This section deals with the act of forcibly taking control of somebody's cardiac conduction system. For a detailed review of this topic, I direct the gentle reader to the Operation Manual for the 5388 Medtronic Pulse Generator, as well as to this article on pulse generator engineering. The topics of discussion are:
- Sensitivity and pacing threshold
- How to check pacemaker sensitivity
- Output and capture threshold
- How to check capture threshold
In the barest sense, the rate setting on the pacemaker is the knob which controls your cardiac output. It is the number of times the pulse generator will produce a pulse.
It can be turned down to 50, to act as a backup in the event of a sudden bradycardia; or it can be turned up to 90 to increase the amount of forward flow. One may wish to fiddle with the rate in situations where cardiac ischaemia is a serious rate-dependent problem, and one may not wish to be going very fast.
The electrodes do not only pace, they also sense the electrical activity at the myocardial surface. One can define the sensitivity of a pacemaker electrode as the minimum myocardial voltage required to be detected as a P wave or R wave, measured in mV. The sensitivity of the pacemaker is actually a setting on the box, where the lower the number, the more sensitive the pacemaker.
The actual maximum sensitivity of the pacemaker is very high - when the electrode is freshly inserted, it can potentially detect very subtle changes in local electrical activity. The general range of sensitivity for a normal pacemaker box is 0.4-10mV for the atria, and 0.8-20mV for the ventricles. However, to use maximal sensitivity settings could cause the pacemaker to mistake various random fluctuations of electrical activity for cardiac activity. This could lead to madness. Either it would not fire at all (convinced the myocardium is depolarising normally) or it would fire constantly, mistaking the electrical interference for atrial activity. Thus, the sensitivity needs to be set intelligently,
- Put the pacemaker in a VVI, AAI or DDD mode (i.e. endogenous cardiac activity should inhibit the pacemaker.
- Change the rate to one which is much lower than the patients native rate.
- Change the output to whatever the minimum setting is; you would not want to get an R on T phenomenon. Capture is not required for this test, only the pacing spikes.
- Observe the sense indicator.
- Keep decreasing the sensitivity (i.e. increasing the mV value).
- Eventually, the sensitivity will be so poor that any of the endogenous electrical activity of the myocardium will no longer be sensed by the pacemaker.
At this stage, the pacemaker, blind to all electrical activity, will assume the patient is in asystole, and will start to pace in a totally asynchronous fashion. Or rather, pacinf spikes will regularly appear, at the rate which you have set.
- Now, the pacemaker sensitivity can be carefully increased (decreasing the mV value)
- Eventually, there will be a sensitivity value so low that the pacemaker senses every p-wave or QRS interval.
- This minimal sensitivity value is the sensitivity threshold.
- most of the time, you tend to leave the sensitivity turned down to half of the sensitivity threshold to ensure that the cardiac electrical activity will be sensed even if the electrode tip overgrows with filth.
- If you turn the sensitivity value down any more than that, you risk oversensing. Oversensing is described in greater detail elsewhere; briefly, it is an inappropriate inhibition of pacing in response to some sort of trivial non-cardiac signals, like the friendly hum of the nearby microwave.
Of course, you need an endogenous rhythm to test the pacing threshold. if your underlying rhythm is asystole, there is no point trying to make the pacemaker sense anything. Instead, the tradition is to set the pacemaker to 2mV.
The output of a pacemaker is the current (measured in milliamperes, mA) which it produces as a brief pulse.
The current is delivered in a brief burst, over about 0.6 milliseconds. During this time, the pulse generator discharges a capacitor into the leads. Voltage between the electrodes increases suddenly to 5V, and drops slightly over this brief period (as determined by the lead impedance) Because the ECG is such a crude tool, the shape of the pulse is never appreciated as anything more than a narrow "spike".
This is empirically the ideal pulse duration. It cannot be an more narrow; the narrower the pulse the higher the voltage required to stimulate the cardiac tissue. And it can be no more prolonged; the duration-response curve flattens out beyond 0.8 milliseconds, so you don't save any voltage by making the pulse last longer.
Of course, the current setting is rather nebulous -the current delivered to the ventricle is determined by Ohms Law, V =IR. The actual delivered current thus depends a lot on the lead impedance. Fortunately, the modern pacemakers can calculate lead impedance and typically adjust the voltage so that the delivered current at least vaguely resembles the number you dialed up on those twiddly knobs.
"Capture" is the nomenclature for the effective stimulation of cardiac depolarisation by the pacemaker. Basically, if the pacemaker fires and the myocardium responds with a normal QRS (or p wave), there is successful capture.
The capture threshold is the minimum current setting required to produce a depolarisation of the paced chamber.
In order to find this minimum current setting one can perform a simple manoeuvre:
- Set the pacemaker well above the native rate, so that the chamber of interest is being paced continuously
- Start reducing the output until a QRS complex no longer follows each pacing spike.
- The minimum output at which there is still consistent capture is the capture threshold.
- Typically, one might want to set the output to about double capture threshold. This way, even if the fibrinous crust overgrows on the surface of your electrode, the patient will continue to be paced.
Why bother with this, one might ask? Why don't we just pace everyone on maximal output setting? Well, high output current has its disadvantages. The higher the current and voltage, the more rapidly does the encrustation of the electrodes occur. There are strong arguments for keeping the output current low.