It is a common misconception that the EKG machines for sale “measure” the patient’s heartbeat. It does not. It detects and records the electrical activity of the nerves of the heart. It is of note that these nerve bundles do not contract themselves. Rather, they set the tone of the contractions and guide the action of the heart. This makes the electrical action of the heart very diagnostic and has made the EKG the essential tool that it is today.
Initially, the EKG machine picks up the signal from the sino-atrial or SA node. This translates into the P wave of the EKG. It goes from the right atrium to the left, causing that part of the heart muscle to contract.
It is notable that the SA node is connected to its counterpart in the ventricles by only one path, and once the signal is in the atrio-ventricular node two important things happen. There is a delay before the AV node sends the signal on to the next part of the heart. This allows the atria to completely empty. Simultaneous contractions would cause either incomplete filling or back flow from the ventricle to the atrium. This delay is seen in the PR segment of the EKG.
The other thing that occurs in the AV node is the further propagation of the signal. This propagation goes into nerve structure called the purkinje fibers. They stimulate the entirety of the ventricles and are seen on an EKG in the QRS section.
The last part of the EKG is called the T wave. This is the part of the cycle that allows the electrical system of the ventricles to repolarize, starting the process over again. The diagnostic value of the EKG is not just in the shape of these waves, but in the length of the sections. Changes in the QRS section can be directly related to changes in the purkinje fibers, which are located in the lower part of the heart, for example.
It is a fact that changes in these electrical impulses are directly parallel to the changes in the heart muscle itself. Sampling the heart itself is not only impractical but could be dangerous in the middle of a cardiac episode, so EKG machines remain an essential diagnostic tool.