Page 181 - Critical Care Nursing Demystified

166 CRITICAL CARE NURSING DeMYSTIFIED

Cardiac Electrophysiology

Electrophysiology of the heart (depolarization and repolarization) is much like
electrical conduction along a wire for an electric light. Transmission of an elec-
trical charge occurs because of the exchange of positive and negative ions from
outside to the inside of the cell. In the heart this process is called depolariza-
tion. Depolarization occurs when a charged cell membrane is altered by the
exchange of positively charged electrolyte sodium. Sodium is allowed into the
cell, changing the membrane from the negative resting state to a positive,
excited state. This excited state is transferred from one cell to another.
Repolarization happens when strong pumps return the positive electrolyte
outside the cell and a resting state returns the cell back to a more negative
condition. This starts with special cells in the SA node creating a rolling wave
from one cell to the next until the entire heart has depolarized. Only after this
electrical event occurs does the mechanical event or heart contraction happen.
So the spark occurs first and then the pump next.
Let us review briefly the electrical conduction system of the heart by return-
ing to Chapter 3 and looking at Table 3–1 and Figure 3–2, cardiac conduction
system components. What is the normal pacemaker of the heart? What is its Downloaded by [ Faculty of Nursing, Chiangmai University 5.62.158.117] at [07/18/16]. Copyright © McGraw-Hill Global Education Holdings, LLC. Not to be redistributed or modified in any way without permission.
normal or native rate? What takes over if the SA node pacemaker fails? If you
can answer these questions without looking, please proceed. If you cannot,
review the information because it provides the basis for what we do when we
perform rhythm monitoring and analysis.

How Does Cardiac Monitoring Really Work?

2 So how does this electricity get captured so the critical care nurse can make
some sense of it all? It requires special equipment to convert tiny amounts of
electricity made by the heart during the electrical cardiac cycle. Although systems
are extremely sophisticated today, the basics of cardiac monitoring require some
standardized equipment no matter what kind of system is used. See Figure 4–1.
If we follow that all-important patient back to the system, we first need
something to take the tiny amounts of electricity from the patient’s heart and
change them into something we can interpret. The conductor of electrical signals
from the patient’s heart is called an electrode. An electrode can come in many
shapes and sizes from a simple tab that looks a lot like duct tape that is used to
perform a 12-lead ECG to a disc that looks like a medication patch. No matter
what it looks like, this electrode conducts millivolts! That is right! Millionths of