How to evaluate leftward axis on an electrocardiogram (ECG) and atherosclerotic ectasia of the thoracic aorta using Transthoracic echocardiography (TTE)?

Ordering TTE for Left Axis Deviation and Thoracic Aortic Ectasia

Order a transthoracic echocardiography (TTE) with specific instructions to evaluate the aortic root, ascending aorta, and aortic arch using parasternal long-axis and suprasternal views, with measurements using the leading-to-leading edge convention at end-diastole, but recognize that TTE has significant limitations for thoracic aortic assessment and will likely require follow-up CT angiography for definitive evaluation of atherosclerotic ectasia. 1

Understanding TTE's Role as First-Line Imaging

TTE is recommended as the first-line imaging technique in evaluating thoracic aortic diseases but is not the definitive modality for comprehensive aortic assessment. 1 While TTE serves as an excellent screening tool for the aortic root and proximal ascending aorta, it has critical blind spots that limit its utility for thoracic aortic pathology. 1

What TTE Can Evaluate

• Aortic root and proximal ascending aorta: TTE provides reliable measurements from the parasternal long-axis view, which is the most frequently used technique for measuring proximal aortic segments in clinical practice. 1

• Aortic arch assessment: The suprasternal view is paramount for thoracic aorta evaluation and should be included in all TTE exams, depicting the aortic arch and three major supra-aortic vessels. 1

• Associated cardiac findings: TTE can assess left ventricular hypertrophy (which may explain the left axis deviation on ECG), aortic valve pathology, and aortic regurgitation. 1

Critical Limitations of TTE for Thoracic Aortic Ectasia

Anatomic Blind Spots

• The distal ascending aorta and proximal arch represent a "blind spot" for TTE due to left mainstem bronchus interposition, making complete evaluation of atherosclerotic ectasia impossible. 1

• The descending thoracic aorta is inadequately visualized: In one study, the mid-portion of the descending aorta was not visualized in 30% of patients. 1

• TTE cannot provide precise diameter measurements of the aortic arch and descending thoracic aorta, requiring confirmation with CT or MRI. 1

Measurement Accuracy Issues

• TTE consistently underestimates maximum aortic root diameter compared to CT angiography and MR angiography in comparative studies. 2

• TTE measurements are 1-2 mm smaller than CT/MRI because echocardiography uses the leading-to-leading edge convention at end-diastole, while CT/MRI includes the aortic wall thickness. 3

• If TTE shows an increase of ≥3 mm per year in aortic diameters, confirmation by CT/MRI should be obtained due to potential measurement error. 2

Proper TTE Ordering Instructions

Specific Views Required

When ordering the TTE, specify the following views:

• Parasternal long-axis view: For aortic root and proximal ascending aorta visualization. 1

• Suprasternal view: Mandatory for aortic arch analysis, which should be included in all transthoracic echocardiography exams. 1

• Modified apical five-chamber view: For additional aortic root assessment. 1

• Short-axis view posterior to the left atrium: To image the descending aorta (though visualization will be limited). 1

Measurement Protocol

• Report aortic diameters using the leading-to-leading edge convention in end-diastole (Class I recommendation). 1

• Measure perpendicular to the axis of flow to ensure standardized measurements. 1

• Document measurements at: aortic annulus, sinuses of Valsalva, sinotubular junction, and ascending aorta at maximal diameter. 1

Why CT Angiography Will Be Necessary

ECG-triggered CT angiography is recommended for comprehensive diagnosis, follow-up, and pre-treatment assessment of the entire aorta, particularly for atherosclerotic ectasia. 1

Advantages of CTA Over TTE

• Complete visualization: CTA provides assessment of the entire thoracic aorta without blind spots, including the descending thoracic aorta where atherosclerotic changes are common. 1

• Superior spatial resolution: CTA offers higher spatial resolution and 3D reconstruction of the entire aortic length, essential for evaluating atherosclerotic plaque and ectasia. 3

• Accurate diameter measurements: CTA measurements are made perpendicular to the long axis using double-oblique technique, providing precise assessment of ectasia. 1

• Atherosclerotic plaque characterization: CTA can assess plaque calcification, ulceration, and mural thrombus that TTE cannot adequately visualize. 1

Clinical Algorithm for Your Patient

Step 1: Order TTE First

• Use TTE as initial screening to assess aortic root, proximal ascending aorta, and cardiac structure (to evaluate left axis deviation). 1
• Evaluate for left ventricular hypertrophy, which may explain the ECG finding. 1

Step 2: Recognize TTE Limitations

• Understand that TTE will provide incomplete assessment of thoracic aortic ectasia, particularly in the arch and descending segments. 1
• TTE may underestimate true aortic dimensions. 2

Step 3: Proceed to CTA

• Order ECG-gated CTA of the chest for definitive evaluation of atherosclerotic ectasia throughout the thoracic aorta. 1
• Specify measurements using inner-to-inner edge convention at end-diastole with double-oblique technique. 1
• Include arterial and delayed phases to assess for penetrating atherosclerotic ulcer or intramural hematoma if clinically indicated. 1

Common Pitfalls to Avoid

• Do not rely solely on TTE for surgical decision-making regarding thoracic aortic ectasia, as it underestimates aortic size and has significant blind spots. 2

• Do not assume a normal TTE rules out significant thoracic aortic pathology, particularly in the descending aorta where atherosclerotic ectasia commonly occurs. 1

• Do not use different imaging modalities interchangeably for serial follow-up without accounting for measurement differences between techniques. 3

• Do not order chest X-ray for aortic assessment, as it has limited sensitivity (64%) and specificity (86%) and cannot rule out aortic disease. 1