Basic Electrophysiology

Question 1

Temperature and SA node firing

Answer 1

Increasing body temperature increases SA node firing by increasing the slope of phase 4. Approximately 10 beats/min increase with 1 degree Celsius elevation in body temperature. Cooling has the opposite effect.

Question 2

Electrolyte imbalance

Answer 2

Imbalance of K and Ca in particular can have serious effects

Question 3

Hyperkalemia

Answer 3

• An increase in K raises the resting potential, which reduces the rate of rise of the fast inward Na current and amplitude of the action potential and slows conduction.
• Results in a reduction of the P wave amplitude, widening of the P-R interval and QRS complex and a decrease in the force of contraction.
• Accelerates repolarization, shortening the duration of the AP.
• Shortens the Q-T interval and gives characteristic tall T-wave peaks.

Question 4

Hypokalemia

Answer 4

• Decreases the resting potential
• Slows repolarization and prolongs the duration of the AP
• There is a flattening of the T wave
• Prolongation of the P-R and QT intervals
• Severe hypokalemia can cause AV block and ventricular fibrillation

Question 5

Hypercalcemia and Hypocalcemia

Answer 5

Changes in the serum Ca levels predominantly alter the myocardial action potential duration.

• Hypercalcemia shortens the ventricular AP by shortening phase 2, thereby shortening the ST segment, which results in the shortening of the Q-T interval.
• Hypocalcemia prolongs phase 2, which prolongs the ST segment and therefore prolongs the Q-T interval.

Question 6

P wave

Answer 6

atrial depolarization

Question 7

QRS complex

Answer 7

Ventricular depolarization. Phase 0.

Question 8

T wave

Answer 8

Ventricular repolarization. Phase 3.

Question 9

PR interval

Answer 9

AV node conduction

Question 10

QT interval

Answer 10

ventricular depolarization and repolarization

Question 11

If you give a drug that will slow phase 0 of the action potential (Na channel blocker), what would occur on the EKG?

Answer 11

Phase 0 will occur over a longer period of time, as a result, the QRS will also occur over a longer period of time and the QRS will be wide. Cell to cell conduction of the action potential would be slower, so conduction speed would be slower.

Question 12

If you give a drug that will prolong phase 3 (K channel blocker), what would occur in the EKG?

Answer 12

Phase 3 will be delayed. Therefore, phase 2 will be longer. The T wave will be delayed, resulting in a longer QT segment.

Question 13

Potassium channel blockers

Answer 13

Increase AP duration and phase 3

Question 14

Calcium channel blockers

Answer 14

L-type Ca, slows rate in SA and AV node

Question 15

Sodium channel blockers

Answer 15

Reduce phase 0 and slope of depolarization

Question 16

Beta blockers

Answer 16

Prevents calcium entry into the cell. Decrease HR, conduction velocity, strength of contraction. Used to treat many CVS conditions: Hypertension (inhibits renin), Angina/MI, Arrhythmias.