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Atrial fibrillation (AF) is the most common sustained arrhythmia encountered in clinical practice. It affects close to 2 million people in the USA and imposes a significant burden on health care costs. Left atrial volume (LAV) has been shown to be a strong predictor of atrial fibrillation in a wide variety of patients ranging from community based healthy adults to those undergoing cardiac surgery. Additionally, it is a strong predictor of success and recurrent AF following catheter ablation for AF. Furthermore, successful catheter based ablation of AF can lead to decrease in LAV and improved mechanical function due to reverse remodeling. While various imaging methods like echocardiography, and computed tomography have been used for LAV assessment, cardiac cine magnetic resonance imaging remains the gold standard. The multi-slice method (MSM) which uses multiple short axis images to assess atrial volumes is the most precise method of measuring LAV and has been previously validated in cadaveric studies.
In the current study, Hof et al. compare the accuracy of the simpler area length method (ALM) with MSM for LAV measurement by MRI. 72 MRI scans from 40 patients with paroxysmal or persistent AF were included in the study. ECG gated cine MRI was done pre and post procedure (few months) using well standardized protocols. Left atrial end systolic volume (ESV) and end diastolic volumes (EDV) were calculated using both the ALM and MSM. Left atrial function was assessed using the formula (EDV-ESV)/EDV. All measurements were done by a single blinded observer. The majority of patients were male (75%) and were relatively young (mean age of 57 years). Only about 25% had hypertension and 10% had structural heart disease indicating that these were largely patients with lone AF. Overall, there was a significant difference in the left atrial volumes measured by the two methods with mean EDV, ESV being 102 mL, 49 mL and 111 mL, 65 mL by ALM and MSM respectively. The ALM underestimated the LAV and overestimated the left atrial EF compared to the MSM. However, a good and significant correlation existed for LAV measured by both methods. The authors concluded that ALM correlates well with the MSM when measuring LAV using MRI and suggest it as an alternative to the time consuming MSM. These results are in line with other studies that compared LAV measurement with 3D-transesophageal echocardiogram and cardiac CT with MSM by MRI.
While discussing the clinical relevance of these finding a few important issues need attention. While increased LAV has been shown to predict incident and recurrent AF, it is unclear how this can be incorporated into the current clinical practice. Do we deny ablation or anti-arrhythmic drugs for these patients because of the high risk of failure? Do they need closer surveillance compared to others following ablation? If so what are the ideal threshold atrial volumes for determining our treatment strategy? Only if the above question is answered, we would be able to assess the clinical relevance of measuring LAV and the impact of the different methods of measurement. Secondly, though a good overall correlation was demonstrated between ALM and MSM, a Bland-Altman plot could have provided more useful information regarding the agreement between the two estimates of LAV. Notable other limitations of the study include the small number of patients studied and the limited scope (lone AF). Finally, though the authors propose that ALM can be less time intensive, they fail to provide details of the time taken for measurement of LAV by the two methods and the amount of time saved by using ALM in their study population.
Hof IE, Velthuis BK, VAN Driel VJ, Wittkampf FH, Hauer RN, Loh P. Left Atrial Volume and Function Assessment by Magnetic Resonance Imaging. J Cardiovasc Electrophysiol. 2010 May 10.
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