BASIC ANESTHETIC MONITORING
The American Society of Anesthesiologists (ASA) standard for basic monitoring notes, “Every patient receiving anesthesia shall have the electrocardiogram continuously displayed from the beginning of anesthesia until preparing to leave the anesthetizing location.” The continual evaluation of the patient’s circulation (also an anesthetic monitoring standard) requires an understanding of the physiologic events represented by the electrocardiogram (ECG). Note that continual is defined as “repeated regularly and frequently in steady rapid succession” whereas continuous means “prolonged without any interruption at any time.”
For an anesthesia provider, the scope of knowledge concerning ECG should at least encompass that contained in the current ACLS manual. It is beyond the scope of this text to include the wealth of information privy to a cardiologist. Rather, the anesthesia provider must know when it is appropriate to consult a cardiologist as well as perioperative recognition of dysrhythmias and myocardial ischemia along with their appropriate treatments. The cardiologist can determine the presence of previously undiagnosed cardiac disease or worsening of preexisting disease and can suggest intraoperative cardioactive drug or pacemaker management or if further evaluation is necessary. History of a recent myocardial infarction (MI) is a significant risk factor for subsequent infarction associated with elective noncardiac surgery. Historically, elective surgery has been delayed 3 to 6 months following MI because of increased risk of mortality and subsequent MI. Best practices suggest delaying elective noncardiac surgery by at least 60 days following MI with risk of complications decreasing over time.
A standard 12 lead ECG is rarely obtained intraoperatively except in specialized cardiac cases. Significant cardiac abnormalities noted during the preoperative anesthesia assessment (arrhythmias, new murmurs, preoperative ECG abnormalities, presence of implanted pacemaker or cardioverter, and previous cardiac events) can trigger cardiologist consultation and delay of surgery. Additionally, pacemaker experts may need to be consulted due to the existence of several different types of implanted pacemakers and cardioverters which can require magnet placement or reprograming to counteract undesirable effects of intraoperative electrocautery. This chapter will discuss the physiologic basis of a normal ECG and the pathophysiology associated with corresponding ECG changes.
ELECTROPHYSIOLOGY OF MYOCARDIAL CELLS
The ECG is a graphic representation of the net electrical activity of the heart associated with the action potentials of the myocardial cells. An understanding of action potentials of individual myocardial cells is necessary to interpret the electrical events depicted in the ECG. The excitable cells of the body, such as muscle and nerve, characteristically exhibit action potentials—changes in the cell membrane’s electrical charge from the preexisting resting membrane potential (see Chapter 1 for more details). All living cells in the body have a resting membrane potential due to the presence of relatively more negatively charged ions (anions) inside the cell than outside. This imbalance of charges is caused by three factors:
The resting cell membrane is 50 to 100 times more permeable to potassium ...