Auscultation: back to the basics

Auscultation is a key skill as part of the physical examination, but recent data suggests that most vets do not continue to develop their skills in the first years after graduation (Mullowney et al., 2021). This suggests a deficit in continuing education provided for vets. This article aims to help provide a revision of some basics of cardiac auscultation, supported with my personal perspective as an experienced operator in veterinary cardiac auscultation.

Choosing the right stethoscope for you

The fact that a huge range of stethoscopes are available on the market suggests that people like different things. Some very low-cost models have good sound quality, and occasionally a higher-cost model does not yield what you might expect for the list price. Some of the “cardiology” models do not add a huge amount of value, but it is advisable to trial a model for a sound quality that works for you as an individual before investing in it.

The diaphragm (Figure 1) of some stethoscopes is a small enough size for human paediatrics and this is preferable for cats and most dogs, whereas larger dogs and horses should be auscultated using a larger, adult-size diaphragm (Figure 2). Some higher-end stethoscopes in the range have an interchangeable diaphragm for flexibility, and some have earpieces which can be changed to better suit the user.

Electronic stethoscopes have recently become more cost-effective and offer some useful additions to the more traditional equipment used by most people. Although they remain uncommon, the capability to amplify sounds and compatibility with hearing aids means that these models offer an option for more inclusivity for clinicians with hearing impairments. Also, the capacity to export sound files helps for recording, teaching and learning.

Many of the smartphone-connected devices generate a phonocardiogram – this is an image that represents the sound heard during the recording, and visualising the sound can be extremely useful in some situations.

Auscultation conditions

To make the best of auscultation as a diagnostic test, it is absolutely necessary to minimise the interference of any factors which could inhibit a good, thorough use of the stethoscope.

Environmental factors:

• Background noise from air conditioning or other building-associated noise (Figure 3)
• Background noise from human conversation in the same room
• Ambient noise from outdoors or other rooms

Patient factors:

• Background noise from hair-coat friction against the stethoscope head
• Referred upper respiratory tract noise, especially brachycephalic breeds (Figure 4)
• Panting, purring or vocalising during auscultation
• Excessive motion or inadequate restraint
• Obesity
• Long or matted hair coat
• Masses (eg lipoma) in the region of auscultation

Although a number of these factors are beyond our control, we can moderate conversation in the room and even move room to a quieter location. We can recruit an extra person to help restrain the patient or to provide sufficient distraction. Most animals that pant or purr heavily during auscultation can be distracted from this for a few moments to obtain a clearer assessment of heart sounds, by a person doing something surprising, the noise of a whistle or somebody opening the door. Holding closed the mouth of a panting dog often does not stop the background noise and may increase patient stress.

One study was published to look at various methods of trying to interrupt cats from purring during auscultation (Little et al., 2013). This reported that 7 percent of cats could not be stopped from purring, but turning on a nearby tap seemed to work effectively in over 80 percent of cases. A commonly used method of gently touching the nasal planum or stroking the larynx during auscultation was not tested in this paper, but seems to be a relatively effective way to cause a break in the purring sounds and thereby hear the heart more effectively.

Structured guide to auscultation procedure

After trying to make the environment work as best you can, the first step in auscultation is palpation of the thorax bilaterally. The apical impulse on the left should be stronger than on the right (if the opposite is true, it suggests significant cardiomegaly). Consider where the apical impulse is most prominent on the left and right – these are the regions referred to as “apex” (Figure 5). In many dogs, this is close to rib spaces 5 to 7, but that is a vague guideline and does not apply consistently across breeds. Also, when the heart is remodelled, the normal position of the apical impulse may be displaced.

Next, place the stethoscope diaphragm over the region of the apical impulse (Figure 5). Consider the sounds heard and initially focus on identifying the S1 and S2 sounds. Move the stethoscope around the apical region – a few centimetres dorsal to where you started, and then a few centimetres caudal. Find the range over where you can hear the heart sounds, and then return to the area of the apex where you found them to be the loudest. Focus again on S1 and S2: listen to see if you can hear any other sounds around these – between them or elsewhere.

At this point, I have a series of questions that I ask myself:

1. Is the heart rate appropriate for the degree of anxiety/illness?
2. Can I hear both sounds clearly? Are they of repeatable intensity between heartbeats?
3. Is there a coarse or musical sound (murmur) between these heart sounds?
4. Is there an additional heart sound like a gallop sound or click (see later)?

After reviewing these questions, I then move the head of the stethoscope to the basilar region – this is a couple of rib spaces cranial to the apex, and slightly dorsal (Figure 5). You may be able to hear the S2 sound a little more prominently in this location. The triceps should be advanced gently cranially in order to effectively hear the base.

Being a visual person, when listening to the heart I find it helpful to picture the heart sounds while auscultating (Figure 6), and see if what I hear fits my expectations – and, if not, what is different and when it is occurring.

Sometimes it can be challenging to differentiate between systole and diastole. Diastole tends to be longer than systole (more difficult to identify at higher heart rates) and is also variable in duration in dogs with sinus arrhythmia. If you can hear a murmur, then – statistically speaking – it is probably present in systole; diastolic murmurs are incredibly rare in companion animals (Figure 7).

What creates the normal heart sounds?

People often refer to S1 and S2 being caused by the atrioventricular (AV) valves (S1) and semi-lunar valves (S2) closing. In reality, the closure of the valves themselves would cause such minor vibrations that they cannot possibly generate the heart sounds. In addition, the AV valves actually close just before contraction of the ventricles, because of eddies in flow just below the mitral valve which are triggered by atrial inflow. The normal heart sounds are, in fact, caused by several events happening at once, which leads to reverberations of the heart and great vessels which are sufficient to be audible using a stethoscope.

• S1: the first heart sound is caused by deceleration of blood against the closed atrioventricular valves at the same time as blood rapidly accelerates through the outflow tracts and into the large arteries
• S2: the second heart sound is caused by deceleration of blood against the closed aortic and pulmonic valves at the same time as the mass of ventricular myocardium relaxes rapidly in early diastole

Summary

Auscultation requires practice and consideration, and it is useful to remind ourselves of the basics and what is to be expected when listening to heart sounds in practice. In the next article of this three-part series (January 2022), we will focus on heart murmurs and their auscultation.