Introduction
Patients admitted to a cardiothoracic critical care unit (CCU) are at increased risk for cardiac arrhythmias and severe conduction disturbances, which represent an important cause of morbidity and can be potentially life threatening. Sustained arrhythmias may occur in up to 20% of critically ill patients admitted to the CCU. Atrial fibrillation (AF) and ventricular tachycardia (VT) represent the majority of tachyarrhythmias, while conduction disturbances with severe bradycardia can account for up to 20%.
These events may represent the primary reason for admission, but are often the result of a series of insults commonly seen in the context of a CCU, such as hypoxia, myocardial ischaemia, infection, sepsis, adrenergic hyperactivity, QT interval prolongation and electrolyte imbalance. Additional causes or triggers include cardiac surgery, mechanical ventilation, mechanical irritation from central venous catheters, inotropic and vasopressor agents and drugs known to prolong the QT interval.
The presence of significant structural heart disease, chronic obstructive pulmonary disease or other significant extracardiac comorbidities, systemic inflammatory response syndrome, sepsis, high central venous pressure and low arterial oxygen tension are known predictors of a higher arrhythmic risk. However, it is often not possible to predict or prevent the occurrence of severe arrhythmias or conduction disturbances and therefore a prompt diagnosis and treatment are required.
Management includes the correction of a known trigger as well as treatment directed at the arrhythmia itself (such as antiarrhythmic drugs, pacing, cardioversion or defibrillation). The impact of a certain arrhythmia depends on the patient’s underlying cardiac and respiratory function and the characteristics of the arrhythmia itself (rate, duration, irregularity), and these parameters will also influence the urgency and type of treatment required.
In addition, patients in the cardiothoracic CCU may have previously implanted cardiac electronic devices such as standard pacemakers, cardiac resynchronisation therapy (CRT) devices and implantable cardioverter-defibrillators (ICD). Although implantation, follow-up and troubleshooting of these devices should be in the domain of a trained cardiologist, it is imperative that intensive care physicians are familiar with the patients’ underlying cardiac diagnoses, the reason for the device implant, the basics of the current cardiac electronic device technology and the most frequent issues relevant in the context of a cardiothoracic CCU.
The purpose of this chapter is to discuss the role of pacing and defibrillation in patients admitted to a cardiothoracic CCU and the most frequently encountered issues involving pacemakers and ICDs.
Pacing in the Cardiothoracic Critical Care Unit
The most common indication for pacing, either temporary or permanent, is bradycardia. Bradyarrhythmias are relatively common in patients admitted to the CCU following cardiac surgery. In most cases, these events are temporary and due to sick sinus syndrome, slow AF or atrioventricular (AV) block.
Causes of Bradycardia
The most common cause of bradycardia in the cardiothoracic CCU is postoperative heart block. Up to 8% of patients undergoing aortic valve replacement were shown to require permanent pacemaker implantation. Local oedema may prolong conduction times, but direct injury to the conduction system during removal of penetrating calcium or insertion of deep stiches placed during valve surgery are the main cause. Postoperative complete AV block is seen in approximately 4% of patients undergoing mitral valve replacement and ring annuloplasty, although any degree of AV block may be seen in nearly 25%. Damage to the AV nodal artery may play a role in these cases. Proximal left anterior descending artery and septal artery disease have also been shown to increase risk of postoperative AV block. Predictors of need for permanent pacemaker implantation include older age, female sex, greater preoperative end-systolic diameter and left ventricular septum hypertrophy, pre-existing conduction system disease, severe annular calcification, prolonged total perfusion time, re-do operations and history of renal dysfunction, hypertension and bicuspid aortic valve. Predictive models have been developed for the prediction of perioperative need for permanent pacemaker implantation.
When to Pace
Acutely, the decision to pace is based on the haemodynamic impact caused by the underlying bradycardia, rather than the specific rhythm disturbance per se. When pacing is required, it may be temporary or permanent. Temporary pacing options include surgically implanted epicardial pacing wires, transvenous pacing leads implanted fluoroscopically or using floatation balloons or, in emergency situations, transcutaneous pacing via external defibrillator pads. Decisions on permanent pacemaker implantation are based on whether the bradycardia is expected to resolve or not.
