The conundrum of left bundle branch block

Stress Test-Induced Left Bundle Branch Block

Left bundle branch block (LBBB) is an uncommon complication of myocardial perfusion imaging using Regadenoson, a vasodilatory agent. The mechanism is poorly understood at this time but could be related to ongoing ischemia. The use of a Regadenoson with the subsequent occurrence of LBBB could be a predictor of coronary artery disease or conduction abnormalities and should be understood by the physician to diagnose and risk stratify patients undergoing myocardial perfusion imaging properly.

Differential effect of regadenoson versus dipyridamole on heart rate in patients with left bundle branch block: How does it affect the results of pharmacological nuclear stress testing?

Background

Myocardial perfusion imaging (MPI) stress test is performed either using exercise as a stress modality or through the use of pharmacological vasodilator agents in those who cannot exercise. Regadenoson and dipyridamole are some of the most common vasodilator agents used. We aim to study the effect of these agents on the heart rate and the imaging results.

Methods

This was a retrospective study which included 187 patients with left bundle branch block. Patients received either dipyridamole or regadenoson during the myocardial perfusion imaging stress test. Charts were reviewed, and patient characteristics were recorded, as well as baseline heart rate, peak heart rate during stress, and angiographic data if available.

Results

Regadenoson increased peak, absolute and relative heart rates significantly more compared to dipyridamole. The peak heart rate for Regadenoson was 94.1 ± 17.36 and for dipyridamole it was 85.38 ± 16.48 BPM (P < 0.001). The relative and absolute heart rate increase in the regadenoson group were 40.75 ± 23.01% and 26.06 ± 13.44 BPM, respectively. The relative and absolute heart rate increase in the dipyridamole group were 24.61 ± 18.25% and 16.23 ± 10.97 BPM. The frequency of reversible septal defects was similar in both groups (54% for Regadenoson vs. 63% for Dipyridamole; P = 0.24).

Conclusions

There is a statistically significant increase in heart rate with the use of regadenoson for MPI compared to dipyridamole. However, the number of septal perfusion defects was similar between the two groups. The effect of this increase in heart rate, while statistically significant, is likely of no clinical significance.

Left ventricular wall motion abnormalities as well as reduced wall thickness can cause false positive results of routine SPECT perfusion imaging for detection of myocardial infarction

AIMS

The relationship between wall thickness, wall thickening, wall motion, and single-photon emission computed tomography (SPECT) results for detection of myocardial infarction has never been systematically evaluated in a clinical setting. In particular, the discussion whether non-ischaemic regional wall motion abnormalities and reduced wall thickness can cause SPECT to be false positive for infarct detection remains unsettled.

METHODS AND RESULTS

We therefore evaluated patients presenting with left bundle branch block (LBBB) and only included them in the analysis if any coronary artery disease (CAD) had been ruled out by angiography. LBBB is known to cause septal wall motion abnormalities as well as to reduce systolic septal wall thickness. Thus, LBBB is a good non-ischaemic clinical model to evaluate the influence of wall thickness and wall motion on the homogeneity of tracer distribution in resting SPECT images. SPECT revealed fixed defects in all 139 patients initially identified for possible enrollment. CAD was found to be present by angiography in 120 patients. The remaining 19 patients without any CAD underwent cardiovascular magnetic resonance (CMR) and were included in the study. Evaluation of SPECT using a 72-segment model revealed septum-related fixed defects in all 19 patients. Every defect was interpreted as myocardial infarction by blinded observers. The comparison of nuclear results to the gold standard CMR demonstrated that none of the fixed SPECT defects did represent myocardial infarcts. Defects, however, exactly matched areas of wall motion abnormalities as well as regions with impaired wall thickness as demonstrated by CMR. On a segmental basis, we found a strong relationship between wall motion and reduced wall thickness on one hand and SPECT defects on the other hand. For example, only 5% of segments with normal wall motion were false positive by SPECT for myocardial infarction, whereas 93% of all dyskinetic segments were found to be false positive (P<0.01). Comparing wall thickness to SPECT results revealed that 58% of segment in which wall thickness was 1 SD below the mean and 93% of segments in which wall thickness was 2 SD below the mean showed fixed defects by SPECT. Conversely, only 0.5% of segments in which wall thickness was above the mean were affected by false positive SPECT results (P<0.01).

CONCLUSION

Wall motion abnormalities as well as impaired myocardial wall thickening and wall thickness can cause false positive results of resting SPECT myocardial perfusion imaging for detection of myocardial infarction in the absence of myocardial infarct scars and CAD.