Looking for Tall P Waves

Sorry for the delayed post. Internship was a bit busy last week, but I will post more regularly.  Contributions also welcome!


Motivation: "Right is height" - this is a phrase often used to describe the EKG changes caused by right atrial enlargement.  But, late last week, while listening to a lecture on EKG and description of P pulmonale, I wondered just how reliable is this sign.  Or, are there other better signs of RA enlargement? For background, right atrial enlargement is classically characterized on the EKG by "P pulmonale" or P wave height greater than 2.5 mm in lead II.

Paper: Evaluation of Electrocardiographic Criteria for Right Atrial Enlargement by Quantitative Two-Dimensional Echocardiography. Kaplan, J. D. et. al. J Am. Coll. Cardiol. (1994); 23:747-52. http://www.ncbi.nlm.nih.gov/pubmed?term=8113560%20

Methods: EKGs of hospitalized patient with mild to severe right atrial enlargement on echo were randomly selected and compared against EKGs of age and gender matched healthy controls.  There were 100 patients with right atrial enlargement and 25 control patients.  EKG were interpreted independently by two cardiologists using calipers and magnifying glasses.

Results:

Cohorts: Of the patients with right atrial enlargement, 52 were in sinus rhythm, 41 were in atrial fibrillation, 5 were in atrial flutter, and 2 in ectopic atrial rhythm.  All controls were in sinus rhythm.  Right atrial enlargement was most commonly associated with tricuspid regurgitation (30%), pulmonary hypertension (28%), and cardiomyopathy (14%).

EKG criteria:

1. P wave height>2.5 mm in lead II (P pulmonale), sensitivity: 6%, specificity: 100%
2. QRS axis > 90 degrees, sensitivity: 34%, specificity: 100%
3. P wave height>1.5 mm in lead V2, sensitivity: 33%, specificity: 100%
4. P wave height>1.5 mm in lead V1, sensitivity: 17%, specificity: 100%
5. R/S ratio > 1 in lead V1 (without RBBB), sensitivity: 24%, specificity: 100%
6. QRS amplitude <6 mm in lead V1, sensitivity: 33%, specificity: 92%

Discussion: The most interesting result of the paper, I thought, was the finding that P pulmonale is very insensitive finding though quite specific.  In fact, with the same specificity, we can look with better sensitivity at the P wave height in V2 to determine right atrial enlargement.  Another interesting finding was that often the most sensitive findings relate more to right ventricular enlargement (like QRS axis > 90 or R/S ratio >1 in V1).  Surprisingly, the specificity of this finding, was quite high meaning that almost all people with right ventricular hypertrophy have right atrial enlargement.  Thus, RV enlargement is a surrogate marker for RA enlargement.

This paper, however, has some important limitations.  First, there were 25 controls to match against 100 patients.  A few more controls would have been more desirable to capture the full range of normal.  A higher number of controls would likely have driven down the "100%" specificity of some of the findings.  Secondly, of the 100 patients with right atrial enlargement, only 52 were in sinus rhythm.  So, for findings relating to P wave morphology, the number of subjects was limited.  Overall, I think that the main points to take away are that P pulmonale has low sensitivity and RV hypertrophy can be used as a surrogate marker for RA enlargement.

EKG - Sensitive for MI?

Motivation: During my final clerkship in medicine, I was plagued by chest pain.  Almost every day, a patient complained of chest pain, and the usual reflex was to order a 12-lead EKG.  Most of the time, the EKG was unchanged, but I did not know what to make of the normal result.  Could this person still be having a myocardial infarction?  The answer was, after all, yes.  At the same time, an unchanged EKG presumably decreased the likelihood of the patient suffering an MI.  So, how sensitive is EKG or serial EKGs for MI?

Paper: Usefulness of Automated Serial 12-Lead ECG Monitoring During the Initial Emergency Department Evaluation of Patients With Chest Pain. Fesmire, F. et. al. Annals of Emergency Medicine (1998), (31): 3-11.

Methods: A prospective observational study in 1000 patients with chest pain admitted at an academic center.  Patients received an initial ECG and automated sequential ECG 20 minutes apart.  The ECG information was compared against the final diagnosis at discharge.  Acute myocardial infarction was diagnosed on the basis of cardiac enzymes, new Q-wave formation, or death within 24 hours of presentation.  Unstable angina was diagnosed if the in-hospital attending diagnosed the chest pain as likely ischemia related.  Investigators examining the ECG were blinded to final diagnoses.  Exclusion criteria for patients included cocaine use, tachyrhythmia, presence of pacemaker, and discharge from ED.

Results:
Acute Myocardial Infarction: Among patients with final diagnoses of acute MI, the initial ECG had a sensitivity of 55.4%.  Serial ECG twenty minutes apart had sensitivity of 68.1%.  The difference in sensitivity between serial ECG and initial ECG was statistically significant (p < 0.001).  The specificity of initial ECG and serial ECG monitoring for acute MI were comparable (94.6% vs. 94.8%, difference not significant).
Acute Coronary Syndrome (ACS): For patients with final diagnoses of ACS (which consisted of MI plus unstable angina), the sensitivity of an initial ECG was 27.5% while the sensitivity of serial ECG was 34.2% (difference statistically significant with p < 0.001).  Serial ECG had higher specificity at 99.4% compared to 97.1% for initial ECG (p < 0.01). 
Mortality: In the study, 17 patients died (8 with initial diagnosis of acute MI, 8 with initial diagnosis of unstable angina, and one patient from non-ACS causes).  17.6% of deaths occurred in patients with no changes in serial ECG.

Discussion: Overall, a one-time ECG is a pretty insensitive tool for detecting acute myocardial infarction (sensitivity of 55.4%) and an even poorer tool for diagnosing ACS (sensitivity of 27.5%).  Serial ECG had higher sensitivities, but the difference to me was not overwhelming.  Rather, I think that the study highlights that if ACS is a real concern, the diagnoses still hinges on a good story and cardiac enzyme monitoring.  The ECG can be used as a triage tool to identify patients with STEMI, but for all patients with ACS in hosptial, the ECG is not an appropriate tool to rule out concern.

Some of the methodological deficiencies in the study may have inflated the calculated sensitivity of ECG.  The study only considered patients admitted to the hospital, but it is known that myocardial infarctions are often missed in the emergency department.  So, it is possible that some patients with MI and normal ECG were discharged.  With inclusion of these patients, the sensitivity would be even lower.