The American College of Cardiology/American Heart Association guidelines recommend permanent pacemaker implantation for patients with postoperative third-degree or advanced second-degree AV block that is not expected to resolve, although the timing for implant is left to the physician’s discretion. Most authors recommend pacemaker implantation 5–7 days after the operation if conduction disturbances persist, especially in patients in whom these disturbances are unlikely to recover, namely those at advanced age, with pre-existing conduction system disease and submitted to valve surgery. Recovery is common in patients with sick sinus syndrome, but unlikely in the case of complete heart block. Predictors of long-term pacemaker dependency are complete AV block as the indication, bypass time longer than 105–120 minutes, preoperative history of syncope and body mass index ≥28.5 kg/m2.
The cost of prolonged occupation of intensive care beds and prolonged hospital stay, as well as the increased morbidity, reduced mobilisation, comfort and safety associated with prolonged temporary pacing should be weighed against the cost and risks of unnecessary pacemaker implantation in patients who would otherwise demonstrate a full recovery.
Basic Pacemaker Functioning
A detailed review of pacemaker function is beyond the scope of this chapter, but certain considerations regarding basic pacemaker types and function are worth mentioning. In its simplest form a pacing system delivers regular electrical impulses to the heart at a programmed rate. The minimum energy required to be delivered by the pacing system in order to achieve electrical capture is called the pacing threshold. Most modern pacing systems are designed to also look for intrinsic electrical activity first, before pacing, in order to minimise any unnecessary pacing but also to prevent delivery of a ventricular pacing stimulus on a T wave, which can be dangerous and trigger arrhythmias. To do this the pacing system has to be able to sense intrinsic electrical activity.
Pacing Mode
Whether permanent implanted systems or temporary external pacing boxes, all pacing systems have different pacing modes that dictate their function. The North American and British Group (NBG) pacemaker code is a three- to five-letter code designed to describe pacemaker mode.
The first letter designates the chamber paced: A stands for atrium, V for ventricle, D for both (dual), O if the pacemaker has been deactivated.
The second letter designates the chamber sensed (A, V, D, O): O represents asynchronous pacing without sensing.
The third letter describes the pacemaker’s response to a sensed signal: I (inhibition) means the pacemaker discharge is inhibited by a sensed signal; T (trigger) means the pacemaker discharge is actually triggered by a sensed signal; D (dual) means both inhibition and triggering responses are available (for example, in a DDD pacemaker, an atrial sensed signal will inhibit atrial pacing but trigger ventricular pacing after a prespecified delay).
Position four refers to the rate-response algorithm and is only relevant for permanent systems. Rate-response means the pacemaker will be able to increase its pacing rate in response to increasing physiological needs. This is possible due to the incorporation of either activity sensors with vibration detectors or minute-ventilation sensors.
Position five is used to indicate whether multisite pacing is present.
A three-letter code is adequate to describe emergency temporary pacing and most forms of permanent pacing in the context of a cardiothoracic CCU. Table 10.1 gives examples of pacemaker modes, which the CCU physician should be familiar with. Although the VVI mode is potentially applicable to all cases of bradycardia, it should be kept in mind that the lack of AV synchrony may reduce cardiac output by up to 25%, which is particularly relevant in patients with structural heart disease. A DDD mode, where possible, will ensure appropriate AV synchrony.
Table 10.1 Pacemaker modes of potential applicability to patients in the cardiothoracic critical care unit
| MODE | Applicability |
|---|---|
| AOO and VOO |
|
| VVI |
|
| DDD |
|
| DDI |
|
Pacemaker Type
Permanent pacemakers can be single chamber (usually right ventricle), dual chamber (right atrium and right ventricle) or biventricular systems. The choice of system is decided based upon underlying cardiac conditions and indication for the device, and will be made by an appropriately trained cardiologist.
Biventricular pacemakers (also referred to as CRT) deserve special mention. These devices are indicated in patients who may not actually have any bradycardia, but have a LV ejection fraction ≤35%, QRS duration ≥120 ms and heart failure symptoms, especially in the presence of left bundle branch block. CRT can improve cardiac output, haemodynamics, heart failure symptoms, functional capacity and quality of life in appropriately selected patients. It is therefore reasonable to expect that a patient on the CCU who already has a CRT device in situ has advanced heart failure, and maintaining correct device function in these patients is even more critical.
