Normal EKG Rhythm, Axis, Intervals, and Waves

Normal Sinus Rhythm

Normal sinus rhythm is the typical rhythm of a healthy heart, distinguished by the following characteristics:

1:1 Ratio of P-Waves to QRS Complexes: Each P-wave is succeeded by a QRS complex, demonstrating that every atrial depolarization triggers a corresponding ventricular depolarization.

Regular Rhythm: The intervals between heartbeats are consistent, indicating a steady and uniform heart rate.

Consistent P-Wave Morphology: P-waves exhibit uniform morphology, typically appearing upright (positive) in lead II and inverted (negative) in lead aVR.

Intervals and Waves

Grasping the different intervals and waves on an EKG is essential for accurate interpretation of cardiac function. Here is an overview of each component.

P-Wave

Represents Atrial Depolarization: The process of electrical activation in the atria.

Components:

• The first part reflects the right atrium.
• The middle part reflects both atria.
• The terminal part reflects the left atrium

Depolarization Pathway:

• Begins at the sinus node in the right atrium.
• Spreads from right to left and slightly downward (angle of 0–75 degrees).

Lead Deflections:

• Leads facing the depolarization wave (I, II, aVL, aVF, V5, V6) show a positive deflection.
• Leads facing away (e.g., aVR) show a negative deflection.
• Lead V1, being perpendicular, records a biphasic P-wave (positive then negative).

Normal P-Wave Characteristics:

• Axis: 0–75 degrees.
• Duration: Less than 0.12 seconds.
• Amplitude:
  • Less than 2.5 mm in lead II (usually the tallest P-wave).
  • In V1:
    • Positive portion less than 1.5 mm.
    • Negative portion less than 1 mm in amplitude and duration.

Note: The P-wave is smaller in amplitude compared to the QRS complex because the atria are smaller and less muscular than the ventricles.

An interesting Exercise: What does a normal P wave look like in lead III, assuming the normal P wave vector is 0 to 75 degrees?

If the P wave axis falls between 0 and 30 degrees, lead III records a primarily negative deflection. If it’s 30 degrees, it’s biphasic. If it’s between 30 and 75 degrees, it records a primarily positive deflection.

PR Interval

• Definition: The time from the start of atrial depolarization (beginning of the P-wave) to the start of ventricular depolarization (beginning of the QRS complex).
• Reflects: primarily the delay in conduction at the AV node.
• Duration: Typically 0.12 to 0.20 seconds (three to five small boxes on the EKG).

PR Segment

Straight line from the end of p-wave to the beginning of QRS- Should be isoelectric (at baseline). Elevations or depressions may indicate underlying pathology.

QRS Complex

• Represents Ventricular Depolarization.
• Duration: Normally less than 0.10 seconds.
• Depolarization Pathway:
  • After the AV node, the impulse travels through the bundle of His.
  • Depolarization starts in the inter-ventricular septum, usually from left to right, handled by the left bundle branch’s septal fascicle..

Q-Wave: Normal vs. Abnormal.

• Small Q-waves may be seen in leads I, aVL, V5, and V6 due to septal depolarization as it travels from left to right, away from these leads.
• Concerning Q-Waves:
  • Any Q-wave in leads V2 to V3 ≥ 20 ms or a QS complex in these leads.
  • Q-wave ≥ 30 ms and ≥ 0.1 mV deep in two contiguous leads where a positive deflection is expected.
  • QS complex in leads I, II, aVL, aVF, V4 to V6 (leads where we expect a net positive deflection)
• Note: Q-waves in aVR are not significant; can be normal in lead III.

QRS axis

The QRS complex amplitude is significantly greater than the P wave amplitude due to the ventricles’ much higher muscular mass compared to the atria. Furthermore, the left ventricle dominates what we see on the EKG due to its larger mass.

Determining the Axis

A good way to determine the axis is by looking at the net vector of the wave being analyzed in 3 leads:

• I: since it is aligned at 0°, any vector between – 90° and +90° will be registered as a net positive deflection. Otherwise, it will be negative.
• II: since it is aligned at 60°, any vector between – 30° and +120° will be registered as a net positive deflection. Otherwise, it will be negative.
• aVF: since it is aligned at 90°, any vector between 0° and 180° will be registered as a net positive deflection. Otherwise, it will be negative.

