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HLT-EMT39-03 · Lesson 2 · 24 clock hours · theory

Electrocardiography Fundamentals: Leads, Waveforms, and Intervals

Objective. Learner will explain the standard 12-lead ECG lead system, correctly identify each waveform component and interval, and apply a systematic rate and rhythm analysis method to normal sinus rhythm strips.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

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What Is an Electrocardiogram?

An electrocardiogram (ECG or EKG) is a graphic recording of the electrical activity generated by the heart with each beat. The heart's conduction system produces electrical impulses that spread through the myocardium in a predictable sequence; electrodes placed on the skin detect those impulses from different angles. As a paramedic, you will use the 12-lead ECG to detect ischemia, injury, infarction, and dysrhythmias. Accurate interpretation begins with a solid understanding of leads, waveforms, and intervals.

The 12-Lead System

The standard 12-lead ECG uses ten physical electrodes to create twelve views of the heart's electrical activity. The leads fall into two groups.

Limb leads use four electrodes placed on the extremities (right arm, left arm, right leg, left leg). They produce six leads:

Precordial (chest) leads V1–V6 use six electrodes placed across the chest. They are unipolar leads that view the heart from a horizontal plane. Standard placement:

Precise electrode placement is essential. Misplaced electrodes can mimic or mask pathology, leading to incorrect treatment decisions — a direct patient safety concern.

ECG Paper and Scale

Standard ECG paper moves at 25 mm per second. Each small box is 1 mm wide (0.04 seconds) and 1 mm tall (0.1 mV). Each large box contains five small boxes, equaling 0.20 seconds horizontally and 0.5 mV vertically. Calibration is confirmed by the standard calibration pulse: a 1 mV signal that deflects exactly 10 mm (two large boxes) tall. Always confirm calibration before measuring intervals.

Waveform Components

P wave: Represents atrial depolarization. In normal sinus rhythm (NSR), the P wave is upright in leads I and II, rounded, and precedes every QRS complex.

QRS complex: Represents ventricular depolarization — the electrical event that triggers the pumping contraction. It consists of:

T wave: Represents ventricular repolarization. Normally upright in most leads. An inverted or markedly peaked T wave can indicate ischemia or electrolyte disturbance.

U wave: A small deflection occasionally seen after the T wave; its clinical significance is variable and is not required for basic interval measurement.

Key Intervals and Normal Ranges

| Measurement | Normal Range |

|---|---|

| P-wave duration | 0.06 – 0.12 seconds (up to 3 small boxes) |

| PR interval | 0.12 – 0.20 seconds (3 – 5 small boxes) |

| QRS duration | Less than 0.12 seconds (less than 3 small boxes) |

| QT interval | Approximately 0.36 – 0.44 seconds; varies with heart rate |

The PR interval is measured from the start of the P wave to the start of the QRS complex. It represents the time for the impulse to travel through the AV node. Prolongation beyond 0.20 seconds suggests AV block.

The QT interval is measured from the start of the QRS to the end of the T wave. A prolonged QT interval is associated with risk of life-threatening dysrhythmias.

The Six-Step Rhythm Analysis Method

Always approach rhythm interpretation in a systematic, stepwise manner. Skipping steps leads to missed diagnoses.

1. Rate: Count the ventricular rate. The 300-divide method: divide 300 by the number of large boxes between two consecutive R waves. For irregular rhythms, count the number of QRS complexes in a 6-second strip and multiply by 10. Normal rate: 60–100 beats per minute.

2. Regularity: Are the R-to-R intervals consistent? Use calipers or a card to compare intervals. A rhythm is regular if all R-R intervals are equal or vary by less than 0.08 seconds.

3. P waves: Are P waves present? Is there one P wave before every QRS? Are they all the same shape? P waves should be upright and uniform in lead II for sinus rhythms.

4. PR interval: Measure the PR interval and determine whether it falls within the normal range (0.12–0.20 s). Is it constant across all beats?

5. QRS duration: Measure the QRS and determine whether it is narrow (less than 0.12 s) or wide. A wide QRS suggests a ventricular origin or conduction delay.

6. Interpretation: Combine all findings and name the rhythm. For this lesson, the expected interpretation for practice strips is normal sinus rhythm: rate 60–100, regular, one upright P before each QRS, PR interval 0.12–0.20 s, QRS less than 0.12 s.

