Do not agree.
The ECG shows sinus rhythm with dextrocardia. The ST-T morphology cannot be reliably assessed. Repeat ECG with limb leads reversed, and place right-sided chest leads.

Note that the limb leads remained unchanged, but the QRS progression in the chest leads has normalized. One can now diagnose probable left ventricular hypertrophy (LVH) with secondary repolarization abnormality vs. anterolateral ischemia. As the repolarization abnormality was identical to that seen two years before, LVH was the more likely diagnosis. Possible abnormalities in the limb leads may have remained obscure.

(1) ECG showing dextrocardia – see above.
(2) Chest X-ray demonstrating a right-sided heart with the ventricular apex pointing to the right – below left.
(3) Abdominal CT showing the liver on the left and the spleen on the right – below right. The abdominal CT was recorded during a prior admission.

Two sets of high-sensitivity troponin measurements remained within normal limits. An echocardiogram showed new moderate biventricular dysfunction with left ventricular ejection fraction (LVEF) calculated at 35%, changed from echocardiogram three years prior in which biventricular function was normal and LVEF was 55%. Given a history that was concerning for ischemia and new biventricular dysfunction, the Cardiology service recommended cardiac catheterization which revealed calcification and minor luminal irregularities of all coronary vessels and a 50% stenosis of the mid left main coronary artery. No stents were placed, and medical management was recommended. The patient experienced several bouts of right sided chest pain reliably relieved by oral Tylenol. Her chest pain was felt to be musculoskeletal in nature. She was discharged in stable condition with follow-up appointments with both her primary care physician and Cardiology.

What is situs inversus, and what does it mean for our patient?
Situs solitus refers to the normal orientation/location of the thoracic and abdominal viscera. In situs solitus, we find a tri-lobed right lung, a bi-lobed left lung, and a heart on the left side with the apex facing left. The liver and gallbladder are on the right, the spleen is present on the left, the descending colon is on the right, the ascending colon on the left, and the inferior vena cava (IVC) runs to the right of the aorta in the abdomen, etc.

In 1 out of 10,000 people, the orientation of the thoracic and abdominal viscera is reversed - a heterotaxy syndrome called situs inversus. Of these patients, the vast majority have situs inversus totalis, in which the heart is in the right chest with the apex oriented toward the right (dextrocardia). The remainder of the thoracic and abdominal viscera are in positions exactly mirroring situs solitus. For example, the right lung is bi-lobed, the left lung is tri-lobed, the liver and gallbladder are on the left side, the spleen is on the right side, the ascending colon is on the left, the descending colon is on the right, the IVC runs to the left of aorta in the abdomen, etc. Approximately 2-5% of these patients will have congenital cardiac anomalies, but the vast majority will be asymptomatic with normal quality of life and life expectancy.

Approximately 1 in 22,000 people in the general population will have situs inversus with levocardia, meaning that, although the thoracic and abdominal viscera are in mirror/reverse position from situs solitus as in situs inversus totalis, the heart remains in levocardia (its normal orientation), located in the left chest with the apex oriented toward the left. Nearly 100% of these patients have congenital cardiac anomalies, usually of the cyanotic variety (i.e. transposition of the great vessels). These patients have a severely shortened life expectancy. In fact, only about 5-13% of patients with situs inversus with levocardia survive beyond 5 years of age.
Of patients with situs inversus totalis, 17-25% will have Kartagener syndrome characterized by chronic sinusitis, bronchiectasis and situs inversus. This syndrome is a result of dynein protein abnormalities causing impaired mucociliary clearance, known as primary ciliary dyskinesia. Interestingly, only 50% of patients with primary ciliary dyskinesia will have situs inversus, and thus only 50% will meet official criteria for Kartagener syndrome.

Based on radiographic, echocardiographic, and ECG evidence proving dextrocardia, CT angiography of the chest confirming a tri-lobed left lung and bi-lobed right lung, and CT abdomen obtained during previous hospital admission, our patient has situs inversus totalis without associated primary ciliary dyskinesia. She had no evidence of congenital cardiac abnormalities on her echocardiogram or catheterization. She had respiratory bronchiolitis associated interstitial lung disease and chronic obstructive pulmonary disease, both secondary to a greater than 25 pack-year history of cigarette smoking, rather than chronic sinusitis. This patient’s coronary disease did not seem to be related to her underlying situs inversus totalis, but rather due to the significant smoking history, diabetes, and hypertension.

