Rhythm & Rate 12

This previously healthy 52 year old man came to the Emergency Department with severe back pain that had been treated with high doses of opioid drugs. Figure_1 is his ECG on arrival, and Figure_2 shows the frontal leads and the rhythm strip



+ Describe the main findings

Figure 2

The main finding are:

  1. The rhythm strip (Figure-2) shows sinus rhythm with slight variation in the R-R intervals (1280 to 1440 msec) that is consistent with sinus arrhythmia. Sinus bradycardia is present, with a heart rate between 42 - 47 beats per minute.
  2. The morphology of the QRS complexes is normal, but some of the QRS complexes in the frontal leads have a low amplitude (low voltage if an alternative term). Figure-2 shows that four of the frontal leads have a QRS amplitude of 5.5 mm (5.5 mV) or less.

Low electrocardiographic QRS voltages (LQRSV) are defined as a decreased zenith-to-nadir QRS amplitude in both the frontal leads and the precordial leads that is less than 0.5 mV (5 mm) in all the frontal leads and less than 1.0 mV (10 mm) in all the precordial leads. The ECG of R-0012 does not meet the criteria for LQRSV

The frontal plane axis, the PR interval, the QRS duration and the QT intervals are all normal. The QRS complexes have normal morphology, and the ST segment and the T waves are normal.

Final ECG diagnosis:

  • Normal ECG with sinus bradycardia and sinus arrhythmia
  • Non specific finding of low amplitude QRS complexes in most of the frontal leads

What could be the cause of the sinus bradycardia? The patient is not a extremely fit person. There is no history of cardiac disease or syncope, the cardiovascular examination is normal, and there are no ECG features of conduction disorder. The patient is not on any cardiac medications that could explain the bradycardia.

We are left with two possibilities:

  • The ongoing and not fully controlled back pain has caused an increased vagotonia
  • Could the regular doses of endone that the patient has received affect the heart rate?

Opioid receptors are present in cardiac cells, and experiments using spontaneously beating isolated rat hearts have found that stimulation of δ- and κ-receptor agonists reduced heart rate and contractility when directly added into the perfusate (Source: Wei Huang. 2013 MD Thesis Direct functional effects of opioid agonists on the isolated perfused rat heart)

Key Points

  • Possible association between opioid use and sinus bradycardia in this case
  • Some of the QRS complexes have a low amplitude but the ECG does not meet the criteria for low QRS voltage (LQRSV) complexes