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Chicago, IL
Dr. Frazin is Director of Echocardiography Laboratories at Sts. Mary and Elizabeth Hospitals and Jesse Brown VA Hospital; and Associate Professor of Clinical Medicine, University of Illinois, Chicago, IL.
Correspondence to: Leon J. Frazin, MD, Jesse Brown VA Hospital, 820 S Damen, Chicago, IL 60612; e-mail: LFrazin{at}aol.com
There are life-threatening complications caused by abnormal venous-to-arterial communications. These may be intracardiac, such as seen in a ventricular septal defect, atrial septal defect, or patent foramen ovale (PFO). Pulmonary arteriovenous malformation (PAVM) is an extracardiac cause of an abnormal communication.
Complications resulting from such abnormalities include cerebrovascular accidents, transient cerebral ischemic events, and cerebral abscesses. PAVM complications include the above complications plus hemoptysis and hemothorax.
The article by Zukotynski et al1 in this issue of CHEST (see page 18) shows that there is a positive predictive value when saline solution contrast transthoracic echocardiography is used to determine the degree of severity of right-to-left shunting caused by PAVMs. Their results utilized CT as the standard. Positive results were scored for delay (number of cardiac cycles) before appearance of bubbles in the left atrium, and were graded for opacification of the left ventricular chamber according to a grading system of 1 thru 4 as proposed by Barzilai et al.2 Grade 1 was minimal left ventricular opacification, and grade 4 was extensive left ventricular opacification with endocardial definition. Conclusions showed that the degree of shunt grade was statistically related to the probability of PAVM.
From a cardiology perspective, there is interest in this study because it may be difficult to differentiate an intracardiac right-to-left shunt as caused by a PFO from a PAVM. There is ample evidence in the literature that transthoracic echocardiography with saline solution contrast is as accurate as transesophageal echocardiography (TEE) with saline solution contrast to determine the presence of a right-to-left shunt.3 However, there is little evidence in the literature that transthoracic echocardiography is equivalent to TEE in determining the source of the right-to-left shunt.
The accepted echocardiographic standard for differentiating a PFO from PAVM relies on a delay of three to eight cardiac cycles or 2 to 5 s of agitated saline bubbles to arrive in the left atrium after right atrial opacification. Earlier opacification would indicate an intracardiac shunt.4
In their study, the authors1 state that bubbles could have appearance in the left atrium as early as three cardiac cycles after right atrial opacification, especially in patients with grade 3 or 4 PAVM. They also noted bubbles appearing in the left atrium in less than three cardiac cycles in patients with a PFO, PAVMs, and false-positive findings. Although three cardiac cycles is the accepted cutoff point, this is where the situation becomes tenuous, and the authors correctly believe that the time of appearance of bubbles is often not a reliable indicator of shunt location. What is thought to be a PFO may be a PAVM; the reverse may be true; and both abnormalities may coexist. Indeed, it only requires one case report5 to illustrate the diagnostic value of TEE when transthoracic echocardiography with saline solution contrast falsely indicates the source of a shunt.
Zukotynski et al1 relied on only one echocardiographer to evaluate contrast echoes. This is a weakness in the study and may be somewhat responsible for inaccuracy of timing and appearance of bubbles. However, it is not infrequent in practice to rely on the opinion of one echocardiographer, and from a practical point of view it indicates the need for TEE: a definitive procedure for determining whether an intracardiac shunt exists.
TEE is a definitive procedure because its near-field advantage provides significantly improved resolution imagery of the right atrium, left atrium, interatrial septum, and adjacent structures compared to current harmonic-based transthoracic echocardiography. TEE allows a determination of whether contrast crosses the interatrial septum and can definitively determine whether an intracardiac shunt exists. TEE allows improved timing when contrast arrives in the right atrium, and improved timing when contrast crosses the interatrial septum and where bubbles cross the interatrial septum.
All maneuvers to enhance right-to-left bubble crossing such as inferior vena cava compression (abdominal), cough, and Valsalva have improved accuracy compared to transthoracic echocardiography. Additionally, information is supplied regarding septal mobility when evaluation of interatrial septal aneurysm is necessary, and color Doppler TEE provides shunt evaluation in the absence of a saline contrast injection.
The Eustachian valve is also visualized by TEE. This structure appears to direct blood flow to the interatrial septum and may prevent PFO closure after birth.6 In addition, TEE allows visualization of the right and left pulmonary veins, which are the exit for bubbles entering the left atrium from a pulmonary arteriovenous malformation.
Zukotynski at al1 used transthoracic contrast echocardiography as an initial screening test to detect PAVMs. They do not mention TEE as a solution to the difficulty of timing contrast appearance and its origin, which they state is problematic in their study.
There are life-threatening complications resulting from intracardiac or pulmonary right-to-left shunts. TEE should be relied on to provide an accurate echocardiographic diagnosis of shunt location when transthoracic saline solution contrast study findings are positive.
Footnotes
The author has no conflict of interest to disclose.
References
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