If you order an ECG, please spend a couple of minutes to analyze it. The first steps include rate, regularity and rhythm. Then go left-to-right, starting with the P waves. The normal sinus P wave cannot be negative in lead I as the SA node is in the upper right atrium and therefore, atrial activation is right-to-left, going towards lead I. If the P wave is negative in lead I, follow the steps below:

Below is an example of right arm – left arm lead reversal. Note that the limb leads look essentially identical to what was seen in dextrocardia (everything negative in lead I, upright P, QRS and T in aVR and negative P, QRS and T in aVL), but in the chest leads, there is normal R-wave progression.

Correct interpretation of the ECGs
ECG #1
- NSR with normal PR, normal QRS axis and RBBB 
- Sinus rate of 85/min with 1:1 AV conduction
 
ECG #2
- Regular bradycardia, still normal PR intervals and normal QRS axis, unchanged RBBB pattern 
- The sinus rate is now 90/min with 2:1 AV block; hence, the ventricular rate is 45/min
- Blue arrows below demonstrate twice as many sinus P waves as QRS complexes

​These ECG findings are consistent with right bundle-branch block and acceleration-dependent second-degree AV block where there is 1:1 AV conduction at slower sinus rates but 2:1 AV block at faster atrial rates. Acceleration- dependent AV block localizes the block to the His-Purkinje system, which is characterized by all-or-none conduction. At a critical sinus rate, only every other sinus impulse is able to conduct through the His-Purkinje system and every other P wave is blocked. On the conducted complexes, there is no change in the PR interval. The blocked P waves are almost never recognized by the interpretation software, which usually reads the rhythm as sinus bradycardia, a clinically trivial dysrhythmia. Therefore, the ECG must be carefully scrutinized in order to identify this ominous rhythm and the correct diagnosis.
 
- In this case, slowing down the sinus rate with carotid massage or IV beta-blocker can paradoxically improve AV conduction.
- Atropine, on the other hand, is contraindicated because it can worsen AV conduction by increasing the sinus rate.
- This finding is concerning for extensive distal conduction disease. Patients with bundle-branch block and acceleration-dependent AV block are at high risk of complete heart block at an anatomically distal level, which may result in asystole and cardiac arrest. 
- This finding requires urgent EP evaluation and is an absolute indication for dual-chamber (AV sequential) pacemaker implantation

​The patient was admitted directly from the outpatient EP office visit to Cardiology for permanent pacemaker implantation. He received a dual-chamber pacemaker. He continued to experience syncopal symptoms following his initial procedure due to intermittent loss of lead capture requiring revision and a brief admission to the cardiac ICU. The remainder of his hospitalization was uncomplicated, and he was discharged shortly thereafter with no recurrence of symptoms. During his first outpatient EP follow-up he was noted to be asymptomatic with no recurrence of syncope and a fairly low pacing burden. EP noted that they anticipate his pacing burden will increase over time.

​This patient had right bundle-branch block and an occult acceleration-dependent 2:1 AV block that was present but unrecognized on the ECGs during multiple prior ED visits for syncope. 
 
Bundle-branch block combined with acceleration dependent 2:1 AV block is indicative of extensive distal conduction system disease and requires urgent EP evaluation for pacemaker implantation to avoid degeneration to complete heart block and potentially, cardiac arrest. 
 
This rhythm is rarely detected by the interpretation software and will be missed on ECG review unless thoughtfully searched for. 

It is well known that patients with bifascicular block are at risk of developing complete heart block. This is especially true for patients with bifascicular block who experience syncope. It is less appreciated, but still well documented that patients with simple left or right bundle branch block and syncope too, as in the current case, are frequently found to have paroxysmal AV block and asystole.1 Based on these data it is recommended that patients with unexplained syncope who have any type of intraventricular block should undergo prolonged monitoring with an implanted loop recorder (class I recommendation).2 In this case, the patient had numerous previous workups by cardiologists and underwent a number of minimal value testing such as echocardiography and stress testing, but was never offered cardiac monitoring.
 
A second important lesson of the case is that 2:1 heart block is almost never recognized by interpretation softwares and is rarely recognized by providers. Most cases are interpreted as sinus bradycardia, which is usually a more benign dysrhythmia. Whenever a patient’s ECG appears to demonstrate regular sinus bradycardia, please carefully search for twice as many P waves than meets the eye by halving the interval between the conducted P waves. This “trick” only takes a few seconds to do. It is most important to search for 2:1 heart block in patients who have bundle-branch block and in patients who present with presyncope or syncope.