Page 40 - Cardiac Nursing

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16 PA R T I / Anatomy and Physiology

there is a continual movement toward equilibrium. When equi- (positively charged) are more concentrated in the sarcoplasm than
librium is reached, forces driving ion movement are balanced, and in the extracellular space. To balance the force of the concentra-
there is no additional net change in the ion distribution. The tion difference, the inside of the myocardial cell would need to be
Nernst equation, discussed later, is useful in understanding the re- approximately 94 mV compared with outside of the membrane.
lationship between electrical and diffusional forces driving ion That charge, 94 mV, is known as the potassium ion equilibrium
movement. It is useful to remember that the permeability proper- potential or the Nernst potential for potassium ion. The resting
ties of the living membrane change continually. myocardial membrane is permeable to potassium ions. The large
concentration gradient is maintained because the actual voltage
Diffusional Force across the membrane between activation cycles, approximately
Particles in solution move, or diffuse, from an area of higher con- –90 mV (inside negative), is close to the Nernst potential for
centration to an area of lower concentration. In the case of un- potassium ions in the myocardial cell, and thus nearly sufficient to
charged, soluble molecules, diffusion proceeds until there is a uni- retain potassium ions within the cell. The slow outward trickle of
form distribution of the molecules within the solution. The potassium ions is corrected by a membrane pump that moves
solution is then said to be in equilibrium. At equilibrium, there is potassium ions back into the cell (and moves sodium ions out of
still particle movement within the solution, but no net change in the cell). If the resting potential were –94 mV, there would be no
overall particle distribution. Charged particles also diffuse. The net potassium ion movement.
diffusion of charged particles is influenced not only by the con- The following illustrates the Nernst equation calculation of the
centration gradient but also by the electrical field. equilibrium potential for potassium ion:
Electrical Force and Current RT [K ] o

Like charges repel, and opposite charges attract. Positively charged E K Ln
FZ K [K ] i
particles flow toward negatively charged particles and regions;
similarly, negatively charged particles are attracted to positive ions where E K equilibrium potential for K
and regions. The electrical (or electromotive) force difference be- R gas constant
tween regions is called the potential difference and is expressed in T absolute temperature
volts (1 mV 0.001 V). The net flow of charges is called current F the Faraday (number of coulombs per mole of charge)
(measured in amperes). Resistance is the opposition to the flow of Z K the valence of K ( 1)

current, measured in ohms. Ohm’s law (electromotive force [K ] o K concentration outside the cell (e.g., 4 mM)

current resistance) describes the relation among current, volt- [K ] i K concentration inside the cell (e.g., 155 mM)

age, and resistance. Converting from the natural log to the base 10 log and replac-
When charged particles have different concentrations in the ing the constants measured at 37°C with numeric values, the
solutions separated by a cell membrane, and some of the particles equation becomes approximately as follows:
are able to permeate the membrane and others are not, an electri-

cal force is established. This force influences the distribution of all [K ] o
other charged particles. The potential difference across biologic E K 61 log 10
[K ] i
membranes is described by comparing the interior of the cell with
the external solution. In the typical quiescent or resting myocar- 4
dial cell, the potential difference is –70 to –90 mV; that is, the cell E K 61 log 10 155 97 mV
interior is negative with respect to the exterior. When positively According to the Nernst equation, the higher the potassium
charged ions move from the extracellular fluid to the intracellular ion concentration in the external solution, the more depolarized
fluid, the current is said to be inward. With inward current, the is the potassium equilibrium potential. If the resting membrane
cell interior becomes less negative, that is, it depolarizes. When were highly permeable to potassium ion, then the higher the ex-
positively charged ions flow into the extracellular space from the ternal potassium ion concentration, the more depolarized would
interior of the cell, the current is said to be outward; the cell re- be the resting potential. If one were to perform such an experi-
polarizes or hyperpolarizes. Movement of negatively charged ions ment, placing an intact muscle cell in a dish bathed in solutions
in one direction is electrically equivalent to an opposite-directed with varying potassium ion concentrations as the potassium ion
movement of positively charged ions. Thus, the inward movement concentration in the external solution is raised, the membrane be-
of an anion such as chloride is called an outward current. This, comes more depolarized. When the concentration of potassium
too, causes repolarization or hyperpolarization.
ion in the extracellular fluid becomes equal to the concentration
Nernst Equation Calculation of Equilibrium Potential in the intracellular fluid, the membrane potential is 0 mV.
for Specific Ions. The Nernst equation is used to calculate the In cardiac surgery, when it is important to have a heart with-
equilibrium potential for a particular ion. If the potential differ- out electrical and mechanical activity, the organ is sometimes per-
ence across the membrane is the same as that calculated by the fused with cool cardioplegic solution. The perfusate typically con-
Nernst equation for a particular ion, then the electrical force tains 15 to 35 mM potassium. As would be predicted from the
would counterbalance the concentration difference of that specific Nernst equation, the cell membranes depolarize. The depolarized
ion. If the membrane were permeable to the ion, there would be cells no longer experience an action potential, resulting in a mo-
no net ion movement. An understanding of the equilibrium po- tionless surgical field.
tential is basic to an understanding of the electrical characteristics Each ion has a different equilibrium potential that depends on
of biologic membranes. the relative concentrations of that ion on the two sides of the
Potassium ion distribution across the sarcolemma provides a membrane (see Table 1-2). In each case, the equilibrium potential
useful example in discussing the Nernst equation. Potassium ions can be calculated using the Nernst equation. For example, given