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(Chest. 2005;128:2615-2618.)
© 2005 American College of Chest Physicians

Correlation of Left Atrial Size With P-Wave Duration in Interatrial Block*

Vignendra Ariyarajah, MD; Kristin Mercado, MD; Sirin Apiyasawat, MD; Puneet Puri, MD and David H. Spodick, MD

* From the Massachusetts Veterans Epidemiology Research and Information Center, Veterans Affairs Boston Healthcare System, and Department of Medicine (Dr. Ariyarajah), Brigham and Women’s Hospital, Harvard Medical School, Boston; Division of Cardiology (Drs. Mercado and Apiyasawat), Department of Medicine (Dr. Puri), Saint Vincent Hospital, Worcester; and Division of Cardiovascular Medicine (Dr. Spodick), University of Massachusetts Medical School, Worcester, MA.

Correspondence to: Vignendra Ariyarajah, MD, Preventive Cardiology, MAVERIC, VA Boston Health Care System, 150 South Huntington Ave, Boston, MA 02130; e-mail: vignendra{at}hotmail.com


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Conclusion
 References
 
Background: Interatrial block (IAB) [P-wave duration ≥ 110 ms] is associated with left atrial (LA) enlargement (LAE) and pathophysiologic derangements that result in atrial tachyarrhythmias, LA electromechanical dysfunction, and embolism. However, there has been no study addressing the direct correlation of P-wave duration in IAB and LAE.

Methods: One hundred eighty-one consecutive patients who were admitted to a tertiary care teaching hospital over 5 consecutive days were screened for past transthoracic echocardiogram evaluations and were then matched with ECGs done within 10 days of these echocardiographic investigations. ECGs were evaluated for presence of IAB, and patients were subsequently classified into two groups: control patients and patients with IAB. Patients were also matched for common comorbidities. Mean, SD of age, Pearson correlation coefficient (r), p values, and multivariate and linear regression analyses were analyzed for the investigated variables of LA size, left ventricular hypertrophy (LVH), posterior wall thickness, septal thickness, and P-wave duration.

Results: From the sample (n = 66; mean age ± SD, 71.3 ± 13.7; female gender, 48.5%), the mean LA size in the control group was 36.7 ± 4.01 mm and for the group of patients with IAB (n = 38) was 42.2 ± 7.25 mm (p = 0.004). Linear regression analysis revealed that P-wave duration was significantly correlated with LA size (p = 0.0002, r = 0.606).

Conclusions: Degree of conduction delay in IAB (P-wave duration) is an independent, direct correlate of LAE, and the regression equation (LA size [in millimeters] = 2.47 ± 0.29 x P-wave duration [in milliseconds]) could be used to estimate LAE.

Key Words: echocardiogram • formula • interatrial block • left atrial enlargement • P-wave duration


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Conclusion
 References
 
Interatrial block (IAB), conduction delay between the atria due mainly to Bachmann bundle (BB) abnormality,12 results in delayed activation of the left atrium (LA) and is depicted as prolonged P waves (≥ 110 ms) on ECG.23 It is an important correlate of LA enlargement (LAE), with 88% of patients with IAB shown to have this echocardiographic abnormality.45 While increased P-wave amplitude on certain ECG leads may indicate right atrial enlargement, indeed most of the literature interprets prolonged P-wave duration as LAE. However, interatrial conduction delay can in fact occur independent of increase in atrial size. IAB has also been associated with significant LA electromechanical dysfunction and is a risk for embolism besides being a potent precursor of atrial tachyarrhythmias, especially atrial fibrillation (AF).678 However, quantitative correlation between LAE and P-wave duration in IAB has not been established.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Conclusion
 References
 
Patients in sinus rhythm admitted over 5 consecutive weekdays to the general medical floor of a tertiary care teaching hospital for various nonacute presentations were screened for echocardiograms. Of the 181 patients (ages 23 to 96 years; 49.3% female), 67 patients underwent prior echocardiograms within 3 years (range, 0.17 to 3.0 years; mean, 1.22 years) of hospital admission. Echocardiographic studies were performed using an ATL HDI 5000W imaging system with a P4–2 scan head and 2.6-MHz transducer (Phillips Medical Systems Company; Bothell, WA), and all except one study, which was transesophageal (excluded from study), were two-dimensional transthoracic echocardiography (TTE) in the parasternal long-axis, four-chamber, two-chamber, and subcostal views with the patient in the left lateral recumbent position. All 66 echocardiograms (TTEs) and their respective reports were reviewed for LA size (diameter in the parasternal long-axis view), septal thickness, posterior wall thickness, and presence of left ventricular hypertrophy (LVH). A diameter of ≥ 40 mm was considered LAE, with ranges of 40 to 49, 50 to 59, and ≥ 60 mm, respectively, accepted as mild, moderate, and severe LAE.

