Electrocardiogram

Chapter 4 Electrocardiogram



HEART RATE CALCULATION


Calculate heart rate from electrocardiogram (ECG) strip as follows.


▪ Each large box = 5 × 5 small boxes

▪ At a paper speed of 25 mm/sec:
□ 5 large boxes (a 25-mm length of paper) pass in 1 second

□ In 1 minute, 5 × 60 boxes (300) pass
• Therefore each large box = 0.2 second

□ 25 small boxes pass in 1 second

□ In 1 minute, 25 × 60 boxes (1500) pass
• Therefore each small box = 0.04 second

Use either of the following methods to calculate heart rate. (Either method can be used in any species, as long as the heart rhythm is regular; this method is not reliable if bradycardia with pauses is present.)


First, start at a complex on a heavy line (see arrows in figures following), then go to the next complex.


▪ Count number of large boxes (5 × 5 small boxes) between consecutive R waves of complexes
□ Divide 300 by this number

▪ Count the number of small boxes between consecutive R waves of complexes
□ Divide 1500 by this number

At a paper speed of 50 mm/sec use the same method but substitute 600 for 300 and 3000 for 1500.



Dog



image This dog’s heart rate can be calculated as 300 ÷ 2.4 = 125. An alternative method is 1500 ÷ 12 = 125.


Cat



image This cat’s heart rate can be calculated as 1500 ÷ 9.5 = 158. An alternative method is 300 ÷ 1.9 = 158.


RESPIRATORY SINUS ARRHYTHMIA



Features



▪ Alternating periods of faster and slower heart rates, often related to respiratory pattern

▪ Increased heart rate during inspiration

▪ Decreased heart rate during expiration and respiratory pause

▪ Usually, normal conformation of P wave and QRS complexes

▪ Accentuated with increased vagal tone or vagotonic procedures (e.g., increased ophthalmic pressure)

▪ May be more noticeable in brachycephalic breeds


image Respiratory sinus arrhythmia in upper strip has been replaced by sinus rhythm after administration of anticholinergic agents (lower strip).


Therapy



▪ Normal finding; no therapy usually necessary

▪ If arrhythmia is pronounced or is associated with marked bradycardia, anticholinergic agents may be administered


VENTRICULAR BIGEMINY



Features



▪ Normal complexes alternate with premature ventricular contractions


image Ventricular bigeminy in a dog after thiopental administration for anesthetic induction.


Cause



▪ Commonly seen after IV barbiturate administration

▪ Directly related to concentration of solution


Therapy



▪ Usually not necessary; condition usually resolves within a few minutes; ensure adequate ventilation and evaluate for excessive depth of anesthesia


SECOND-DEGREE ATRIOVENTRICULAR BLOCK



Features



▪ Some P waves not followed by QRS complexes and T waves

▪ Mobitz type I block
□ PR interval becomes progressively longer until block occurs

□ P waves usually normal

□ QRS and T usually normal when present

▪ Mobitz type II block
□ PR interval is constant

□ P waves usually normal

□ QRS more often abnormal


image Second-degree atrioventricular block in a dog. This is an example of a type II block (PR interval constant).


image Another example of a second-degree atrioventricular block in a dog.


image Second-degree atrioventricular block in a horse after xylazine premedication. Note the lack of a pulse wave corresponding to the missing QRS-T complex.


Causes



▪ Interrupted conduction from atria to ventricles

▪ Electrolyte imbalances

▪ Some drugs
□ Xylazine, other alpha2 agonists (common)

□ Low-dose atropine IV

□ Digitalis toxicity

▪ May occur in young, healthy dogs and horses


Therapy



▪ Not necessary unless rate of normally conducted impulses is very low or if QRS complexes are abnormal

▪ IV administration of anticholinergic agents

▪ Stop digitalis if toxicity is the cause

▪ Cardiac pacing for unresponsive cases (e.g., severe bradycardia that does not respond to anticholinergic agents)


RIGHT BUNDLE BRANCH BLOCK



Features



▪ Wide QRS complex

Related posts:

  1. Blood and Blood Substitutes
  2. Case Management
  3. Fluids
  4. Drug Combinations

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Oct 14, 2016 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on Electrocardiogram

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