1.1.16
Pacemaker nomenclature – antitachycardia (implantable cardioverter-defibrillators)
The implantable cardioverter-defibrillator (ICD) code has four positions.
Position I indicates the chamber shocked.
Position II indicates the antitachycardia pacing chamber.
Position III indicates the method of antitachycardia detection. Haemodynamic detection includes sensing of blood pressure or transthoracic impedance.
Position IV indicates the antibradycardia pacing chamber, in case defibrillation results in bradycardia.
Perioperative considerations in patients with ICDs
Preoperative – a multidisciplinary approach is essential. Perioperative management of an ICD should ideally be developed in collaboration with the cardiology and surgical teams, however, this may not always be feasible. In the out-of-hours setting, review of the patient’s information card and/or medical records should provide helpful information such as indication for treatment and functionality of the device. A CXR will help in determining the type of implantable cardiac device and if all leads are intact.
Intraoperative – identification of potential sources of electromagnetic interference (EMI) is important to minimize device malfunction. Commonly encountered factors associated with EMI include electrocautery (diathermy), evoked potential monitors, nerve stimulators, and fasciculations. Generation of EMI may cause inappropriate defibrillation. To minimize this risk, antitachycardia functions should be suspended. Variability observed with magnet application to ICDs is less than that observed with pacemakers. For the majority of ICD devices, magnet application temporarily inhibits arrhythmia detection and discharge, with rapid resumption of antitachycardia functions with magnet removal. However, the use of a magnet, by non-experts, with certain devices may result in unanticipated results such as permanent programming changes, changes to antibradycardia functions or no change in function at all. Best practice advises perioperative input from an electrophysiologist or cardiologist. Continuous intraoperative haemodynamic monitoring and immediate availability of an external defibrillator are essential.
Postoperative – continuous monitoring and external defibrillator availability must be continued until ICD function is resumed. A review by an electrophysiologist is recommended prior to termination of cardiac monitoring.
1.1.17
Preload, contractility and afterload
The definitions of preload, contractility and afterload were developed from experiments looking at isolated muscle fibres in vitro, allowing individual definitions to be produced. In vivo, these factors are interlinked, being dependent upon and affected by each other, making measuring them individually more difficult.
The stroke volume is determined by all three variables: preload, contractility and afterload. These, together with the heart rate, determine myocardial performance. Preload can also give an indication of how well a myocardium is performing. A heart requiring a higher preload to generate a cardiac output is not performing as well as one that is generating the same cardiac output with a lower preload.
In vivo, direct measurement of initial myocardial fibre length is not possible and therefore preload cannot be determined. As such, various surrogate markers have to be used. The volume in the ventricle at the end of diastole gives an indication of fibre stretch just before the onset of contraction. This can be measured by echocardiography and is called the end-diastolic volume.
The pressure generated in the heart chambers for a given volume is dependent on the chamber’s compliance, with the end-diastolic pressure often being referred to as the ‘filling pressure’. The right atrial pressure can be inferred from the CVP giving an indication of the filling pressure of the right side of the heart. The left-sided pressures are more difficult to measure and require a pulmonary artery catheter to obtain their values.
Contractility is difficult to define in isolation, being affected by all the factors that affect myocardial performance. Most factors that increase contractility do so by increasing the intracellular calcium concentration. Inotropy is often used synonymously with contractility.
Afterload can be represented by the mean arterial pressure during systole, or by measurement of the end-systolic pressure.
1.1.18
Pulmonary artery catheter trace
A pulmonary artery catheter is a balloon-tipped, flow-directed multi-lumen catheter, initially inserted through a central venous introducer sheath. During its placement a trace is produced demonstrating the pressures as the catheter moves through the chambers of the right heart and into the pulmonary circulation. The pulmonary capillary wedge pressure (PCWP) is used as a surrogate for the left atrial pressure.
Continuous pressure monitoring is used, via the distal lumen of the catheter, to guide correct insertion and produce the trace seen above. The balloon is inflated once the catheter has reached the right atrium and is allowed to float with the flow of blood to reach the pulmonary circulation. The right atrium pressure waveform is similar to the CVP waveform. On reaching the right ventricle the wave will oscillate between 0–5 mmHg and 20–25 mmHg. The catheter will then pass through the pulmonary valve and enter the pulmonary artery. The systolic pressure will remain the same as the right ventricle, but the diastolic pressure will rise to about 10–15 mmHg owing to the presence of the pulmonary valve. A PCWP is obtained by allowing the catheter’s balloon to occlude a pulmonary vessel. The trace will look similar to the CVP waveform, but with a range of 6–12 mmHg. The measurement should ideally be taken in West Zone 3 of the lung (where the pulmonary artery pressure is greater than both the alveolar and pulmonary venous pressures, ensuring a continuous column of blood to the left atrium) and at the end of expiration.
Pulmonary artery catheters can also be used to measure cardiac output (by means of an integral thermistor), mixed venous oxygen saturations, right-sided heart pressures and the right ventricular ejection fraction. It can also be used to derive systemic and pulmonary vascular resistances and the cardiac index.
1.1.19
Systemic and pulmonary pressures
The heart consists of two pumps in parallel: the low pressure right side that pumps into the pulmonary circulation, and the higher pressure left side that pumps into the systemic circulation.
The CVP approximates to the pressure in the right atrium and oscillates between 0 and 5 mmHg. In the right ventricle, the systolic pressure increases to 20–25 mmHg, with the diastolic pressure remaining similar to that in the right atrium. The presence of the pulmonary valve increases the diastolic pressure in the pulmonary artery to 10–15 mmHg, while the systolic