Each of these echocardiograms were then matched for 12-lead ECGs at 25 mm/s with 10 mm/mV standardization done within 10 days (range, 0.08 to 10.0 days; mean, 1.67 days) of these studies. Reasons cited for ECG evaluations were similar and consistent with nonacute presentations and their respective echocardiographic evaluations. ECGs were evaluated for IAB using the greatest duration of P waves in any of all 12 leads measured with standard calipers under < 10-fold magnification. For increased specificity, P-wave duration ≥ 120 ms was used as the criterion to diagnose IAB. Patients were subsequently classified into two groups: control patients and patients with IAB. The onset of the P wave was defined as the junction between the T-P isoelectric line and the beginning of the P deflection and the offset as the junction between the end of the P deflection and the PR segment. Patients in both groups were also matched for common existing comorbidities such as hypertension, coronary artery disease (CAD), congestive heart failure (CHF), hyperlipidemia, diabetes mellitus (DM), AF, COPD, treated hypothyroidism, nonischemic cardiomyopathy, valvular heart disease, cerebrovascular accidents, and other arrhythmias such as bradycardia, ventricular tachycardia, ventricular fibrillation, and unspecified arrhythmias.

Data are expressed as mean ± 2 SDs for continuous variables and as frequencies for categorical variables. Differences between groups were assessed using {chi}2 statistics for categorical variables and analysis of variance for continuous variables. Multivariate analysis using stepwise linear regression was performed with Pearson correlation coefficient (r), and a graph was constructed with the corresponding regression equation to determine independent variables and correlates such as LA size, LVH, posterior wall thickness, septal thickness, and P-wave duration. A p value < 0.05 was considered significant. Statistical analysis was performed using statistical software (SPSS version 10.0; SPSS; Chicago, IL). Institutional Review Board approval and patient consent were obtained for this study.


   Results
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 Abstract
 Introduction
 Materials and Methods
 Results
Conclusion
 References
 
From the sample (n = 66, mean age ± SD, 71.3 ± 13.7; female gender, 48.5%), 38 patients had IAB (57.6%) and 28 patients did not (control) [Table 1 ]. Overall, hypertension was present in 46 patients (69.7%), CAD in 32 patients (48.5%), hyperlipidemia in 22 patients (33.3%), CHF in 20 patients (30.3%), DM in 15 patients (22.7%), and AF in 13 patients (19.7%) among the common comorbidities. When these comorbidities were compared between the groups, hypertension (31 patients; 81.6%; p = 0.014), CAD (23 patients; 60.5%; p = 0.023), and cardiomyopathy (9 patients; 23.7%; p = 0.024) were significantly more prevalent in the group with IAB (Table 1).


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  		Table 1.. Clinical Characteristics of Study Groups*

 
The mean LA size for the control group was 36.7 ± 4.01 mm and for the group of patients with IAB was 42.3 ± 7.25 mm (p = 0.004) [Table 1]. Of the patients with IAB (n = 38), 21 patients had LAE (55.3%, p = 0.031), compared to 8 patients (28.6%) in the control group. In the group of patients with IAB, 12 patients (31.6%) had mild LAE, 8 patients (21.1%) had moderate LAE, and 1 patient (2.6%) had severe LAE. Septal thickness and LVH (76.5%) were more prevalent in the group of patients with IAB (p = 0.032 and p = 0.016, respectively), while the prevalence of increased posterior wall thickness was no different between groups (Table 1).

Although there was no significant quantitative correlation between P-wave duration and septal or posterior wall thickness, multivariate stepwise linear regression showed P-wave duration to be significantly quantitatively correlated with LA size (p = 0.0002, r = 0.606) and that P-wave duration was an independent factor for LAE. Therefore, LA size could be calculated using a simple formula (Fig 1 ): LA size (in millimeters) = 2.47 ± 0.29 x P-wave duration (in milliseconds).



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  		Figure 1.. Correlation between P-wave duration and LA size in IAB patients.

 
In the group of patients with IAB, there were 14 ECGs with P-wave durations of 120 ms. However, only two ECGs had LAE (mild) on their respective echocardiograms, with LA sizes of 40 mm and 43 mm, respectively. Moreover, mild LAE was only frequent when P-wave durations were ≥ 130 ms (Fig 1).

Discussion and Limitations
IAB, highly prevalent in the general hospital population,910 is considered a specific marker for LAE.4567 Indeed IAB is interpreted by most authors as either "LAE" or "LA abnormality" rather than what it is, a conduction delay, which we show is correlated to both. Moreover, no study has conclusively demonstrated a correlation between P-wave duration in IAB and LAE. Our age- and sex-matched investigation showed a very significant correlation between them. Using the formula, LA size (in millimeters) = 2.47 ± 0.29 x P-wave duration (in milliseconds), an approximate estimate of echocardiographic LA size as measured by parasternal long-axis diameter can be made and could supplement echocardiographic studies when determining LAE (Fig 1). However, in our retrospective study, we used stored data of echocardiograms and could not utilize techniques that could have been available in a prospective investigation, such as measurements of the outlined endocardial surface of the LA walls at specific freeze frames using a digitized graphic ball.6 As such, LA volumes that would give a better estimate of LAE could not be calculated. Instead, we relied on standard LA parasternal long-axis diameters for LA size measurements, which tends to underestimate LAE.11 In our defense, however, there have been studies12 that showed a fair correlation between two-dimensional echocardiographic findings and angiographic estimation of LA parameters. We are, however, currently structuring a prospective trial to further evaluate the true value of the proposed formula.

