Electrocardiogram (ECG)
The heart is made up of four chambers, a left and right atrium (pl. atria) on top, and a left and right ventricle on the bottom. Located in the right atrium is the sinoatrial (SA) node, which serves as a natural pacemaker to initiate the electrical signal that causes the heart to contract. The electrical signal travels from the sinoatrial node to the atrioventricular (AV) node and then to the ventricles. This causes contraction of the atria first, followed by contraction of the ventricles. This leads to the characteristic thump-thump sound of a heartbeat.
In 1901, Willem Einthoven conducted a series of experiments designed to measure electrical activity in the heart. He discovered that he could infer heart activity by measuring electrical potential across the heart without actually applying electrodes to the heart itself. He attached the electrodes to arms and legs and the graph of the resulting current is what we call an electrocardiogram (ECG or EKG). In 1924, he received the Nobel Prize in medicine for his work.
Fig. 5 Einthoven's Triangle of ECG electrode placement
Images from Wikimedia Commons; heart: Eric Pierce CC BY-SA, chest: Patrick J. Lynch, medical illustrator; C. Carl Jaffe, MD, cardiologist CC-BY
Fig. 5 shows the relationship between the placement of pairs of electrode and the direction of the vector measured by each pair relative to the heart. The particular vectors are called "leads". A common misconception is that leads refer to the number of electrodes. Although ECG electrode wires are sometimes called "leads", a "twelve lead" ECG measures 12 vectors - it does NOT have 12 wires. The letters at the corner of the triangle represent electrode placement positions (RA=right arm, LA=left arm, and LL=left leg).
When a depolorizing wave moves through the heart, its average trajectory can be described by a vector. Fig. 6 shows an early stage in the generation of a heartbeat. A depolarizing wave moves from its source at the sinoatrial node. The small black arrows show the directions taken by the wavefronts and the large white outline arrow shows the average trajectory of the wave. A lead I ECG measures in the direction shown by the vector from - to +. It measures the component of the average trajectory in the direction of its vector. Since the average trajectory is largely in the direction of the lead I vector, a lead I ECG would measure this wave effectively. In contrast, a lead III ECG, whose vector is nearly perpendicular to the average trajectory shown, would be relatively ineffective at measuring this wave.
A lead II ECG effectively measures activity along the heart's axis, and is therefore commonly monitored. Different leads provide different "views" of heart activity. Some are more useful for diagnosing particular conditions than others.
Fig. 6. Vector measured by lead I (- to +) and its relationship to the depolarizing wave generated by the sinoatrial (SA) node Image from Wikimedia Commons by Eric Pierce CC BY-SA
For routine heart monitoring, a three-lead ECG based on Einthoven's triangle is common. This provides three different views of the heart's activity. For more intensive heart monitoring, a twelve-lead system that requires 5 electrodes is used. Simultaneously monitoring the various leads provides a much more detailed picture of heart activity. For the sake of simplicity, we will be performing a single lead ECG (lead I) in today's lab.
An ECG trace graphically illustrates the movement of the electrical signal through the heart. When a depolarizing wave moves toward the positive electrode of a lead, it produces an upright waveform. When a depolarizing wave moves away from the positive electrode, it produces an inverted waveform.
There are five components to a single heartbeat that are traditionally recognized on an ECG: the P, Q, R, S and T waves (Fig 7). The P wave indicates contraction of the atria. It is generally upright because it usually moves in the direction of the lead vector. The QRS complex indicates contraction of the ventricles. It is more complicated than the P wave because the path of the depolorizing wave changes direction as the ventricular contraction progresses (Fig. 7). It is also usually larger in amplitude because the signal moving through the ventricle is stronger than the signal originating in the SA node. Fig. 8 shows that there is a wide variety of normal QRS complexes and that it may not be possible to see all three of its possible components. The T wave results from ventricular repolarization, where the muscle tissue of the ventricles is returned to its resting potential.
Many disorders of the heart can be diagnosed by examining an ECG. It should be noted that doctors and other medical personnel have been highly trained in the interpretation of ECG readouts. ECGs vary depending on their lead and the person. Therefore you should not be alarmed if your ECG does not look like the example.
