Quiz: Interpreting cardiac waveforms

An electrocardiogram (ECG) is a tracing of the electrical activity in your patient’s heart. This prevalent test can give EMTs, paramedics, nurses and doctors the ability to assess the heart’s electrical conduction system. Without a good understanding of how individual waves, intervals and segments correlate to the heart’s mechanical action, providers are only getting half the picture.

An ECG comprises many different waves, intervals and segments, which, when combined, give us a complete cardiac complex. A wave is any deflection above or below the isoelectric line – several waves are present in the standard ECG complex, such as a P wave, R wave, and T wave.

An interval is simply a measurement of time between two points on the ECG. Measurements can be instrumental, especially the PR interval measurement when looking for a first-degree heart block.

A segment identifies a period of electrical inactivity within the heart. One of the most useful ECG segments is the ST segment, which helps providers diagnose ST elevation myocardial infarctions or a STEMI.

There are five waves in total found in an ECG complex:

• P wave
• Q wave
• R wave
• S wave
• T wave

The P wave is the first upright waveform seen on the ECG and represents atrial depolarization. We can conclude from this waveform that electrical impulses have left the SA node and have traveled through the internodal pathways and Bachmans Bundle before stopping at the AV node.

The Q wave is the first downward deflection seen on the ECG, and it follows after the P wave. The Q wave represents the depolarization of the interventricular septum. The interventricular septum is the area of cardiac muscle between the left and right ventricles. It is home to the left and right bundle branches responsible for the depolarization of the septum. The Q wave is the first evidence providers have that the electrical impulses have passed through the AV node.

The R wave is likely the most effortless wave to identify on the ECG. The R wave is the second upright waveform, and in normal ECGs, it is the largest. The R wave represents ventricular depolarization, which is the largest and most forceful contraction of the heart.

The S wave is the negatively deflected waveform that begins immediately upon the R wave coming back down to the isoelectric line. The S wave indicates that the Purkinje Fibers have received the electrical impulses and have depolarized the last portion on the ventricles.

Lastly, the T wave is an upright waveform that appears on the ECG after the entire QRS complex. The T wave represents ventricular repolarization. The atrium also repolarizes too, but it occurs during the QRS complex and is buried in the R wave.

ECG interpretation: Cardiac intervals

There are two intervals that are useful when evaluating an ECG. The first is the PR interval, measured from the beginning of the P wave to the beginning of the Q wave. If there is no apparent Q wave present, the measurement starts at the beginning of the P wave and ends at the R wave. The PR interval is a measurement of how long the electrical impulses remain in the atrium, including the brief hold or pause at the AV node. In normal ECG complexes, the PR interval measures between 0.12 and 0.20 seconds.

The second interval that is useful for measuring is the QT interval. The QT interval represents the entire time it takes for the ventricles to depolarize and then reset or repolarize. QT intervals vary depending on age and gender, but typically they range between 0.36 and 0.44 seconds.

It is not as common to measure segments as measuring intervals, but there are two segments worth noting. The first is the PR segment, located at the end of the P wave but before the QRS complex. This segment represents how long the electrical impulses were held in the AV node before being passing through to the ventricles. Please don’t confuse this with the PR interval, which measures the amount of time it takes the impulses to travel from the SA node and pass through the AV node.

The second common segment is the ST segment. This segment is typically looked at by providers when trying to diagnose a STEMI. The ST segment is the period between ventricular depolarization and the beginning of repolarization.

Figure: ECG waves, intervals and segments

Test your knowledge. Take the test below to see how your ECG cardiac waveform knowledge measures up.