EKGs

EKG LEADS

The cardiac conductionThe transmission of nerve impulses along neurons. system consists of nodes and fibers. These are essentially neuronsThe functional cells of the nervous system that transmit signals.. They conduct action potentials through the myocardium. That being said some students expect an EKG to look like an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission.. However what’s reflected in an EKG is a composite of all action potentials that are happening in the heart. We have action potentials of cardiac muscle fibers that are just about to contract. At the same time, action potentials of cardiac muscle fibers are starting to relax. This is why you don’t see beautiful action potentials.

EKG

Each one of these waves or features on an EKG corresponds to a step in the cardiac cycle. We have the P wave, the QRS wave (which is this big thing here), and the T wave. That’s it—just those three things. And each one of those things corresponds to a significant step of the cardiac cycle.

The P eave reflects atrial systole.  This is when the atria are generating pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. to force blood through the AV valves.  The QRS wave reflects ventricular systole.  The heart is forcing blood from the ventricles to the arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr. There’s also the T wave, which students often think is the third contraction. It’s not a contraction but the recovery of the ventricles. So they create a depolarizationThe loss of electrical charge across a membrane, triggering an action potential. as they recover.

EKG Intervals

We look at each peak or wave on an EKG. We also look at the intervals between those waves. The most important intervals are listed on this diagram here. The duration between the start of the P wave and the start of the R wave is the PR interval. It reflects the time it takes for an action potential generated by the pacemaker cellsThe basic structural and functional units of life. in the sinoatrial node. This potential travels to the cells in the AV node. This interval and the P wave can help us identify issues with the SA node. They also help determine if there’s something wrong with the atrial myocardium.

The QT interval reflects the time that it takes only the myocardium of the ventricles to have their contraction. This can be very important to discovering myocardial infarctions or areas of the myocardium that have been denied oxygen.  If any ventricular cardiomyocytes are dead then this QT interval will be affected.

 The ST interval provides a look at ventricular filling. The duration of the ST interval can tell you how long the heart is taking to fill in diastole. This is important because if the heart doesn’t fill then it has nothing to contract. The length of the ST interval reflects the volume of the EDV or the end diastolic volume. This also connects to the concept of preload, which is how much stretch the heart can accomplish in diastole. Again if the heart stretches more it would be able to fill more. But there is an optimal value for this stretching.

Calculating HR

Heart rate can be calculated from an EKG paper.  Those little blocks on the background of the EKG strips represent time.  Can you see the thicker outlines of the larger boxes?  Those represent one second.  Both of these strips represent 5 seconds of heart contractions.  Let’s consider the strip on the top.  We seem to have 6 cardiac cycles or 6 heart beats per 5 seconds. So, it’s like I hooked up my patient for 5 seconds and took an EKG.  They have 6 QRS waves for these 5 seconds or 6 cardiac cycles for these 5 seconds.  Heart rate is always reported in bpm or beats per minute.  There are some calculations we have to do to get how many beats are in one minute.  Let’s make two fractions.  One will be 6 over 5 to represent our EKG strip’s 6 beats per 5 seconds.  The other fraction will have a question mark up here and 60 seconds down here.  Cross multiply 6 times 60 and then divide by 5 to get 72 beats per minute.  If I hooked up this patient and let it run for one minute, I’d see 72 QRS waves. 

Stop this video and do the calculation for the bottom EKG.  Did you get 96?  We’d have 8 over 5 and question mark over 60.  Doing that cross multiply and divide, we get 96 beats per minute.