Implantable Cardioverter Defibrillators
ICDs are seeing more widespread use within cardiology and are therefore likely to be seen in patients on the cardiothoracic CCU. They are implanted in patients at high risk of life-threatening ventricular arrhythmias, most commonly due to structural heart disease such as ischaemic or dilated cardiomyopathies. All ICDs have pacemaker functions in addition to their defibrillator function. Accordingly, they can be single, dual or biventricular systems.
Figure 10.1a illustrates a simple single chamber pacemaker with a solitary lead in the right ventricle. Figure 10.1b shows a more complex CRT-D device with leads in the right atrium, right ventricle and in a branch of the coronary sinus (for left ventricular pacing).
Figure 10.1 Single chamber pacemaker (a) and cardiac resynchronisation therapy defibrillator (b). (a)
White arrow, right ventricular lead placed in the interventricular septum. (b) Black arrow, right atrial lead placed in the right atrial appendage; grey arrow, single-coil ICD lead placed in the right ventricular apex; white arrow, left ventricular lead placed in a branch of the coronary sinus; black arrow, high-energy device.
Pacing Related Complications
Pacing related complications can either be as a consequence of the implant procedure itself or due to abnormal or unexpected pacemaker function. Although intensive care physicians do not implant or program pacemakers, it is important to be aware of potential environmental factors that may impact on pacemaker function and possible acute complications related with the temporary or permanent pacing system implantation.
Implant Complications
With any transvenous system, permanent or temporary, the following acute implant related complications may occur: myocardial perforation with pericardial effusion and tamponade, pneumothorax, haemothorax, lead displacement or acute infection.
Other Complications
Other potential issues involving patients with pacemakers include the precautions needed when performing direct current cardioversion or defibrillation, the applicability and utility of magnet application, the possibility of electromagnetic interference and apparent or real system malfunction with failure to pace, failure to capture, failure to sense or increasing pacing threshold.
Pacemaker–Patient–Environment Interactions and Special Considerations
Certain precautions should be taken when performing specific procedures or manoeuvres in patients carrying cardiac electronic devices and this list is by no means exhaustive:
When externally defibrillating a patient with a pacemaker the external electrodes should not be placed close to the pacemaker, the minimal effective energy should be used and the device must be checked afterwards.
Certain situations may cause a temporary rise in pacing threshold and thereby impair device function, including electrolyte disturbance and immediately after CPR.
Subclavian puncture should be avoided ipsilateral to implanted devices when inserting a central venous line due to the risk of lead insulation tear.
Pulmonary artery catheters should also be avoided, if possible, in patients with recent device implantations due to the risk of lead displacement.
Atrial pacing spikes might be misinterpreted as QRS complexes by intra-aortic balloon pumps and interfere with triggering. Using arterial waveform is an alternative.
The use of electrocautery can cause interference with both pacing and ICD systems. In pacemakers it might result in inappropriate inhibition of pacing as the device misinterprets the electrical interference as intrinsic rhythm. In ICDs it may result in the delivery of shock therapy as the high frequency noise from electrocautery may be interpreted as a tachycardia requiring treatment.
Troubleshooting Pacing and ICD Systems
Temporary pacing, either with epicardially or transvenously placed leads, is often needed following cardiac surgery. Problems associated with temporary pacing include the displacement of the lead, myocardial perforation, which may lead to tamponade, increasing pacing threshold, lack of capture and lack of sensing. When lack of pacing or lack of capture are detected, all connections should be immediately checked, the ventricular output set to maximum and the generator programmed into asynchronous mode. Lack of capture is usually the result of lead displacement or threshold rise.
Whilst there are many similarities to the approach taken with troubleshooting a permanent pacing system, it is a more complex device and often will require the assistance of a physician familiar with pacemakers. Pacing and sensing problems can originate in the generator, the pacing leads, the lead–myocardium interface or the patient. Table 10.2 lists the most frequent troubleshooting issues encountered in patients with pacemakers, their most frequent causes and recommendations to overcome the problem. Figure 10.2 illustrates ECGs of different pacemaker malfunctions.
Table 10.2 Troubleshooting issues encountered in patients with pacemakers
| Causes | Manifestations | Troubleshooting | |
|---|---|---|---|
| Failure of output |
|
|
|
| Failure to capture |
|
|
|
| Rapid pacing |
| Rapid ventricular paced rhythm – there is ‘tracking’ of the atrial sensed events |
|
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