Chapter 11: The Normal Electrocardiogram

           

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The Normal Electrocardiogram

An electrocardiogram (ECG) is a graphic recording of electrical potentials generated by the heart during each cardiac cycle. As electrical impulses pass through cardiac tissue, current spreads to surrounding tissues and reaches the surface of the body. Electrodes placed on the skin detect these minute voltage changes, allowing clinicians and students to analyze the electrical activity driving cardiac mechanical events.

Understanding the components of a normal ECG waveform—including the P wave, QRS complex, and T wave—is fundamental to evaluating heart function. This text breaks down the mechanisms of depolarization and repolarization, the timing and voltage calibrations of standard ECG paper, the correlation between electrical signals and physical contractions, and the methods used to record these vital signals accurately.

Characteristics of the Normal Electrocardiogram

• Cardiac impulses generate electrical currents in surrounding tissues.
• Current spreads to the surface of the body.
• Skin electrodes record these electrical potentials as electrocardiograms.
• Normal ECG consists of P, QRS, and T waves.
• QRS complex contains Q, R, and S waves.
• P wave represents atrial depolarization before atrial contraction.
• QRS complex represents ventricular depolarization before contraction.
• P wave and QRS complex are depolarization waves.
• T wave represents ventricular repolarization and recovery.
• T wave occurs 0.25–0.35 seconds after depolarization.
• Electrocardiogram records both depolarization and repolarization waves.

Depolarization Waves versus Repolarization Waves

• Depolarization reverses normal negative internal cell potential.
• Inside becomes slightly positive; outside becomes negative.
• Traveling depolarization creates potential difference between electrodes.
• Halfway depolarization creates maximum positive wave deflection.
• Complete depolarization returns recorded wave to zero baseline.
• Equal external negativity produces no potential difference.
• Wave resulting from spreading depolarization is a depolarization wave.
• Repolarization restores positive charges to cell surface.
• Halfway repolarization creates negative wave deflection.
• Complete repolarization returns recorded wave to zero baseline.
• Equal external positivity produces no potential difference.
• Wave resulting from spreading repolarization is a repolarization wave.

Monophasic Action Potential and ECG Waves

• Ventricular monophasic action potential lasts 0.25–0.35 seconds.
• Action potential upsweep is caused by rapid depolarization.
• Action potential return to baseline is caused by repolarization.
• QRS complex appears at action potential onset.
• T wave appears at action potential end.
• No potential is recorded during complete depolarization.
• No potential is recorded during complete polarization.
• Current flows only during partial polarization and depolarization.
• Body surface records current during partial polarization states.

Relationship of Contraction to ECG Waves

• Depolarization precedes muscle contraction chemical processes.
• P wave occurs at beginning of atrial contraction.
• QRS complex occurs at beginning of ventricular contraction.
• Ventricles remain contracted until after T wave ends.
• Atria repolarize 0.15–0.20 seconds after P wave ends.
• Atrial repolarization occurs during QRS complex recording.
• Atrial T wave is obscured by QRS complex.
• Atrial T wave is seldom observed in ECGs.
• Ventricular repolarization wave forms normal T wave.
• Ventricular repolarization begins 0.20–0.35 seconds post-QRS.
• Ventricular repolarization extends over 0.15 seconds duration.
• Prolonged duration makes T wave prolonged in shape.
• T wave voltage is lower than QRS complex.

Voltage and Time Calibration of ECG

• ECGs are recorded on calibrated paper grids.
• Calibration lines are pre-ruled or printed simultaneously.
• Ten small divisions vertically represent 1 millivolt.
• Upward direction represents positive electrical potential.
• Downward direction represents negative electrical potential.
• Vertical lines represent time calibration divisions.
• Standard paper speed is 25 millimeters per second.
• Horizontal 25 millimeters equals 1 second interval.
• Dark vertical lines represent 0.20 second segments.
• Small horizontal boxes represent 0.04 second intervals.

Normal Voltages in the Electrocardiogram

• Recorded voltages depend on electrode placement locations.
• Direct precordial electrode records 3–4 millivolts QRS.
• Membrane monophasic action potential measures 110 millivolts.
• Limb leads record QRS voltage of 1.0–1.5 millivolts.
• P wave voltage ranges between 0.1–0.3 millivolts.
• T wave voltage ranges between 0.2–0.3 millivolts.

ECG Intervals and Heart Rate Determination

• P-Q interval measures atrial to ventricular excitation time.
• P-Q interval begins at P wave start.
• P-Q interval ends at QRS complex start.
• Normal P-Q interval duration is about 0.16 seconds.
• Termed P-R interval when Q wave is absent.
• Q-T interval measures ventricular contraction duration.
• Q-T interval extends from Q wave to T end.
• Normal Q-T interval duration is about 0.35 seconds.
• Heart rate equals reciprocal of beat time interval.
• One second beat interval equals 60 beats per minute.
• Normal adult QRS interval is about 0.83 seconds.
• Normal resting rate is about 72 beats per minute.

Methods and Recorders for Electrocardiograms

• Cardiac potential shifts occur in under 0.01 seconds.
• Recording apparatus must respond rapidly to voltage changes.
• Modern clinical systems use computer-based electronic displays.
• Direct pen recorders write signals on moving paper.
• Ink pen systems connect to electromagnet mechanisms.
• Heat-sensitive paper turns black from heated stylus tip.
• Electrical-current paper turns black under electrical stylus tip.