How to measure heart rate from ecg (electrocardiogram)

How to measure heart rate from ECG:

Before going to calculate heart rate from an ecg strip. Lets me give a short introduction to ECG and its leads.

Electrocardiogram (ECG)

Graphic recording of electrical potential generated due to transmission of depolarization wave (or cardiac impulse) through the heart, and due to its spread into surrounding tissue and body surface. Is called electrocardiogram (ECG).

Father of ECG

Einthoven (1903)

Significance

(1) Site of pace-maker is recorded

(2) Heart. Rate can be calculated

(3) Rhythm of heart can be recognised

(4) The voltage produced due to potential changes in the heart can be calculated.

(5) Helps to diagnose heart diseases.

Waves of ECG

(I) P Wave: produced by atrial depolarization.

(2) QRS Complex: Produce by ventricular depolarization.

(3) ST Segment and T Wave: Produce by ventricular repolarization (Ganong)

(4) Atrial T Wave: It is a depression in PR segment, usually obscured by QRS complex. Produced by atrial repolarization; Seen in (a) Sinus tachycardia (b) Hypertrophied atria.

(5) U Wave

(Inconstant). Produced by slow repolarization of the papillary muscle. Seen in hypokalemia

PQ or PR Interval

It shows

(1) Time interval b/w onset of atrial contraction and onset of ventricular contraction. OR

(2) AV nodal delay

Value

1. 16 second (120-210 msecond)

QT Interval

It shows a duration of ventricular contraction (from the

beginning of Q (or R) wave to the end of T wave).

Value

0.35 second

Total Time Period Of ECG

it shows time for one heart beat.

Value

0.83 second

Calculation of Heart Rate From it

In 1 sec. Heart beats-1 time

In 60 sec. Heart beats-1/0.83 time

So, Heart Rate = 72 beats / min

Calculation of Heart Rate From ECG  

(1) Calculate smallest squares b/w  2 successive R waves and divide it by 1500. The result is Heart Rate.

(2) Calculate large squares b/w 2 successive R waves and divide it by 300, Result is Heart Rate.

(3) If the cardiac rhythm is irregular, then count QRS complexes in 15 large squares, multiply it by 20, Result is Heart Rate.

Recording of ECG

Apparatus

Electrocardiograph

Kinds

Two

(1) Pen recorder(old)

(2) Oscilloscope(modern)

Electrical Current In Partly Depolarized Heart

(1) Base is negative

(2) Apex is positive

Electrocardiographic (ECG) Leads

Conventional electrode system that carries electrical potential from body surface to electrocardiograph while recording ECG, are called electrocardiographic

leads

Types

(1) Bipolar limb leads

(2) Precordial (chest) leads

(3) Augmented unipolar limb leads.

Bipolar Limb Leads

Here ECG is recorded from two specific electrodes connected to limbs.

Types

Three

(1) Lead l

+ ve terminal j Left arm

-ve terminal + Right arm

(2) Lead II

-ve terminal + Right arm

+ ve terminal + Left leg

(3) Lead III

+ ve terminal + Left leg

-ve terminal + Left arm

Precordial (Chest) Leads

Here ECG is recorded by placing + ve electrode to one of six separate positions on chest over heart and by connecting -ve (or indifferent) electrode simultaneously to right arm, left arm and left leg,

Six Precordial Positions

V1 = 4th intercostal space 1 inch away from right

sternal border

V2 = 4th intercostal space 1 inch away from left

sternal border

V3 = Mid-point b/w V2 and V4

V4 = 5th intercostal space at left mid-clavicular

line

V5 = Point where anterior axillary line cuts

perpendicularly the horizontal line extended

from V4

V6 = Point where mid-axillary line cuts

perpendicularly the same horizontal litre

extended from V4

 

Note

(1) QRS complex of V1 and V2 is -ve because they are towards base of heart (-ve)

(2) QRS complex of  V4, V5 and V6 is + ve; because they are towards apex of heart(+ ve)

Augmented Unipolar Limb Leads

Here two limbs are connected to -ve terminal while the third limb to + ve terminal; this positive terminal is known as follows:

(1) aVR Lead: When + ve terminal is connected to right arm

(2) AVL Lead: When + ve terminal is connected to left arm

(3) aVF Lead: when + ve terminal is connected to left leg