Shortness of breath after gunshot wound: can you make the diagnosis?
Posted Sep 12 2013 1:10pm
Today’s post is a very good case presented by Richard Huang, a third-year medical student at St. George’s University. Reading through this case and thinking about it as you go along is a great way to test your clinical knowledge, as well as your understanding of the underlying pathophysiologic concepts. Thanks, Richard! So without further ado, let’s get on with the case.
You are working the overnight shift at an inner city emergency room. You hear sirens, and an ambulance pulls up with an 18-year-old male with a gunshot wound to the front of the chest just above the diaphragm. Upon preliminary examination, he is dyspneic, cyanotic, diaphoretic, and cold. His vitals are blood pressure 65/45 mm Hg, heart rate 150 beats/minute with pulse amplitude of 1/4, and respiratory rate 33 breaths/minute.
Q. Given the patient’s initial presentation, which two organs are most likely to be injured?
A. The location of the injury and the patient’s initial signs and symptoms point towards injury of either the heart or the lung. Take a look at each sign or symptom in red above and formulate your opinion on whether it suggests involvement of the heart or lung (or both). Then hover for a few seconds over the sign or symptom for a short discussion.
The final tally is nine signs and symptoms pointing to a heart problem (with five pointing exclusively to a heart problem) and four signs and symptoms pointing to a lung problem (with none pointing exclusively to a lung problem). The landslide demonstrated by the tally may make us jump to the conclusion that the gunshot wound damaged the heart, leading to all the problems that we see.
Not so fast! All we know is that there is something preventing the heart from pumping properly to maintain cardiac output. We do not know if the cause is intrinsic to the heart or if it is due to outside forces, such as the lungs. But with what we know about the patient, we can generate a couple of differential diagnoses.
Given that the patient suffers from an gunshot wound to the chest, we might suspect that the bullet ruptured the myocardium, causing blood to spill into the pericardial space with each pump of the heart, leading to cardiac tamponade.
In cardiac tamponade, the blood in the pericardial space compresses and constricts the heart, preventing it from fully filling during diastole. This creates a filling defect, which reduces the stroke volume of the heart, leading to decreased cardiac output.
Since the pericardial space is now blood-filled, there is increased distance between the heart and the external chest wall. Thus, upon auscultation of the precordial area, heart sounds are muffled.
Also, since the heart has difficulty filling during diastole, there is back pressure into the venous system, which can be observed as distended jugular veins in the neck or quantified as an elevated jugular venous pulse (JVP).
These three classic signs of cardiac tamponade – muffled heart sounds, elevated JVP, and weak peripheral pulse – are commonly known as Beck’s triad.
Alternatively, if the bullet damaged the lung, it could have caused a pneumothorax, or more specifically, a tension pneumothorax given the patient’s clinical presentation.
In a tension pneumothorax, there is one-way flow of air into the pleural cavity – essentially like having a one-way valve attached to the pleura. With each inspiration, the valve opens to allow air into the pleural space, but with each expiration, the valve closes, trapping the air in the pleural space. With each breath taken, then, there is more and more air trapped in – and expanding – the pleural space.
Since the space within the thorax is limited, the expanding pleural cavity compresses and displaces the surrounding structures, such as the ipsilateral lung (which may be displaced and collapse under the increased pleural pressure), and the trachea and respiratory tree (which may deviate towards the contralateral side).
The heart may also be displaced (causing the displacement of the normal positions of heart sounds) and compressed (leading to a filling defect on diastole). If this happens, the heart would have reduced cardiac output (leading to all the signs and symptoms seen in our patient).
As is the case in cardiac tamponade, a filling defect of the heart due to tension pneumothorax could cause distended neck veins and elevated jugular venous pressure. However, there are two signs that are exclusive to tension pneumothorax that are not found in cardiac tamponade. In tension pneumothorax, the side of the lung lesion should show diminished breath sounds and should be tympanic to percussion (due to collapse or shifting of the lung).
Now that we have our differential diagnosis of cardiac tamponade and tension pneumothorax, we need to complete the physical exam, looking for further signs and symptoms that would point us towards either diagnosis. In this patient, further physical examination showed muffled heart sounds and an elevated jugular venous pressure – signs which, as discussed above, point towards cardiac tamponade. If the physical examination had revealed diminished breath sounds and tympany on percussion on one side of the chest, tracheal shift towards the other side, and an elevated jugular venous pressure, the most likely diagnosis would have been tension pneumothorax.