= cardiac output
[Hb] = haemoglobin concentration (g.dl–1)
1.34 = maximal O2 carrying capacity of 1 g of Hb measured in vivo (Hüfner’s constant) (ml.g–1)
SpO2 = arterial haemoglobin oxygen saturation (%)
PaO2 = arterial oxygen tension (kPa)
0.0225 = ml of O2 dissolved per 100 ml plasma per kPa
Oxygen flux is defined as the amount of oxygen delivered to the tissues per unit time. Oxygen delivery to the tissues is governed by two fundamental elements: cardiac output and arterial oxygen content. Arterial oxygen content comprises the sum of oxygen bound to haemoglobin and oxygen dissolved in plasma. The normal clinical range for oxygen flux is 850–1200 ml.min–1, with measurement requiring pulmonary artery (PA) catheter insertion.
Oxygen flux may be optimized, without invasive PA pressure measurement, if the modifiable variables are considered.
Cardiac output (CO) – determined by heart rate, preload, contractility and afterload. These factors may be negatively affected by pathological states and drugs (i.e. anaesthetic agents, vasopressors). Optimization may include heart rate control, correction of volume status and administration of vasoactive medications. Direct treatment of disease states should also be implemented.
Haemoglobin concentration – correction of anaemia will result in an increase in arterial oxygen content. Paradoxically, this may have a deleterious effect on oxygen flux due to the changing rheology of blood in the vascular compartment.
Haemoglobin oxygen saturation (SpO2) – may be adversely affected by hypoxia due to hypoventilation, diffusion impairment and ventilation/perfusion inequality. Carbon monoxide poisoning and methaemoglobinaemia should be considered where appropriate. Optimization should focus on the use of supplemental oxygen to maximize alveolar oxygen tension (although the effect will be minimal in shunt) and specific treatment of the cause.
Arterial oxygen tension – influences SpO2 and volume of oxygen dissolved in plasma. Increasing PaO2 has a finite effect on SpO2 once maximal saturation is reached. Dissolved arterial oxygen increases proportionally with an increase in PaO2. This increase becomes clinically significant at hyperbaric pressures.
1.1.15
Pacemaker nomenclature – antibradycardia
The pacemaker code has five positions.
Position I – chamber paced.
Position II – chamber sensed (detection of spontaneous cardiac depolarization).
Position III – response to sensing on subsequent pacing stimuli.
Position IV – presence or absence of an adaptive-rate mechanism in response to patient activity. The previous pacemaker code included a programmability hierarchy (i.e. simple vs. multi), which is now deemed unnecessary.
Position V – presence and location of multisite pacing. This is defined as stimulation sites in both atria, both ventricles, more than one stimulation site in a single chamber or any combination of these.
Pacemakers and diathermy
If possible, diathermy should be avoided in patients with pacemakers. However, if diathermy is required, bipolar is safer (as the current travels between the two instrument electrodes). This should be used in short bursts at the lowest energy settings.
When diathermy is used intra-operatively, a variety of untoward events may occur. These include inappropriate pacemaker inhibition (failure to pace), system reprogramming, and permanent pacemaker damage. With the design of newer units, these events are becoming increasingly rare. The most frequent interaction is pacemaker inhibition caused by misinterpretation of diathermy electrical activity as intrinsic cardiac activity. If the pacemaker has a ‘D’ or ‘I’ in position III, the pacemaker becomes inhibited and does not pace. The clinical effect depends on the duration of electrical stimulus, the underlying cardiac rhythm and the degree of dependency on the pacemaker. Ideally, to avoid adverse diathermy interaction, pacemakers should be evaluated by a clinical electrophysiologist prior to surgery to develop a perioperative device management plan. This plan should consider re-programming to a fixed-rate mode, whether (and how) a magnet could be used and recommendation for follow-up post-operatively.
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
