Pharmacology · chapter 16 · Introduction to the Cardiovascular System
Conduction of Electrical Impulses
The 14 things this section of the textbook says that you are most likely to be asked about — quoted word for word, not retold. Then 6 practice questions with the reasoning.
Key points
Peripheral and apical pulse
The heart rate can be measured by palpation of the pulse on an artery such as the radial artery (known as a peripheral pulse) or by auscultation on the chest at the apex of the heart (known as an apical pulse).
The SA node is the pacemaker
The pacemaker of the heart is the sinoatrial (SA) node in the atrium, which possesses automaticity, or the ability to spontaneously generate an electrical impulse that initiates the heartbeat.
Normal sinus rhythm rate
The origination of the impulse from the sinoatrial node in the atrium and the pathway of conduction is what leads to a normal heart rate (60–100 beats per minute) and rhythm, which is called normal sinus rhythm.
Monitor electrolytes; low potassium
Given how central electrolytes are to cardiac function and the electrical conduction system of the heart, it is important to monitor clients’ blood levels to ensure they are sufficient. Hypokalemia, or a low potassium blood level, is a risk factor for dysrhythmias and should be avoided.
Backup pacemakers are slower
The atrioventricular node, bundle of His, and Purkinje fibers also possess automaticity; however, their lower rate of spontaneous impulses (varying at 20–40 beats per minute) are suppressed by the higher rate of the sinoatrial node.
Ions in pacemaker cells
In pacemaker cells, the rapid depolarization phase of the action potential is mediated by calcium influx, and repolarization is mediated by potassium efflux (i.e., leaving the cell).
Path of the impulse
The cells of the cardiac conduction system receive the spontaneous impulses from the sinoatrial node and conduct, or carry, the signal through the atrioventricular node and then the bundle of His, which then splits into the left and right bundle branches.
Refractory period
A refractory period follows depolarization when the sodium channels that usually mediate depolarization are inactive. During the refractory period, the cell cannot be depolarized until the refractory (or inactive) period is over.
Calcium triggers contraction
This causes release of comparatively massive amounts of calcium into the cytoplasm from the sarcoplasmic reticulum in a process called calcium-induced calcium release. The high levels of calcium cause downstream contraction of the cardiac myocytes.
What an ECG monitors
Electrocardiography (EKG or ECG) is a common diagnostic tool that allows health care professionals to monitor various aspects of the client’s heart including rate, rhythm, and the presence of ischemia. It is also used for monitoring medications.
The P wave
The P wave represents atrial depolarization, which is when the electrical impulse travels from the sinoatrial node through the atria to cause atrial contraction.
QRS complex and T wave
The QRS complex represents ventricular depolarization, which is the electrical conduction signal that causes ventricular contraction or ventricular systole. Finally, the T wave represents ventricular repolarization.
ECG in normal sinus rhythm
On the electrocardiogram of a client in normal sinus rhythm, every P wave is followed by a QRS complex, which is followed by a T wave.
Regularly versus irregularly irregular
If the heart rhythm is irregular, it can be described as regularly irregular, meaning the time between heartbeats varies in a pattern, or irregularly irregular, meaning the time between heartbeats varies without any identifiable pattern.
Terms to know
- Automaticity
- Automaticity describes a cell’s ability to spontaneously generate an electrical impulse that allows it to function as the pacemaker of the heart.
- Depolarization
- Depolarization is the cell membrane potential increasing or becoming more positive as compared to its surroundings.
- Action potential
- An action potential describes rapid depolarization of the cell, followed by repolarization (the cell membrane potential decreasing back to the resting voltage).
- QT interval
- The interval from the beginning of the QRS complex to the end of the T wave is called the QT interval.
- Dysrhythmia
- Dysrhythmia s describe when there is a problem with the heart rate and/or rhythm.
Practice questions
Stuck on select-all-that-apply? How to take them one option at a time.
The graded version needs JavaScript. Every question is below anyway, with the answer and why.
Every question here, with the answer
A client's potassium level comes back low. Why is the nurse concerned about the heart?
- Potassium has no effect on the conduction system.
- Hypokalemia is a risk factor for dysrhythmias.
- Low potassium increases cardiac contractility.
- Low potassium makes the SA node stop generating impulses.
Answer: Hypokalemia is a risk factor for dysrhythmias.
Electrolytes are central to the conduction system, and hypokalemia is a risk factor for dysrhythmias, so blood levels are monitored. The other statements are not supported by the section.
A nurse reviews a client's ECG strip. Which component represents ventricular depolarization?
- The baseline after the P wave
- T wave
- QRS complex
- P wave
Answer: QRS complex
The QRS complex represents ventricular depolarization, which causes ventricular contraction. The P wave is atrial depolarization, the T wave is ventricular repolarization, and the baseline after the P wave reflects transmission through the AV node.
A client's monitor shows a heart rate of 72 beats per minute at regular intervals, with every P wave followed by a QRS complex and a T wave. How should the nurse interpret this rhythm?
- Irregularly irregular rhythm
- Normal sinus rhythm
- Tachycardia
- Bradycardia
Answer: Normal sinus rhythm
Normal sinus rhythm is a rate of 60–100 beats per minute at regular intervals, with each P wave followed by a QRS complex and a T wave. The rate is neither too fast nor too slow, and the rhythm is regular.
A nurse is caring for a client on a cardiac monitor. Which statements about the conduction system are accurate? Select all that apply.
- The AV node, bundle of His and Purkinje fibers also possess automaticity.
- The SA node normally fires at 60–100 beats per minute.
- The Purkinje fibers normally set the heart rate in a healthy heart.
- Atrial repolarization is clearly seen as its own wave on the ECG.
- The SA node is the pacemaker of the heart.
Answer: The AV node, bundle of His and Purkinje fibers also possess automaticity., The SA node normally fires at 60–100 beats per minute., The SA node is the pacemaker of the heart.
The SA node is the pacemaker with an intrinsic rate of 60–100, and the AV node, bundle of His and Purkinje fibers have slower automaticity that is suppressed. In a healthy heart these lower tissues do not show their automaticity, and atrial repolarization is not visualized.
The nurse is teaching a student about the uses of a 12-lead ECG. Which can the ECG help health care professionals monitor? Select all that apply.
- Heart rhythm
- Medication effects
- Serum potassium level
- Presence of ischemia
- Heart rate
Answer: Heart rhythm, Medication effects, Presence of ischemia, Heart rate
The ECG allows monitoring of rate, rhythm and the presence of ischemia, and it is also used for monitoring medications. Potassium is a blood level that is measured with a blood test, not read from the ECG.
A nurse assesses a client's pulse and finds the time between beats varies without any identifiable pattern. How should the nurse document this rhythm?
- Normal sinus rhythm
- Regularly irregular
- Irregularly irregular
- Bradycardia
Answer: Irregularly irregular
Irregularly irregular means the time between heartbeats varies without any identifiable pattern. Regularly irregular varies in a pattern, normal sinus rhythm is regular, and bradycardia describes a slow rate, not the pattern.
Where every quote comes from
Section 16.3 Conduction of Electrical Impulses of Pharmacology for Nurses by Tina Barbour-Taylor, Leah Mueller (Sabato), Donna Paris, Dorie Weaver, OpenStax, 2024. Read the whole section free at openstax.org.
The textbook is licensed under CC BY-NC-SA 4.0. This page quotes it word for word and adds the headings, the order and the practice questions; references to the book's figures and stray spaces left by its formatting are removed. The page is shared under the same licence. Nothing here is sold.