5 The Electrocardiogram

The Electrocardiogram Objectives Define the term electrocardiogram Describe the basics of EKG monitoring List the types of EKG leads Discuss the relevance of Einthoven’s triangle Identify and explain the grids and markings on a representative strip of EKG graph paper

The Electrocardiogram Objectives (continued) Describe the relationship of the following EKG waveforms to the electrical events in the heart P wave PR interval QRS complex ST segment T wave

The Electrocardiogram Graphic representation of the electrical activity of the heart Electrocardiograph Machine used to record the electrocardiogram, or EKG machine EKG Graphic tracing of electrical activity of the heart, not mechanical activity

The Electrical Basis of the EKG Electrical activity is sensed by electrodes placed on the skin surface Recorded in the form of an electrocardiogram Cardiac monitor depicts electrical impulses on monitor screen or oscilloscope

The Electrical Basis of the EKG Electrical impulses present on the skin surface are very low voltage; impulses are amplified by an EKG machine The printed record of the electrical activity of the heart is called a rhythm strip or an EKG strip

EKG Leads Electrode Leads An adhesive pad that contains conductive gel and attaches to patient’s skin Leads How electrodes are connected to the cardiac monitor 3 leads must have a positive, a negative, and a ground

EKG Leads

EKG Leads

EKG Leads

EKG Leads Einthoven’s triangle Imaginary inverted triangle formed around heart by proper placement of bipolar leads Top formed by lead I, right side formed by lead III, and left side formed by lead II Each lead represents a different look at, or view, of the heart

EKG Graph Paper Leaves the machine at constant speed of 25 mm/s Time Measured on horizontal line Amplitude or voltage Measured on vertical line on graph paper

EKG Graph Paper Electrocardiographic Paper divided into small squares 1 mm in width Time interval of 0.04 seconds

EKG Graph Paper Electrocardiographic paper Darker lines divide paper every fifth square Vertically and horizontally Large squares measure 5 mm in height and width Represents time interval of 0.20 seconds Five small squares in each large square

EKG Waveforms Wave or waveform refers to movement away from the baseline or isoelectric line (beginning and ending of all waves) Positive deflection Above isoelectric line Negative deflection Below isoelectric line

EKG Waveforms Electrical impulse leaves SA node, produces waveform on graph paper One complete cardiac cycle = P, Q, R, S, (QRS complex) and T wave

P Wave First wave produced by electrical impulse from SA node Smooth, rounded upward deflection Depolarization of left and right atria 0.10 seconds in length

PR Interval Time impulse travels from SA node through internodal pathways in atria toward ventricles Time interval from start of P wave to start of QRS 0.12-0.20 seconds in length

QRS Complex Consists of Q, R, and S waves Represents conduction of impulse from bundle of His through ventricular muscle Ventricular depolarization

QRS Complex Q wave R wave S wave First downward deflection R wave First upward deflection Largest deflection seen in lead I and II S wave Downward deflection after R wave Measures less than 0.12 seconds (3 small boxes)

ST Segment Time interval during which ventricles depolarized and repolarization of ventricles begin Isoelectric or consistent with baseline

T Wave Follows ST segment Represents ventricular repolarization Slightly rounded, positive deflection Resting phase of cardiac cycle

Summary of EKG Waveforms P wave Atrial depolarization QRS complex Ventricular depolarization, atrial repolarization T wave Ventricular repolarization