CCMA · EKG and Cardiovascular Testing · 7 min read
Cardiac Conduction, Waveforms and EKG Paper
- Trace the pathway of electrical impulses through the cardiac conduction system and identify the intrinsic rates of native pacemaker sites.
- Identify individual EKG waveforms, segments, and intervals, including their normal durations and underlying physiological events.
- Explain standard EKG paper grid measurements for time and voltage, including appropriate paper speed and gain adjustments.
The Cardiac Conduction System and Natural Pacemaker Sites
The heart relies on a specialized network of cardiac tissue to generate and distribute electrical impulses. This process begins at the cellular level with depolarization and repolarization. Depolarization is an electrical event caused by ion exchange across cell membranes that triggers the mechanical contraction of heart muscle. Repolarization is the electrical recovery process that follows depolarization and can occur while the heart muscle is still contracting. Together, these coordinated cycles allow the chambers of the heart to pump blood effectively.
The primary pacemaker of the heart is the sinoatrial (SA) node, which is located in the upper wall of the right atrium. Under normal physiological conditions, the SA node spontaneously generates electrical impulses at an intrinsic rate of 60 to 100 beats per minute. Because it possesses the fastest firing rate in the heart, it sets the pace for the entire cardiac cycle. From the SA node, electrical impulses travel across the internodal pathways of the atria to reach the atrioventricular (AV) node.
The AV node is positioned in the floor of the right atrium near the interatrial septum. A critical function of the AV node is that it briefly delays conduction. This intentional pause allows sufficient time for atrial contraction and ventricular filling to finish before ventricular contraction begins. An everyday comparison is an usher pausing incoming theatergoers at the door for a few seconds so that the previous group can clear the aisle and find their seats before the main performance starts. If the SA node fails to generate an impulse, the AV node can act as a backup pacemaker, possessing an intrinsic rate of 40 to 60 beats per minute.
After passing through the AV node, electrical impulses travel through the bundle of His into the interventricular septum. Within the septum, the bundle of His splits into the right bundle branch and the left bundle branch. Extending outward from these bundle branches are the Purkinje fibers, which rapidly distribute impulses throughout the ventricular myocardium. Should both the SA node and AV node fail, the Purkinje network possesses its own backup intrinsic pacemaker rate of 20 to 40 beats per minute.
Common mistakes
- Measuring the PR interval from the end of the P wave instead of from the beginning of the P wave to the beginning of the QRS complex.
- Confusing horizontal and vertical grid values on EKG paper, such as mistaking one small horizontal square (0.04 seconds) for voltage or mistaking one large square (0.20 seconds) for 1.0 second (which requires five large squares).
- Independently interpreting prolonged intervals, ST-segment shifts, or prominent U waves rather than relaying machine alerts and tracings to the provider for clinical evaluation.
- Failing to check machine calibration or failing to document nonstandard paper speed (such as 50 mm/s) or gain settings (such as 0.5x or 2x) on the tracing for provider review.
Key points
- The cardiac conduction system begins at the SA node (intrinsic rate 60 to 100 bpm), travels through internodal pathways to the AV node (intrinsic rate 40 to 60 bpm) where conduction delays to allow ventricular filling, passes through the bundle of His and bundle branches, and terminates in the Purkinje network (intrinsic rate 20 to 40 bpm).
- Depolarization is an ion-exchange electrical event that triggers muscle contraction, while repolarization is the recovery process returning cells to a resting state.
- Key waveforms include the P wave (atrial depolarization; smooth, rounded, upright in lead II), QRS complex (ventricular depolarization; obscuring atrial repolarization), T wave (ventricular repolarization), and occasionally a U wave (prominent in hypokalemia, interpreted by provider).
- Normal PR interval duration is 0.12 to 0.20 seconds (measured from start of P wave to start of QRS complex), and normal QRS duration is less than 0.12 seconds (typically 0.06 to 0.11 seconds).
- The J point connects the QRS complex to the ST segment; ST elevation or depression may indicate myocardial injury or ischemia, requiring the medical assistant to promptly relay machine alerts to the provider without independent interpretation.
- Standard EKG paper horizontal grid measures time (one small square = 0.04 s; one large square = 0.20 s; five large squares = 1.0 s) at a standard speed of 25 mm/s (50 mm/s widens waveforms for rapid rhythms).
- Standard vertical gain (10 mm/mV) yields a 10-mm mark for a 1.0-mV pulse (one small square = 0.1 mV; one large square = 0.5 mV), with half-standardization (5 mm) used for overlapping waves and double-standardization (20 mm) used for low voltage, both requiring documentation by the medical assistant.
Practice this section
Explained CCMA EKG and Cardiovascular Testing questions, no account needed. CCMA EKG and Cardiovascular Testing practice test
The full lesson continues with 4 more sections with exam tips, audio in English and Spanish, 5 Check yourself questions and 20 flashcards.
- 1 month
$41.99$20.99 - 3 months
$122.99$61.49 - Lifetime
$251.99$125.99
One payment, no subscription.
See CCMA plansAlready a member? Log in
Sample lesson: Electrodes, Leads and 12-Lead Placement25 practice questions on this exam
Each CCMA practice test has 25 original questions with the answers explained. No account needed.
Open the CCMA practice testsOriginal lesson written for CCMA practice · updated October 31, 2026. Exam names are trademarks of their owners; tests.global is not affiliated with them.