the first letter of your pacemaker is always the site where potential pacing takes place. a = atrial, v = ventricular, d = atrial and ventricular.
the second letter of your pacemaker is always the site where the sensing takes place. a = atrial, v = ventricular, d = atrial and ventricular. the letter o (sometimes referred to as zero) means there is no sensing possibility in the pacemaker in the way it is set now.
the third letter of your pacemaker tells you what it will do when it senses something. i = inhibition, t = triggering, d = inhibiton and triggering. the letter o means the pacemaker isn't programmed to respond to sensing.
the first three letters
the three first letters are generally the most common ones that get "played" with. generally with temporary pacemakers we'll only have three letters available. the most common ones that i've know post cardiac surgery have been:
voo - this means the pacemaker paces in the ventricle (v)(most often the right ventricle) and doesn't sense anything (o) and as a result there is no action in the third letter either (o). this pacemaker mode means you get ventricular pacing regardless of the own rhythm of the patient. this can cause dangerous situations if the patient has an own rhythm.
as a result voo pacing is mostly a thing from the past (generally voo was done on the first pacemakers as they simply couldn't sense). in nowadays practice you could decide to take voo as a mode if your patient has a complete asystoly underlying, but vvi would still be the safer option.
the settings needed for this mode: desired heart frequency (in bpm) and ventricular pacing output (in mv). voo at 45 bpm would mean the pacemaker just paces ventricularly at a frequency of 45 bpm. voo at 70 bpm would mean ventricular pacing at 70 bpm.
vvi - this means the pacemaker paces in the ventricle (v), senses ventricular activity (v) and when it senses this ventricular activity will stop pacing (i) (inhibition). this means that when the patients own ventricular activity is faster than the desired frequency on the pacemaker it will stop pacing and the patient's own rhythm will surface. once their rhythm falls below the desired frequency the pacemaker takes over. keep in mind as it is ventricular sensing it won't differentiate between normal a-v conducted rhythms (say sinus rhythm) and ventricular originated rhythms (say aivr - accelerated idioventricular rhythm).
vvi is a setting that we see often on temporary pacemakers for third degree av-block. where the pacemaker will be set for example at vvi - 75 bpm which means that if the own ventricular activity falls below 75 bpm pacing will take over. in my hospital we'll generally reduce the bpm on the vvi mode when we are close to removing it e.g. vvi - 40 bpm to check if the patient goes into third degree avb still or if the problem has resolved. one could of course just turn the pacemaker off, but if your patient goes into asystoly or really slow ventricular responses (say below 25) they'll loose consciousness etc. so it is a bit friendlier to keep vvi at a moderately low bpm first.
the settings needed for this mode: desired heart frequency, ventricular pacing output and ventricular sensing sensitivity (this is set in mv). the ventricular sensing sensitivity is the amount of current the heart has to originate in the ventricle in order for the pacemaker to pick up on the own frequency (generally much lower than the ventricular pacing output).
aoo - this means the pacemaker paces in the atrium (a), doesn't sense anything (o) and as a result can't give follow-up actions (o). a pacemaker in this mode will just keep on pacing the atrium at the desired frequency regardless of the own activity of the heart. in order for this mode to be effective it requires a patient to have a functional atrioventricular conduction, otherwise there is no ventricular response to the atrial pacing.
aoo is a setting i have mostly seen post cardio surgery where there is a ventricular rhythm due to malfunction of normal atrial activity/sinus activity. but when the atria get paced they'll send their signals through the av-node to the ventricles. but for safety issues one would better select aai as a mode.
the settings needed for this mode are the desired atrial frequency in bpm and atrial pacing output in mv. aoo - 90 bpm means the atria get paced 90 times a minute so the patient needs to be carefully monitored if his ventricles contract at 90 bpm as well, otherwise there isn't a 1:1 av-conduction.
aai - in this mode the pacemaker paces in the atrium (a), senses in the atrium (a) and when it senses atrial activity it will stop pacing (i). if the patient's own atrial activity is faster than the desired rate on the pacemaker it means the pacemaker will stop pacing. if the patient's own atrial activity is slower than the desired rate on the pacemaker the pacemaker will start pacing.
aai requires intact av-conduction just like aoo pacing. and it is important to keep in mind that in this mode the pacemaker doesn't look at ventricular activity at all. the decision to pace is completely dependent on atrial activity.
the settings needed here are the desired pacing frequency in bpm, atrial pacing output in mv an atrial sensitivity in mv. generally sensitivity is much lower than pacing output and atrial sensitivity is generally lower than ventricular sensitivity.
doo - in this mode there is pacing in both the atrium and ventricle (d) (generally separated by an interval that mimics the pause in the av-node), but no sensing (o) and thus no follow-up action (o).