IAB, as seen in this study, is a probable manifestation of underlying organic heart disease that potentially affects BB conduction,1 such as CAD, cardiomyopathy, and hypertension.6 Although it might have been better if the control patients and the patients with IAB were matched for comorbidities, this study did not discriminate between disease as the unbiased sample population consisted of patients who were not acutely ill who were admitted over 5 consecutive days for various presentations. We also did not account for body surface area or body mass index, which arguably might affect accuracy of LA parameters or its significance. However, conclusive associations between these factors and LA parameters have not been established.1213 Also, patients with IAB and the control patients were both subject to the same screening process for past echocardiogram (TTE) evaluations from the total 181 patients who were admitted and considered for our study.

Consistent with our proposed formula for LA size estimation, P-wave durations of 120 ± 10 ms on the ECG did not correlate with LAE, although by definition P-wave duration ≥ 110 ms denotes IAB.23 As such, although IAB suggests LAE, this consistently held true when P-wave durations were > 130 ms. It is uncertain, therefore, given such a high prevalence of IAB, whether the "textbook" definition for normal P-wave durations should no longer be < 110 ms314 and should be extended to accommodate P-wave durations of 120 ms as well, hence redefining P-wave duration ≤ 120 ms as normal. However, more likely, is the possibility that in IAB, conduction delay via the BB that is housed at the roof of the atria215 occurs proportionately to increasing intra-atrial tension buildup and, thus, precedes LA dilatation.


   Conclusion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Conclusion
References
 
IAB is significantly associated with LAE,45 a precursor that potentially leads to atrial tachyarrhythmias, LA electromechanical dysfunction, and embolism.678 Severity of these consequences may be related to the degree of conduction delay in IAB and directly translates to P-wave duration being an independent predictor and significant correlate of LAE. The formula, LA size (in millimeters) = 2.47 ± 0.29 x P-wave duration (in milliseconds) [Fig 1] could be used to estimate LAE and supplement echocardiographic (TTE) evaluations.


   Footnotes
 
Abbreviations: AF = atrial fibrillation; BB = Bachmann bundle; CAD = coronary artery disease; CHF = congestive heart failure; DM = diabetes mellitus; IAB = interatrial block; LA = left atrium/atrial; LAE = left atrial enlargement; LVH = left ventricular hypertrophy; TTE = transthoracic echocardiography

Received for publication April 28, 2005. Accepted for publication May 23, 2005.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Conclusion
 References
 

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  4. Munuswamy, K, Alpert, MA, Martin, RH, et al Sensitivity and specificity of commonly used criteria for left atrial enlargement determined by M-mode echocardiography. Am J Cardiol 1984;53,829-832[Medline]
  5. Hazen, MS, Marwick, TH, Underwood, DA Diagnostic accuracy of the resting electrocardiogram in detection and estimation of left atrial enlargement: an echocardiographic correlation of 551 patients. Am Heart J 1991;122,823-828[CrossRef][Medline]
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  8. Leier, CV, Meacham, JA, Schaal, SF Prolonged atrial conduction: a major predisposing factor for the development of atrial flutter. Circulation 1978;57,213-216[Abstract/Free Full Text]
  9. Jairath, UC, Spodick, DH Exceptional prevalence of interatrial block in a general hospital population. Clin Cardiol 2001;24,548-550[Medline]
  10. Asad, N, Spodick, DH Prevalence of interatrial block in a general hospital population. Am J Cardiol 2003;91,609-610[Medline]
  11. Khankirawatana, B, Khankirawatana, S, Porter, T, et al How should left atrial size be reported? Comparative assessment with use of multiple ecocardiographic methods. Am Heart J 2004;147,369-374[CrossRef][ISI][Medline]
  12. Murray, JA, Kennedy, JW, Figley, MM Quantitative angiography: II. The normal left atrial volume in man. Circulation 1968;3,577-580
  13. Gutman, J, Wang, YS, Wahr, D, et al Normal left atrial function determined by 2-dimensional echocardiography. Am J Cardiol 1983;51,336-340[CrossRef][ISI][Medline]
  14. Fuster, V, Alexander, RW, O’Rourke, RA Part 4: Rhythm and conduction disorders. Hurst’s the heart 11th ed. 2004 McGraw-Hill Companies. New York, NY:
  15. Chung, EK Anatomy, electrophysiology, and hemodynamics. Principles of cardiac arrhythmias 1973,14-28 Williams and Wilkins. Baltimore, MD:



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