Electrocardiography


Electrodes and Leads

Leads are differences in Voltages between Electrodes
There are 12 possible Leads
	Extremity Leads (or Limb leads)
	Chest (or Prechordial leads)

Limb Leads (6)
	Bipolar
	Unipolar

Bipolar

Voltage is measured with respect to another electrode

	Electrodes = Right Arm (RA), Left Arm (LA) and Left Leg (LL)
	Einthoven's Triangle
		Lead I = LA - RA
		Lead II = LL - RA
		Lead III = LL - LA

	Relationship (I + III = II)
			    I = LA - RA
		        + III = LL - LA
		      I + III = LL - RA = II
 	Right Leg (RL) is a ground

Unipolar

	Voltage is measured with respect to zero

	Electrodes (aVR, aVL, and aVF) 
		a = Augmented (increased by 50%)
		V = Voltage
		R, L, & F = Right, Left, and Foot

	Relationship
		if combined together, Leads I, II, and II would be equal Zero (I + III - II = 0)

	Leads are combined in system, and individual electrodes are compared to zero (0)

	Polarity exists (positive aVF  is down, positive aVR is up and right, positive aVL is up and left)

Chest (Prechordial) Leads

	Voltage is also measured with respect to zero (unipolar)
	Positive pole of electrode points toward electrode, negative is across body

	Electrodes
		V1, V2, V3, V4, V5, & V6
		V1 = 4th  intercostal space, right of sternum
		V2 = 4th  intercostal space, left of sternum
		V3 = Midway between V2 and V4
		V4 = Midclavicular line in 5th intercostal space
		V5 = Anterior axillary line in 5th intercostal space
		V6 = Midaxillary line in 5th intercostal space

	Viewing the QRS:
		using the V1 the Q and R waves are small
		moving toward V6, Q and R increase and S decreases
		therefore, V1 = RS complex view, V6 = QR complex view
		transition lead is where R and S are equal

Reading the ECG

At 25 mm/second:
	each millimeter is 0.04 seconds (small box)
	every 5 millimeters is a dark line (0.2 seconds)

Heart Rate Determination:

1. Count the number of cardiac cycles that occur every 6 seconds and multiply this number by 10.  A vertical mark is on the top of the ECG paper every 3 seconds.
2. Divide the constant, 300, by the number of large time boxes between two (2) successive R waves.
3. Divide the constant, 1500, by the number of small time boxes between two (2) successive R waves.

Bradycardia = HR less than 60
Tachycardia = HR greater than 100

Basic Electrocardiography

P wave: Atrial depolarization
QRS Complex:  Ventricular Depolarization
ST Segment	}
T Wave  	} Ventricular Repolarization
U Wave		}

P Wave
	normally has amplitude less than .25 mV
	normally has duration of less than 0.12 seconds
	best detected in leads II, III, aVF, and sometimes V1

PR Interval
	beginning of P wave to beginning of QRS complex
	normal PR interval is 0.12 - 0.2 seconds (adults)
	impairment in AV junction prolongs interval ( > 0.2 seconds: first-degree heart block)

QRS Complex
	may be missing a Q, R, or an S wave
	if initial deflection is negative, it is a Q wave
	the first positive deflection is the R wave
	a negative deflection following an R wave is the S wave
	any extra waves are R' if positive or S' if negative

	QRS Complex Interval:
		normally less than 0.1 seconds
		may be slowed due to block in bundle branches, etc.

ST Segment
	end of QRS complex to beginning of T wave
	is usually isoelectric (flat on the baseline)
	elevation or depression > 0.1 mV is abnormal
	myocardial infarction produces deviations
	beginning of ST segment is sometimes called the J (junction) point

T Wave
	normal is asymmetrical shape
	should rise (or fall) gradually and then return to baseline rapidly
	myocardial infarction can lead to symmetrical shape

QT Interval
	beginning of QRS complex to the end of the T wave
	interval depends upon heart rate (RR interval)
	as heart rate increases, both the RR and QT intervals decrease
	
U Wave
	small rounded deflection following the T wave
	often is merged with the T wave and imperceptible
	unknown why they occur
	may predispose patients to ventricular arrhythmia's

Examples of Possible Arrhythmia's

	Right Atrial Enlargement (RAE)
		excessively high P wave
	Left Atrial Enlargement (LAE)
		excessively wide P wave

Right Ventricular Hypertrophy (RVH)
	tall R wave (especially in V1)
	other deviations as well
	Left Ventricular Hypertrophy (LVH)
		tall S waves in right chest leads and tall R waves in left chest leads
		decreasing ST segment (indicates Left Ventricular Strain)

	Right or Left Bundle Branch Blocks (complete or incomplete)
	Myocardial Ischemia (low oxygen)

Myocardial Infarction (death)

	There are hundreds more, especially when the occur simultaneously