a pacemaker in this setting has the advantage over aoo and voo that there is no need for an intact av-conduction (it can even be dangerous) and that you'll have the atrial kick that augments cardiac output where you loose this with voo.
in this mode you need to enter a desired atrial pacing output (mv), ventricular pacing output (mv), desired pacing frequency (bpm) and "av-interval" (in ms).
ddd - this is a mode where there is atrial and ventricular pacing (d), senses both atrial and/or ventricular activity (d) and either triggers a pacing or stops pacing (d) depending on what it senses.
a pacemaker in this setting can basically do it all ^^.
if there is no own heart activity of the patient or it is at a slower rate than the desired pacing frequency it will start out by pacing in the atrium. then it waits to see if the ventricles contract on their own (intact av-conduction). if the ventricles contract on their own it will inhibit the ventricular pacing that was planned and let the ventricles contract on their own.
if there is no ventricular response after the atrial pacing and the interval has passed it will pace the ventricles as well.
if there is spontaneous atrial activity the pacemaker senses this and sees if there is a own ventricular response. if there is a ventricular response by the patient the pacemaker won't do anything (inhibition). if there isn't a ventricular response by the patient the pacemaker will decide to pace the ventricles (triggering).
if there is ventricular activity the pacemaker will sense this too. if the frequency of the ventricular activity is below the desired frequency it will start pacing in the atrium and base ventricular pacing on what it senses (inhibition or triggering). if the ventricular activity is above the desired frequency it will inhibit pacing activity.$
the settings on this mode are: desired pacing frequency (bpm), ventricular and atrial pacing output (mv), sensitivity in the atrium and ventricle (mv), "av-interval" (ms).
fixed rate vs on demand
pacemakers that don't sense and just pace are fixed rate pacemakers. so this implies to the settings doo, aoo and voo. the danger of these setting is that their pacing spike can cause r-on-t phenomena.
say in doo/voo the ventricular spike is in the relative refractory period of the ventricle it can cause a pvc. this is an r-on-t and can cause a vt or vfib. the same mechanism can happen in the atria where aoo can cause afib.
with on demand pacemakers (ddd, vvi, aai and all other combo's) this problem is far less likely because the activity is sensed and the pacemaker spike is stopped due to the sensing. the same problem can still arise if your pacemaker doesn't sense the activity correctly and still fires.
the fourth letter
the fourth letter is an odd one and knows many more letters than just r and o (generally the o is left off and the pacemaker will simply be called ddd and very rarely dddo). the letter r stands for rate responsive and in every day practice it is the only letter commonly used
rate responsive means that the pacemaker decides what the best rate is for the patient. generally there is a resting frequency (e.g. 70 bpm) but the pacemaker has ways to detect body activity or breathing activity. when these activities go up the pacemaker figures you are in need of a faster heart rate and will pace faster to accommodate for this.
the fifth letter
the fifth letter tells us what the pacemaker does when it senses a tachycardia. there are different possibilities. when it does nothing it gets the letter o. when the pacemaker will try to solve the tachycardia by pacing (p). a normal pacemaker can't do anything but o or p. when a patient has an aicd (automatic implantable cardioverter/defibrillator) the fifth letter can be s (this means the pacemaker will shock when it senses a tachycardia) or tries both pacing and shocking (d). of course one has to enter a desired frequency at which antitachycardia measures are taken.
some pacemakers haave a hysteresis function as well. hysteresis means that the pacemaker allows for slightly lower frequencies of own heart activity. an example say a pacemaker would be vvi 70/min. but the own heart rate of the patient is 60 bpm but sinusrhythm. chances are that cardiac output is bigger with the sinusrhythm at 62 bpm than a ventricular paced rhythm at 70 bpm (due to loss of atrial kick). a pacemaker that is vvi 70 bpm but with hysteresis to 60 bpm will alow for sinusrhythms of 60 bpm and above to exist even when you put the pacemaker at vvi 70 bpm. on top of this being better for cardiac output it saves battery function on the pacemaker.
maximum tracking rate
a ddd pacemaker will try to keep 1:1 av-conduction as best as it can. e.g. say the atrial frequency is 100 the pacemaker will pace ventricularly at 100 bpm too as long as there is no own ventricular activity. but at a certain atrial frequency it will be dangerous for the pacemaker to keep 1:1 av-conduction. the frequency where the pacemaker stops giving 1:1 av-conduction is called te maximum tracking rate. depending on what you program the pacemaker to do it might start giving 2:1 av-conduction or switch to a different mode (e.g. vvi). this maximum tracking rate has to be set on pacemakers.