Information obtained from a 2D view of the LA consists of left atrial size and area as well as other pathological conditions. Normal variations in the left atrial size and volume occur during the cardiac cycle. The largest LA volume occurs immediately prior to mitral valve opening at end-ventricular systole
. The shape of the LA at this point is the most spherical so the area can be determined more accurately at this phase. Left atrial size is normally 38 + 6 mm in the anterior-posterior dimension and 39 +7mm in the medial-lateral dimension
. Left atrial size measurements and views have been standardized in TTE. TEE measurements of left atrial size to the TTE have only recently been correlated. Of the 4 TEE views of the LA, the ME AV SAX view is the only view that reliably visualizes the entire LA and correlates best with TTE measurements of LA size. Compared to TTE, TEE underestimates LA size by 6-16%
.
Other disease processes can alter the accuracy of the left atrial internal dimension. Compression of the left atrium from a dilated aortic root or ascending aorta, mediastinal masses, or pericardial tamponade from fluid or compression from a thrombus can decrease left atrial size.
The left atrial size can be estimated from the area-length method, transthoracically from the apical two and 4 chamber views. The area of both views are obtained from planimetry. The length of the left atrium is measured from the mitral annulus to the opposite left atrial wall in the 2 or 4 chamber view. The formula is:
Left Atrial Volume = ( 0.85 x A1 x A2 ) / L
4CV Area
2 CV Area
4CV Length
Area-Length Atrial
Volume Calculation
Atrial volume can also be calculated from the three dimensions in a prolate ellipse formula:
Left Atrial Volume = ( D1 x D2 x D3 ) * 0.523
4CV Length
2 CV Length
Area-Length Atrial
Volume Calculation
Left atrial size has also been compared to the aortic root, yielding a LA-Root ratio. In normal patients, the ratio is 1:1.
There are pathologic conditions that increase and decrease left atrial size. Left atrium dilates in common pathologic conditions such as mitral stenosis, mitral regurgitation, left ventricular failure, right-to-left shunts, VSD, atrial fibrillation. When the left atrium dilates it does so asymmetrically with medial and lateral walls bowing outward. Dilation of the left atrial is an important determinant for atrial fibrillation and the success of cardioversion. Left atrial enlargement greater than 50 mm dimension indicates the potential for chronic atrial fibrillation. Dilation occurs in Hypertrophic Obstructive Cardiomyopathy (HOCM) secondary to mitral regurgitation and diastolic dysfunction.
Rare causes of LA enlargement are giant atrium with/without prolapse and cor triatriatum.
Pathologic states which lead to decreased atrial size include; left-to-right shunts, total or partial anomalous pulmonary venous return, marked dilation of the aortic root, extracardiac masses, and hypoplastic left heart.
Left atrial volume calculation has been suggested as a way to calculate the severity of mitral regurgitation because there is a difference between normal patients and patients with mitral regurgitation regarding left atrial emptying volume. Although large emptying volumes (>40 ml) correlate with severe MR compared to catheterization regurgitant fraction, smaller volumes have not been effectively differentiated from normal patients. This and the fact that calculating the left atrial emptying volume is time consuming and difficult make a poor choice for determining the severity of mitral regurgitation.
Emptying Volume
Emptying volume is the difference between the maximal left atrial volume and minimal left atrial volume. The maximal left atrial volume occurs just prior to the onset of the left atrial contraction which is just before the P wave on the ECG. The minimal left atrial volume occurs at the end of atrial systole or just prior to the onset of the QRS complex on the ECG. Using the above atrial volume formulas the left atrial emptying volume can be calculated.
LA Emptying Volume = LA Volume (LA End Diastole) - LA Volume (LA End Systole)
