It is quite often that you see the words ‘bilateral air entry’ written in the notes in the assessment of a patient’s respiratory system. In many cases this is a simplification of what can be heard through the stethoscope.
One might argue that in some instances it is sufficient to establish ‘air entry’ as opposed to establishing the particular nature of the breath sounds that are heard. For example if there is ‘air entry’ then we might assume that there is aerated lung underlying the stethoscope and therefore no pneumothorax, effusion or collapse.
But this is a simplification that does not account for the nuanced nature of respiratory auscultation. Let us consider a recent event where a patient with a large left-sided pneumothorax was deemed to have ‘bilateral air entry’ as written in the notes. When I assessed this patient I found them to indeed have evidence of bilateral breath sounds, or ‘bilateral air entry’, but would argue that their respiratory examination was entirely consistent with a large pneumothorax.
There was reduced chest expansion, hyper-resonant percussion note, and bronchial breath sounds heard over the left hemi-thorax. But why if there is a large pneumothorax can we hear breath sounds overlying it?
The textbooks (and consequently most medical students) state that over a pneumothorax breath sounds are absent. This maxim may explain why the doctor assumed that there was ‘air entry’ and therefore aerated lung underlying their stethoscope. They indeed did hear evidence of air movement somewhere near their stethoscope but this did not reflect aeration of lung. What this reflected was sounds conducted from the large airways nearby, i.e. the trachea and the contralateral main bronchus.
This doctor actually heard bronchial breath sounds (as opposed to vesicular) and assumed that they were synonymous with air entering lung.
If you apply your stethoscope anywhere in the vicinity of moving air you will hear evidence of that air moving, whether you are listening over the neck or upper abdomen you may indeed hear some evidence of ‘air entry’. However, in order to accurately establish if there is aerated lung underneath your stethoscope you must understand the distinction between bronchial and vesicular breath sounds.
Bronchial breath sounds are typically described as harsh sounds, they are higher pitched than vesicular sounds and occur in discrete (i.e. separated) inspiratory and expiratory ‘packets’. There is a gap in-between the inspiratory and expiratory phase, reflecting the moment when airflow ceases (at the peak of inspiration) and then immediately changes direction (at the onset of expiration).
If you want to hear bronchial breath sounds then apply your stethoscope over the patient’s trachea whilst they breathe. Notice how these sounds are clearly biphasic. When teaching students I liken these sounds to that of Darth Vader’s breathing in the Star Wars movies, or to the movement of a ball thrown directly upwards in the air (at some point it will stop moving, and then fall in the opposite direction).
Bronchial sounds are a normal finding over any airway which is non-expansile i.e. the trachea or main bronchi. If bronchial sounds are heard where there should be aerated lung then either that lung is pathologically rigid, or it is collapsed and you are just hearing the conducted bronchial sounds that were always there in the background. Be careful not to assume that this means ‘air entry’.
What you should hear overlying normal lung tissue is vesicular breath sounds. These sounds are softer, more ‘attenuated’, and do not occur as two ‘packets’ but as one which tapers off towards the end of expiration. See the diagram below to illustrate this:
By attenuation I mean that the harsh and quite noticeable gap between the inspiratory and expiratory ‘packets’ I previously described is not there. Instead there is a more smooth transition between inspiration and expiration.
The reason for this is that the airways through which air is flowing within normal lung are expansile and non-rigid (the alveoli). This means that at the moment of reversal of air flow (the end of inspiration) these small airways which have become distended with air now discharge their elastic recoil (probably asynchronously) leading to an ‘attenuation’ of the inspiratory-expiratory gap.
Airflow, as far as our stethoscope is concerned, seems to be maintained throughout the respiratory cycle until it tapers off at the end. Those millions of alveoli discharging their recoil will soften the distinction between inspiration and expiration, hence what you see in the diagram above.
Cardiovascular physiology is analogous to this. Consider the heart pumping into the arterial system: during systole blood is ejected into the aorta, the aorta then expands to accommodate this (due to the elastic fibres found in its tunica media). Then as systole ends and diastole begins the elastic recoil of the arteries is discharged leading to a maintenance of blood flow during diastole. This is what physiologists describe as the ‘Windkessel’ effect – the elasticity of these vessels dampens the fluctuations in blood pressure throughout the cardiac cycle.
Imagine if the heart pumped into a rigid metal pipe (or if air was flowing through a rigid pipe, like the trachea) – blood flow would be biphasic (quite like bronchial breathing), the heart would probably fail and the organs would not receive a steady flow of blood throughout the cardiac cycle.
This same ‘Windkessel’ effect can be applied to respiratory physiology, and though it may not accurately explain the aerodynamics of breathing (I am no expert in respiratory physiology) it helps us to understand how elasticity of small vessels (i.e. alveoli) maintains a more steady flow of air and therefore produces breath sounds with no intervening gap.
As discussed in my article entitled ‘A Stethoscope Is Just A Tube’ we must know what to listen for when we use this very simple, useful, but potentially misleading piece of kit. If you auscultate without an idea of what to expect then you will miss all but the most obvious clinical signs.
During chest auscultation we should be listening for breath sounds, and these should be documented as being vesicular or bronchial. If we don’t make this distinction we may be misled by the ever-present flow of air through the patient’s trachea and bronchi, and we might miss a pneumothorax.