Basic ECG/Pacemaker Timing

 Last update: 17 October 2000

The Basic Timing Intervals
	Typically, electrocardiograms (ECG's) are performed with a paper speed of 25mm/sec.  When this is the case, each "large" box represents an interval of 200 milliseconds (mS)or 0.2 seconds. The "small" boxes or timing marks within each 200 mS section, represent an interval of 40 mS or 0.04 seconds.  One millisecond is equal to 1/1000th of 1 second, therefore, 1000 mS is equal to one second.  One second of time, as displayed on the ECG strip, is equal to 5 large boxes.  Pacemaker timing is always described in mS intervals.  An interval is defined as the time between two consecutive events. A pacemaker rate is the average of several intervals calculated for one minute of time.  By applying the "60,000" rule, it is easy to convert seconds to milliseconds or mS to seconds. 60,000 divided by the rate returns the millisecond interval.  If 60,000 is divided by the mS interval, the appropriate rate is given.

Sensitivity  The Sensitivity setting is the parameter that allows the pacemaker to "see" intrinsic events.  The illustration shows a device programmed to 4 mV of sensitivity (indicated by black line).  The "R" waves that fall below the black line cannot be sensed.  Common practice requires a 2:1 safety margin for sensing; the 6 mV "R wave" does not meet the criteria and, although sensed, I have color coded it with an amber color as a warning to proceed with caution.  Signals of 8 mV or greater are well within the normal sensing range of this device.

The Refractory Period  The Refractory Period is a brief period of time when the device is not allowed to look for intrinsic events.  Refractory periods are usually divided into two segments; the absolute portions and the relative (or technical) portion.  Nothing can be sensed during the absolute segment.  The relative refractory period allows sensing to occur; however, the pacemaker does not react to the event by resetting an escape interval.  In the illustration at the left, the Alert period represents when "normal" sensing is allowed to occur.

Output  The electrical output of a pacemaker is determined by the amount of Voltage applied to the heart and the amount of time the Voltage is applied.  All implantable pacemakers are constant Voltage output devices.  The pacemaker output consists of the following parts:
1.	The Voltage Output - this is the electrical "pressure" that is applied to electrons to get them moving down the pacing lead wire
2. .	The Pulse Width - this is the time in milliseconds that the Voltage is applied
3.	The Discharge Slope - this is dependent upon the resistance placed on the flow of electrons and other electrical variables.