A cardiac surgical procedure is the most acute application of basic dynamic physiology that exists in medical care. Basic physiologic concepts of electromechanical activation and association, loading conditions, inotropy, etc all affect a successful outcome. Working knowledge of these fundamental concepts is imperative to maintain and return a patient to normal function. The purpose of this chapter is to present a manageable outline of cardiac physiology that can be used in daily practice, as a framework against which pathologic processes can be measured, assessed, and treated.
CELLULAR COMPONENTS AND CELLULAR ACTIVATION
The heart beats continuously based on the unique features of its component cells. A cardiac cycle begins when spontaneous depolarization of a pacemaker cell initiates an action potential. This electrical activity is transmitted to atrial muscle cells and to the conduction system which transmits the electrical activity to the ventricle. Activation is dependent on components of the cell membrane and cell which induce and maintain the ion currents that promote and spread electrical activation.
The activity of cells in the heart is triggered by an action potential. An action potential is a cyclical activation of the cell comprised of a rapid change in the membrane potential (the electrical gradient across the cell membrane) and subsequent return to a resting membrane potential. This process is dependent on a selectively permeable cell membrane and proteins that actively and passively direct ion passage across the cell membrane. The specific components of the myocyte action potential are detailed in Figure 3-3. The myocyte action potential is characterized by a rapid initial depolarization mediated by fast channels (sodium channels), then a plateau phase mediated by slow channels (calcium channels). Further details of this process are introduced as their components are described.
The cardiac cell is surrounded by a membrane (plasmalemma or more specific to a muscle cell, sarcolemma). The structural components of the sarcolemma allow for the origination and then the conduction of an electrical signal through the heart with subsequent initiation of the excitation-contraction coupling process. The sarcolemma also participates in the regulation of excitation, contraction, and intracellular metabolism in response to neuronal and chemical stimulation.
The sarcolemma is a phospholipid bilayer that provides a barrier between the extracellular compartment and the intracellular compartment or cytosol. The sarcolemma, which is only two molecules thick, consists of phospholipids and cholesterol aligned so that the lipid, or the hydrophobic, portion of the molecule is on the inside of the membrane, and the hydrophilic portion of the molecule is on the outside (Figure 3-1). The phospholipid bilayer provides a fluid barrier that is particularly impermeable to the diffusion of ions. Small lipid-soluble molecules such as oxygen and carbon dioxide diffuse easily through the membrane. The water molecule, although insoluble in the membrane, is small enough ...