Safety Points

Key terms9

DepolarizationThe electrical change across a cardiac cell membrane that triggers muscle contraction; recorded as waveforms on the ECG.
PR intervalThe time from the beginning of the P wave to the beginning of the QRS complex, representing conduction through the AV node; normally 0.12–0.20 seconds.
QRS complexThe ECG waveform representing ventricular depolarization, consisting of the Q, R, and S waves; normally less than 0.12 seconds in duration.
QT intervalThe time from the start of the QRS complex to the end of the T wave, representing total ventricular electrical activity; prolongation is associated with dysrhythmia risk.
Precordial leadsThe six chest leads (V1–V6) that view the heart's electrical activity from a horizontal plane using electrodes placed across the anterior chest.
Augmented limb leadsUnipolar leads (aVR, aVL, aVF) that compare one limb electrode to a combined electrical reference point to produce a view of the heart's electrical axis.
RepolarizationThe recovery phase of a cardiac cell, during which the membrane returns to its resting charge; represented on the ECG by the T wave.
CaliperA tool used in ECG analysis to measure and compare intervals and waveform spacing precisely on a rhythm strip.
Normal sinus rhythm (NSR)The expected baseline cardiac rhythm: rate 60–100 bpm, regular, one upright P wave before each QRS, PR interval 0.12–0.20 s, QRS less than 0.12 s.

Worked examples4

1. On a rhythm strip, two consecutive R waves are separated by exactly 4 large boxes. What is the heart rate?
  1. Recall the 300-divide method: Rate = 300 ÷ (number of large boxes between R waves).
  2. Count the large boxes between the two R waves: 4 large boxes.
  3. Divide: 300 ÷ 4 = 75 beats per minute.
Answer: The heart rate is 75 beats per minute, which falls within the normal range of 60–100 bpm.
2. You measure the PR interval on a strip and count 4 small boxes from the start of the P wave to the start of the QRS. What is the PR interval in seconds, and is it normal?
  1. Each small box on standard ECG paper = 0.04 seconds.
  2. Multiply: 4 small boxes × 0.04 s = 0.16 seconds.
  3. Compare to the normal range: 0.12–0.20 seconds.
  4. 0.16 seconds falls within the normal range.
Answer: The PR interval is 0.16 seconds, which is within the normal range of 0.12–0.20 seconds.
3. You count 15 QRS complexes in a 6-second rhythm strip. What is the heart rate?
  1. Use the 6-second strip method: Rate = (number of QRS complexes in 6 seconds) × 10.
  2. Multiply: 15 × 10 = 150 beats per minute.
  3. Compare to normal: 60–100 bpm. This rate is above normal (tachycardia).
Answer: The heart rate is 150 beats per minute, which is faster than normal sinus rhythm and indicates tachycardia.
4. The QRS complex on a strip spans 2.5 small boxes. What is the QRS duration in seconds, and is it normal?
  1. Each small box = 0.04 seconds.
  2. Multiply: 2.5 × 0.04 s = 0.10 seconds.
  3. Compare to the upper limit of normal: less than 0.12 seconds.
  4. 0.10 seconds is less than 0.12 seconds, so this is a normal, narrow QRS complex.
Answer: The QRS duration is 0.10 seconds, which is within normal limits (narrow complex).
In class: Using graph paper calibrated to standard ECG scale, measure P-wave duration, PR interval, QRS duration, and QT interval on five printed normal sinus rhythm strips and compare findings against published normal ranges. · On a torso manikin, practice correct limb and precordial electrode placement for all 12 leads using the manufacturer's placement guide, and verify placement with an instructor before printing a test strip. · Apply the six-step rhythm analysis method (rate, regularity, P waves, PR interval, QRS, interpretation) to ten normal and near-normal strips, recording findings in a structured analysis worksheet.
Materials: ECG graph paper, calipers, printed rhythm strip sets, 12-lead ECG machine or trainer unit, electrode pads, torso manikin or standardized patient volunteer, ECG measurement reference card
Check: Learner applies the six-step method to five strips without instructor prompting, correctly measures all intervals, and accurately interprets each rhythm; 80% accuracy (4 of 5 strips fully correct) required to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

EMS Operations / Patient Assessment
1. Which electrode placement is correct for lead V4 in a standard 12-lead ECG?
Cardiology / Rhythm Interpretation
2. On standard ECG paper running at 25 mm/second, one large box represents how many seconds?
Cardiology / Rhythm Interpretation
3. Which waveform on the ECG represents ventricular depolarization?
Cardiology / Rhythm Interpretation
4. A paramedic measures 6 large boxes between two consecutive R waves. Using the 300-divide method, what is the ventricular rate?
Cardiology / Rhythm Interpretation
5. In the six-step rhythm analysis method, which step immediately follows assessment of rate and regularity?
Cardiology / Rhythm Interpretation
6. Which of the following PR interval measurements falls outside the normal range and suggests a possible conduction delay?
EMS Operations / Patient Safety
7. A student places the V1 electrode at the fourth intercostal space on the left sternal border instead of the right. What is the primary concern?
Cardiology / Rhythm Interpretation
8. Which of the following best describes normal sinus rhythm?
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