calculations based on pulsed-wave doppler and the other uses PISA, or Proximal Isovelocity Surface Area. IMHO, these measurements are either flawed, or are difficult for a non-advanced echocardiographer to make, but I still provide them for context since conceptually they are helpful for understanding just how severe regurgitant lesions can be.

Hemodynamic method:

Assuming no other vavlular lesions, the regurgitant volume of an AI lesion can be calculated by comparing the volume of blood flowing through the LVOT in one cardiac cycle to the volume that flows through either the mitral valve, or the RVOT.

Regurgitant volume = Stroke Volume (LVOT) - Stroke Volume (MV or RVOT)

Regurgitant fraction = Regurgitant Volume / Stroke Volume (LVOT) x 100%

The LVOT stroke volume is calculated using pulsed-wave doppler (PWD) through the LVOT to calculate the LVOT VTI and the LVOT diameter. If this doesn't ring a bell, check out the bottom of this section. The MV or RVOT stroke volumes are calculated using the same exact principles. If using the mitral valve inflow, the cross-sectional area of the mitral valve annulus is measured and multiplied by the VTI of the PWD inflow tracing with the gate also at the mitral valve annulus (not the leaflet tips!!!). If using the RVOT, the RVOT cross-sectional area is calculated by measuring the diameter of the RVOT. It is then multiplied by the VTI obtained by PWD through the same location of the RVOT.

If using either of these methods, it is essential that the PWD gate be placed in the exact same location as the corresponding cross-sectional area that is measured, otherwise the continuity equation is not valid.

This method is simpler, but is plagued by the fact that it relies on calculated cross-sectional areas of complex structures. This LVOT is relatively circular, and the assumptions made in these equations is more valid. The mitral annulus and the RVOT are more complex however, and subsequently difficult to determine an accurate cross-sectional area.

Proximal Isovelocity Surface Area (PISA) Method

This is by far the most complicated but also coolest-looking method of grading the severity of regurgitant lesions. It uses color flow doppler and aliasing velocities, along with a bit of math. Feel free to skip this if it makes your brain hurt - its applicability in the ICU or non-cardiac ORs is incredibly low. But it does look cool (😎) and it is a good way of testing your understanding of basic ultrasound physics.

Image a regurgitant lesion. As blood flows from a relatively large chamber such as the proximal aorta, towards a relatively small orifice such as a small hole between aortic cusps, the law of conservation of math (yay physics!) tells us that this blood must speed up as it approaches the regurgitant orifice. This creates theoretical "shells" centered on the regurgitant orifice, with progressively faster velocities as the radius of the shells decreases. We can "see" these shells by taking advantage of aliasing velocities.

What are aliasing velocities? (see section on pulsed-wave and color flow doppler in particular)

This is best done by adjusting the nyquist baseline down so that the aliasing velocity is more easily visualized as blood flow approaches the regurgitant orifice. The baseline of the nyquist should be adjusted down so that blood flowing in the same direction as the regurgitant jet aliases at the lower frequency (this almost certainly makes no sense in writing so see the associated picture if interested).

Flow Reversal in the Descending Thoracic Aorta

Aortic Stenosis

Anatomy

Physiology

Gradients