Upper Rate Behavior Upper rate behavior refers to how a dual-chamber pacemaker will behave when the atrial rate exceeds the programmed maximum tracking rate The sum of the AV interval plus the PVARP yields what is called the total atrial refractory period and defines the highest rate the pacemaker will track an atrial event before 2 : 1 AV block sets in It is important to try to program a pacemaker so as not to have a patient go from a heart rate of 180 beats/min immediately into 2 : 1 AV block and at 90 beats/min The pacemaker should be programmed so as to achieve the upper tracking rate first, prior to the TARP, and have the patient gradually slow the heart in a Wenckebach fashion rather than 2 : 1 AV block (Fig 22.38) FIG 22.38 Tracing of upper rate behavior The pacemaker rhythm is atrial sensed-ventricular paced (AS-VP) (arrows) The blunt arrow demonstrates an atrial beat that falls into the postventricular atrial refractory period (refractory) mode and as a result a ventricular paced beat does not follow Troubleshooting Knowledge of the device's capabilities and how it relates to the patient's clinical status is key to successful troubleshooting A systematic approach will facilitate success The most common patient complaints are palpitations, tachycardia, or recurrence of the patient's preimplant symptoms One should perform an assessment of the original indication, pacemaker dependence, activity immediately preceding the clinical event, and detailed symptoms as well as duration encountered during the event Often symptoms of pacing malfunction are subtle, such as fatigue, weakness, confusion, neck pulsations, activity intolerance, shortness of breath, palpitations; these may, in fact, be asymptomatic, especially in children From a device perspective, the most common findings are failure to sense, failure to capture, output failure, or a change to magnet rate The first step during interrogation of the device is to identify all basic components including manufacturer, device model, lead models, and programmed settings Utilize other sources through the process, such as chest x-ray for comparison of postimplant findings and current device findings on anteroposterior and lateral views Perform an ECG with evaluation of rhythm, pacing, and interval timing interpretation Routine Holter evaluation can be helpful in detecting subtle interactions between the paced rhythm, and the intrinsic rhythm that can go unrecognized For example, in a patient with varying degrees of AV conduction who, during interrogation, has 1 : 1 intrinsic AV conduction, undersensing may go undetected Given the vulnerability of the lead system, a thorough evaluation must be performed To identify the patient's intrinsic rhythm and ability to tolerate testing maneuvers, it is helpful to first start with sensing To function properly, the device must appropriately sense the intrinsic rhythm Most devices perform automatic threshold, sensing, and impedance checks and provide a graph of these data for evaluation The sensing setting is the amplitude in millivolts (mV) that serves to recognize the presence of the intrinsic beat In order to identify the sensing threshold, the pacing rate is gradually decreased to permit intrinsic beats to occur When this is done one should note the characteristics and presence of the underlying rhythm At times sensing is not possible, if there is no underlying rhythm or the intrinsic rate is less than 30 beats/min Threshold testing (Fig 22.39) identifies the minimum electrical stimulus required to consistently capture the myocardium and trigger atrial or ventricular depolarization The threshold is the measurement of amplitude in volts (V) and the pulse width is measured in milliseconds To perform atrial threshold testing, the pacing rate must be faster than the patient's intrinsic atrial rate Ventricular testing can be performed by a variety of methods If there is a single-chamber ventricular pacemaker, the technique is similar to atrial testing If there is complete AV block with a dual-chamber pacemaker, there is no need to change the pacing rate When AV conduction is present, testing of the ventricular capture threshold can be assessed by programming the AV interval to a timing shorter than intrinsic conduction When capture thresholds are tested, the output is incrementally decreased until loss of capture occurs The cessation of the test results in reversion back to immediate pretest paced or sensed values Once you have identified the atrial and/or ventricular capture thresholds, the output should be programmed to two times the measure threshold or three times the pulse width In some cases it may be helpful to draw a strength duration curve to optimize settings and optimize capture while avoiding excessive battery depletion Another programming feature present in most device models is the ability to set the threshold to adaptive mode This will enable the device to perform automatic threshold testing and adjust the output depending on the detected threshold at that time, preventing the development of exit block The development of exit block due to the loss of myocardial capture can be influenced by clinical status, hemodynamic instability such as viruses, medications, severe electrolyte abnormalities, metabolic disorders, pH balance, and new myocardial fibrosis FIG 22.39 Ventricular threshold test In this example, the amplitude is kept stable at 2.0 V and the pulse width is progressively decreased Capture is demonstrated by a ventricular pacing artifact followed immediately by a wide QRS complex with a T wave in the opposite direction of depolarization At 0.12 ms, each pacemaker artifact is followed by a captured beat At 0.09 ms, only the first beat is captured Two further pacing artifacts are not followed by ventricular depolarization, and later there is a ventricular escape beat The threshold is described as 0.12 ms at 2.0 V VP, ventricular pace Implant Considerations Many challenges are involved in considering device therapy for small patients and those with congenital heart disease Decisions must be individualized to the patient's complex anatomy, size, residual shunts, myocardial function, fibrosis, long-term venous access, risk for thrombus formation, endocarditis, and longterm arrhythmia risk The type of device system implanted is guided by individual patient circumstances Selection regarding a single- versus dualchamber versus biventricular system, unipolar leads or bipolar leads, and MRI