ECG interpretation and lead placement

ECG uses:

ECG interpretation is important knowledge for; Pre anaesthetic screening Evaluating cardiac responses to drug therapy. Screening for drug toxicity. Diagnosis of Cardiovascular disorders, Metabolic disorders and electrolyte imbalances in the body.

What is an ECG?

Graphic recording of the voltage and direction of electrical activity during depolarization and repoplarization of cardiac muscle cells plotted against time.

What can an ECG give information about?

Anatomical orientation of the heart, The relative size of the heart chambers, Heart rate, Rhythm, Origin of excitation Speed of the impukse, Decay of excitation, Disturbances in the above events.

ECG lead placement

Standard recording of ECG; the Einthoven Limb ecg lead placment

Consists of 3 leads. Lead 1; right foreleg compared with left foreleg. Lead 2: right foreleg compared with left hindleg. Lead 3; left foreleg compared with left hindleg.

Augmented unipolar Goldberg leads From extremities, both hind and forelegs, but electrodes from 2 extremities are coupled and serve as indifferent electrode with respect to the third, different electrode. aVR right forleg (+) compared with average voltage of left foreleg and left hindlimb (-), aVL left foreleg (+) compared with average voltage of right foreleg and left hindleg (-) aVF left hindleg (+) compared with average voltage of roght foreleg and left foreleg (-) (a = augmented)

Positioning the animal

For successful ECG interpretation and measurement positioning is important. Therefore normally performed in an animal in right lateral recumbancy on a rubber topped table or a blanker to prevent 60Hz (cycle) interference.

The limb should be perpendicular with the body or slightly separate

Lead Placement

The skin can be wetted before or after electrodes are placed, usually with alcohol or ECG paste to improve electrical contact. Electrodes are connected to the skin with crocodile clips for correct ecg lead placement.

Yellow Electrode: Clip to the skin of the Left foreleg at elbow.

Red Electrode: Attach to skin opposite on the right elbow.

Green Electrode: Attached to skin of left stifle

Black Electrode: Attached to Right Stifle.

White Electrode is for earthing.

Calibration of the ECG

Once animal holder and patient are comfortable and electrodes are in place.

A 1-mV signal is sent through the device by pressing a button. The ECG machine is calibrated so that this 1-mV signal produces a 1-cm vertical deflection on the paper. (1cm=1mV). By calibration I this manner, the height of a the wave deflections can be measured in mV. Thus every 1mm = 0.1mV. This calibration can be altered to make the complexes larger or smaller. For example, too small = can increase sensitivity to 2cm = 1mV. If too small, reduce to 0.5 = 1mV

Speed

Most clinical ECG machines record at 25mm/sec and 50mm/sec. Usually all should be recorded at a paper speed of 50mm/sec. Mark each lead on the paper at the time of recording. The lead that produces the largest deflections should then be chosen and recorded at 25mm/sec to analyse Cardiac rhythm. If HR is very rapid, 50mm/sec widens the complex to ease accuracy of measurement.

Understanding the ECG

The contraction of the heart chambers is coordinated by several regions in the heart that are composed of myocytes with specialized automaticity sand conduction properties. The Sino atrial (SA) node (pacemaker) has the fastest intrinsic pacemaker rhythm it is usually the site of inititation of the cardiac electrical impulse during a normal cardiac beat. The impulse then rapidly depolarizes both the left and the right atria as it travels to the atriventricular (AV) node. After the delay at the AV node, the impulse moves rapidly down the His Bundle and Pirkinje Fibres to simultaneously depolarize the right and left ventricles.

Reading an EKG

The P wave represents depolarization of atrial tissue. The wave for atrial depolarization is not visible because it is masked by succeeding waves. The Q wave, The R wave and the S wave together constituting the QRS complex represent ventricular depolarization. The T wave comes after the QRS complex which reflects repolarisation of the ventricles. The P-Q interval represents primarily the conduction time of the AV node and the His Bundle.

For further online information on reading an ECG click here

ECG Interpretation

Should include: Determination of Heart rate, Cardiac Rhythm Rhythm disturbances Measurement of amplitude and intervals, And the determination of mean electrical axis.

To determine Heart Rate from an ECG

Usually the heart rate is counted over 3 seconds. To determine the number of complexes in 3 seconds identify a segment of the ECG recorded at 50mm/sec that is 150 mm in length and count the number of complexes within it. Three seconds is 1/20 of a minute so multiply the number of complexes by 20 to give the heart rate in 1 minute.

Cardiac rhythm determination

Sinus rhythms are those which originate from the sinus node.. The sinus rhythm in a dog is the normal rhythm.

A sinus rhythm looks like; P waves are positive in front of each QRS complex and a consistent relationship between P and QRS complexes should be present. P-R intervals should be equal.

Small irregularities in sinus rhythms are termed Sinus Arrhythmia’s.

Sinus Tachycardia; sinus rhythms exceeding upper limit of of normal Sinus Bradycarida; slower than the established normal limit.

The P wave

Should be measured from first upward deflection from baseline to the return to the baseline. Max width = 0.04 sec abd max Amp is 0.4mV Left atrial enlargement = P waves are wider than normal and often notched (P mitral). Right atrial enlargement = Pwaves of increased amplitude ( P pulmonale) often associated with chronic pulmonary disease causing pulmonary hypertension. Bi atrial enlargement = P waves are tall and wide, may be notched or slurred.

P-R interval

From first upward deflection of P wave to first deflection of the QRS complex. Norm in a dog = 0.06-0.13 P-R abnormalities occur primarily when an AV node abnormality exists. P-R interval is shortened when the AV node is bypassed P-R interval is prolonged when the AV node is diseased, and conduction through it is prolonged. Per oral admin of Digitalis = prolongation of P-R interval.

QRS Complex

Beginning of first deflection of QRS complex from baseline and QRS complex ends when returns to baseline. R wave Max width = 0.06 secs (large breeds) 0.05 (toy breeds). R wave max amp = 3.0 mV (large breeds) 2.5 mV (small breeds). Depth of S wave = max 0.35 mV. A high amp R wave which alters QRS = increased ventricular muscle mass in left ventricular enlargement.

S-T interval should arise from same level as baseline or T-P segment and normally isoelectric or slightly concave or convex (period of time from end of ventricular depolarization – QRS complex- to onset of ventricular repolarisation – T wave) A significant deviation or alteration in shape of S-T segment suggests myocardial hypoxia.

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