By looking at the wave in each of these leads and looking for an overlap, we can determine the axis of a wave.

I	II	aVF	Overlap	QRS Axis
+	+	+	0° to 90°	Normal
+	+	–	0° to -30°	Normal
+	–	+	N/A	Not possible
+	–	–	-30° to -90°	Left axis deviation
–	+	+	90° to 150°	Right Axis Deviation
–	+	–	N/A	Not possible
–	–	+	150° to 180°	Right Axis Deviation
–	–	–	180° to -90°	Extreme Right Axis Deviation

Normal, early, and poor R-wave progression

• R Wave Progression in Precordial Leads:
  • V1 and V2: Show deep S-waves due to right ventricle overlay.
  • V5 and V6: Show tall R-waves due to left ventricle overlay.
  • Transition Zone: R-wave becomes taller than the S-wave, usually in V3 or V4.
• Early R-Wave Progression: Transition occurs in V1 or V2.
• Late R-Wave Progression: Transition occurs in V5 or V6.

Normal vs. Low voltage

• Normal Voltage: R-wave height plus S-wave depth should be at least 5 mm in limb leads. Also, it needs to be at least 10 mm in precordial leads to avoid low voltage designation.

J-Point

• Definition: The point where the QRS complex transitions into the ST segment. The QRS complex represents the depolarization of the ventricles. The ST segment, on the other hand, represents the period when the ventricles are depolarized, but have not yet begun to repolarize.
• Significance:
  • Used as a reference for assessing the ST segment.
  • Correct identification is crucial for detecting ST segment elevations or depressions, which can indicate myocardial infarction or ischemia.

ST Segment

• Definition: Extends from the end of the QRS complex to the beginning of the T-wave.
• Represents: The period when the ventricles are depolarized but have not yet started repolarizing.
• Normal Characteristics:
  • Usually horizontal or gently upsloping.
  • At the same level as the TP segment (isoelectric line).
• Abnormal Elevations (Measured at the J-point):
  • Must be ≥ 0.1 mV in most leads.
  • In leads V2 to V3:
    • ≥ 0.15 mV in women.
    • ≥ 0.2 mV in men ≥ 40 years.
    • ≥ 0.25 mV in men < 40 years.
• Abnormal Depressions: Horizontal or downsloping ST-segment depression ≥ 0.5 mm below baseline measured 0.08 seconds (2 small boxes) after the J-point in two contiguous leads.

T-wave

• Represents Ventricular Repolarization.
• Normal Characteristics:
  • Positive in leads with a positive QRS complex.
  • Negative in leads with a negative QRS complex.
• Amplitude: Usually 1/3 to 2/3 the height of the corresponding R-wave.
• Shape: Typically asymmetric with a slow upstroke and rapid downstroke.
• Abnormalities:
  • Peaked T-Waves: T-waves taller than 5 mm in limb leads or 10 mm in precordial leads may be concerning.
  • Inverted T-Waves: Symmetric inversion of more than 1 mm may indicate pathology.

QT Interval

• Definition: The time from the start of ventricular depolarization (beginning of QRS complex) to the end of ventricular repolarization (end of T-wave).
• Measurement:
  • From the beginning of the QRS complex to where the T-wave returns to baseline.
  • Use a tangent line on the downslope of the T-wave if the end is not clear.
• Duration:
  • Varies with heart rate; usually about 40% of the cardiac cycle.
  • Corrected QT (QTc): What is normal is dependent on the rate and QT is not very predictive of an adverse outcome at extremes of heart rate or in bundle branch blocks.
    • Adjusted for heart rate using the formula: QTc = QT / √RR.
    • Normal QTc Range: 0.35 to 0.46 seconds.
    • Quick Assessment: The QT interval should be less than half the preceding RR interval.

U-Waves

• Definition: A small wave that may follow the T-wave.
• Origin: Not fully understood.
• Characteristics: Typically a small upward deflection. Smaller than the T-wave and may be difficult to see.
• Clinical Significance: Prominent U-waves (> 2 mm) may be seen in conditions like hypokalemia and bradycardia.

TP Segment

• Definition: The segment from the end of the T-wave to the beginning of the next P-wave.
• Significance: Considered the true isoelectric line and used as a reference point to compare elevations or depressions in the PR and ST segments.