Anesthesia in dogs with heart murmurs: are things really so concerning? The answer is that it depends. Whether anesthesia in dogs with heart murmurs is something to worry about varies based on several factors such as the degree of the dog's heart murmur and other factors such as the reason the dog is being anesthetized in the first place. Veterinarian Dr. Ivana Crnec discusses heart murmurs in dogs and provides important information pertaining anesthesia in dogs with heart murmurs.
Heart Murmurs in Dogs
Normally the heart produces two sounds – a "lub" sound and a "dub" sound. These sounds develop as a result of the valves closing and the blood circulating through the heart. Every other sound, distinct from the normally appearing "lub" and "dub" sounds is considered a murmur.
The simplest way of explaining heart murmurs is by defining them as abnormal heart sounds or extra vibrations. They develop when the normally smooth blood flow through the heart is disturbed and the intensity of the disturbance is significant enough to produce an audible noise, often referred to as a "wooshing" sound. Heart murmurs can easily be distinguished from normal heartbeat sounds with a regular stethoscope.
Many heart diseases, conditions and defects can lead to abnormal blood flow and consequently murmurs. Generally speaking, the disturbances that lead to murmurs can be categorized in three major groups: 1) disturbances caused by abnormal valves or vibrations, 2) disturbances caused by obstructions and diseased valves or dilated vessels, and 3)disturbances caused by regurgitant flow.
In dogs, the most common causes include heart valve deficiencies, defects in the heart walls, dilated cardiomyopathy (weakened heart wall muscles), heartworm disease, endocarditis (inflammation of the heart valves) and tumors.
Heart murmurs are serious and require particular attention. However, certain murmurs are classified as innocent. Innocent heart murmurs are common in rapidly-growing, large breed puppies. They are not related to a heart problem, are usually classified as grade 3 or less and tend to disappear on their own when the puppy is about 4 to 5 months of age.
Classification of Heart Murmurs in Dogs
Heart murmurs can be classified or described based on many criteria and parameters. The most important parameters include: type or timing, intensity or grade, configuration or quality.
Based on the timing, or when during the heartbeat does the murmur occur, there are three different types of murmurs: systolic murmurs which occur during a beat when the heart muscle contracts, diastolic murmurs, which occur in between beats when the heart muscle is relaxed, and continuous or to-and-fro murmurs, which occur throughout the entire heartbeat cycle.
Based on the intensity, heart murmurs are graded on a scale of 1 to 6.
Grade 1 – least serious grade, the murmur is barely detectable with a stethoscope (it takes a very well-trained ear to notice this murmur).
Grade 2 – soft and quiet but noticeable with a stethoscope.
Grade 3 – murmurs of intermediate loudness.
Grade 4 – loud murmur that tends to radiate widely and it is therefore audible on both sides of the chest.
Grade 5 – very loud murmur. In addition of being audible with a stethoscope its vibrations can be felt by holding a hand on the chest wall.
Grade 6 – the most serious grade, the murmur is extremely loud and audible even without a stethoscope. Its vibrations can be felt on the chest wall.
The configuration describes how the murmur sounds. Based on this criteria, there are four main configurations:
Plateau murmurs – manifest with a uniform loudness and are usually caused by aortic valve insufficiency. They are also called regurgitant murmurs because they develop when blood flows back through an abnormally altered valvular orifice.
Crescendo-descrescendo murmurs – their quality is variable and tends to change from louder to quieter. They are usually caused by aortic and pulmonic stenosis. They are also called ejection murmurs because they occur due to turbulent forward flow.
Descrescendo murmurs – they start of loud and then grow quieter. They are usually caused by aortic valve insufficiency and ventricular septal defects.
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Machinery murmurs – they produce very loud and continuous noise, just like a machine. This type of murmur is most commonly observed in patients with patent ductus arteriosus.
Manifestation of Heart Murmurs in Dogs
Signs that indicate a dog has heart issues include the following: unexplained coughing, bluish to grey gums and tongue, altered heartbeat, fast, difficult and noisy breathing, fatigue, weakness, loss of stamina and decreased exercise tolerance, fainting episodes and sudden collapses, abdominal distension (a so-called pot belly), loss of appetite.
Heart murmurs are relatively easy to diagnose – all it takes is a high-quality stethoscope and a well-trained ear. Once the vet performs the auscultation, based on what he hears, how loud it is and where it is coming from, he will be able to determine the type, intensity and configuration of the murmur.
With the dog’s overall health status, age and breed taken into consideration, the vet will make a list of possible underlying causes.
Determining the right underlying cause requires some additional testing like radiographs, echocardiograms, electrocardiograms, blood tests (including heartworm test) and blood pressure tests.
With this information on hand, a vet can be better capable of assessing whether anesthesia in dogs with heart murmurs is something worthy of pursuing or not.
Anesthesia in Dogs with Heart Murmurs
Anesthesia is defined as loss of sensation, particularly pain. The goal is to ensure a state of reversible unconsciousness. However, delivering a safe anesthesia can be challenging even in normal, healthy dogs.
Dogs with cardiovascular dysfunctions, such as heart murmurs, are at an increased anesthetic risk. Veterinarians often debate whether providing a safe anesthesia in such dogs is an impossible endeavor or a challenge with manageable risk.
There are three steps to minimizing the anesthesia-related problems in dogs with heart murmurs.
The first is a thorough pre-anesthetic evaluation. Determining the type, intensity and configuration of the murmur is of paramount importance for figuring out its underlying cause. Once the cause is determined it will be easier to minimize its potentially negative effects.
The second step is modified and individually tailored premedication, induction and maintenance protocols. Modifications are required in terms of type of drugs, dosages and route of administration. This is because different types of drugs have different effects on the heart’s function.
The third step is special monitoring during anesthesia – many parameters need to be carefully and frequently monitored. However, the most important parameters include: electrocardiography which is used to display the electrical impulses that initiate each heartbeat. The pattern of impulses is referred to as heart rhythm and it provides important information on how the heart works.
Blood pressure which indicates the force which the flowing blood exerts on the arterial walls. It indicates the quality of tissue perfusion with blood and blood oxygenation, which is measured through pulse oximetry and it indicates the level of blood saturation with oxygen.
Since heart murmurs are serious and potentially life-threatening, the risk should always be carefully measured. It should therefore be well evaluated whether the risk of non-anesthetizing the patient and not-performing the procedure is greater than the risk the heart murmur may initiate.