Cardiac Waveform Monitoring: A Paramedic and EMT’s Guide

The cardiac waveform. It’s been plastered on shirts and worn around the necks of teenagers for years. When I first started in EMS my sister got me a waveform necklace. I proudly wore it until one day I realized it wasn’t “cute” at all. It represented a seriously messed up heart — long PR interval, wide QRS complex, significant ST elevation. A whole shebang of conditions that are impossible to actually combine. I still wear it, but if anyone asks, it’s a mountain scene.

cardiac monitoring

PQRST isn’t just for patient history anymore. Lets build up from the baseline (See what I did?) looking at both the electrical activity and also the underlying mechanical activity. Remember: just because you have a rhythm on the monitor doesn’t mean the heart muscle is actually contracting.

P Wave

p wave illustration cardiac monitoring

The first movement away from the baseline you see is the P wave.

  • Electrically — The P wave is the first deviation you will see from the baseline and represents atrial depolarization of the right and left atria from an impulse generated at the SA node. The point where the wave returns to the baseline is the end of the P wave. There is no visible wave that represents repolarization of the right and left atria, because it is buried in the larger QRS complex.
  • Mechanically — The atria contract and push blood into the ventricles.

Normal P waves are smooth and rounded, however abnormal P waves can be notched, peaked, or inverted.

QRS Complex

QRS complex illustration cardiac monitoring

In a normal sinus rhythm, the next wave will be the Q wave followed closely by the R wave and S wave. These three waves tend to travel in a group, so we refer to them as the QRS complex, but that doesn’t mean that every QRS complex has all three waves.

Q Wave

q wave illustration cardiac monitoring

The Q wave is the first negative deflection (below the baseline) in the cardiac waveform (unless preceded by a R wave). Unlike the P wave, the Q wave is always negative.

  • Electrically — The electrical impulse that just made its way through the atria continues on its merry way and depolarizes the interventricular septum from left to right.
  • Mechanically — The left and right ventricles contract and eject blood out of the heart into the arteries exiting the heart.

R Wave

R wave illustration cardiac monitoring

The R wave is the first positive deflection (above the baseline) in the QRS complex. Always positive.

  • Electrically — Simultaneous left and right ventricular depolarization.
  • Mechanically — The left and right ventricles contract and eject blood out of the heart into the arteries exiting the heart.

S Wave

s wave illustration cardiac monitoring

The S Wave is the first negative deflection below the baseline in the QRS complex. Always negative.

  • Electrically — Simultaneous left and right ventricular depolarization.
  • Mechanically — The left and right ventricles contract and eject blood out of the heart into the arteries exiting the heart.

T Wave

t wave illustration cardiac monitoring

The T wave follows the QRS complex (or any variation of Q, R, and S waves).

  • Electrically — Ventricular repolarization.
  • Mechanically — T waves occurs at the last phase of ventricular systole. The end of the T wave denotes the start of diastole where the heart rests and prepares for the cycle to repeat.

Normal T waves are slightly asymmetrical, however abnormal T waves can be peaked or inverted.

U Wave

Generally you don’t see U waves (especially in standard 4 lead monitoring), but if you do, they will look like a small notch directly after the T wave. They are rare enough that their exact meaning is still mysterious, but one commonly accepted theory is that they represent repolarization of the Purkinje fibers.

U waves can best be seen when the heart rate is slow.  An abnormal U wave might be flat or inverted, but it will always mirror the preceding T wave and occur before the next P wave.

So, now that you know what is happening behind the scenes of a cardiac waveform you can relax knowing….

cardiac monitoring

…. okay, not THAT relaxed!