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Why Is My Dog's Pulse or Heart Rate Abnormal ?

Why Is My Cat's Pulse or Heart Rate Abnormal ?

To see what normal blood and urine values are for your pet, go here

For an explanation of causes of most abnormal blood and urine tests go here

To see how tests are often grouped, go here

Ron Hines DVM PhD

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Your Pet’s Heart Rate

Your pets pulse = the rate its heart is beating. The special muscle (cardiac muscle) that makes up your pet’s heart doesn’t contract or beat the way ordinary muscle does. Instead, a system of nerves extend through the walls of its four chambers firing timed electrical impulses that originate from a group of specialized cells in in the upper right corner of your pet’s heart (the sino-atrial SA node).

A second node, the atrioventricular or AV node conveys the impulse to the two, more muscular, lower chambers (ventricles). The time during which you pet’s heart is contracting is called systole. The time it is relaxed is diastole.

The SA node receives messenger chemicals and nerve impulses that govern the rate at which it beats; but it will beat without them. Those impulses come via the autonomic nervous system and they are of two kinds: sympathetic impulses, which increase heart rate and parasympathetic impulses, coming by way of the pet's vagus nerve, which decrease heart rate.

Reasons Your Pet’s Heart Would Beat Faster (Tachycardia) :

Whenever the sympathetic nerves that connect your pet’s heart to its spinal cord predominate in activity over the vagus nerve, the pet’s heart beat and cardiac function will increase. The chemical that is released to cause this change in the SA node’s firing rate is norepinephrine (noradrenaline). Calcium plays an important role in this process as well.

All causes of sympathetic nerve stimulation will increase your pet’s heart rate. That stimulation occurs with excitement and physical activity or when exhaustion depletes your pet's muscle glycogen stores.

All causes of Increased body temperature (fever, exertion, heat stroke) increases heart rate. So does the malignant hyperthermia that occasionally occurs during anesthesia.

Dehydration can increases your pet’s heart rate due to a lack of sufficient blood volume to keep its body well oxygenated. Anemia or the sluggish circulation of congestive heart failure (CHF) can have the same effect.

Sympathetic nerve stimulation increases in inspiration and decreases in expiration due to changing vagal nerve tone. Heart rate goes up slightly during inspiration.

Increased blood pressure in the left upper chamber (left atrium) of your pet's heart increases heart rate (Bainbridge reflex).

Medications can increase your pet’s heart rate as well. They include Allerest or caffeine and medications that contain phenylephrine or phenylpropanolamine.

Various antihistamines as well as pseudoephedrine (Sudafed), theophylline and other stimulants increase heart rate and pulse. Your dog or cat ’s heart rate can go up or down after atropine or scopolamine-containing medications.

Reasons Your Pet’s Heart Would Beat Slower (Bradycardia) :

Parasympathetic nerve stimulation, primarily coming from your dog or cat ’s brain through its vagus nerve, decreases your pet’s heart rate.

The chemical that is released is acetylcholine, which shifts potassium out of heart muscle cells that form the SA node. That decreases the rate at which they send their beat impulses to the rest of the heart. These parasympathetic effects predominate in your pet when it is emotionally tranquil (mellow), resting or sleeping.

Stimulation of the blood pressure sensors (baroreceptors located in your pet’s arterial walls) decreases heart rate. These receptors respond to stretch by increasing vagal nerve parasympathetic “tone” which releases acetycholine slowing your pet’s heart rate. The opposite is also true.

Medications can slow your pet’s heart rate too (eg ACE inhibitors for heart disease such as benazepril and enalapril, acepromazine, digoxin and clonadine)

Heart rate can go down or up after scopolamine or atropine-containing medications are given.

Complementary Tests :

EKG, chest x-ray, cardiac ultrasound, CBC/WBC and blood chemistry values, further tests depending on initial results.

.................... DxMe