Cardiac output is important to a patient because if the heart is not functioning properly there may not be enough oxygen pumped through the body. The electrocardiogram is a noninvasive procedure using ultrasound waves that charts the rate and rhythmic pulse of the heart over time. (Each small box on the EKG paper spans .04 seconds of time.) The human heart is comprised of four major sections: the left and right atria at the top and the left and right ventricle at the bottom. Through all of these flows electricity provided by the Electrical Conduction System (ECS). As the electricity flows into the heart, the heart contracts, polarizes, depolarizes and recovers (repolarizes), all of which are marked on the EKG graph.
Look at the first section of rhythm on the EKG. This interval shows a wave that usually looks like a little hill, then a break before a spike. This wave is called “P.” (The waves noted on an EKG monitor are conveniently named alphabetically.) When the ECS begins electrical output at the top of the right atrium it flows through atria and is held for a moment before being dispersed through the ventricles after which the heart contraction occurs. This electrical insurgence into the atria causes interval “P.” Note that from the start of the “P” wave to the start of the “QRS” is called the “PR” Interval. Normal PR Intervals are .12 to .20 seconds in length.
Look at the next wave. The “QRS” is the spiking interval on the EKG caused by electricity going through the ventricles (depolarization). It should be equal to or less than .10 seconds. On an EKG if the spikes are an even distance apart this indicates a regular ventricular rhythm.
See that right after the spike that there is a pause before another hill is shown. This is where the pulse occurs. The last hill in the series on the EKG is the "T Wave." It shows the recovery of the ventricles through repolarization.
Count 30 medium size boxes (ones with 25 little boxes inside) on the EKG graph paper starting with the box after a QRS spike. (This represents 6 seconds of time.) Count the number of spikes that occur within the 30 boxes. Multiply by 10. This gives you the ventricular rate (heart rate). Normal heart rate is 60 to 80 beats per minute. Then count the number of P intervals the same way for the PR rate. There should be one P Wave for every QRS.