Volume , Issue

In the last installment of this column, we covered some non-urgent applications of point of care ultrasound (POCUS) in the wilderness. This month, we’ll introduce POCUS as a versatile tool for more critical situations. 

The main critical care goal in wilderness medicine is rapid stabilization, followed by immediate evacuation to definitive care. POCUS may facilitate these goals, but should not interfere with patient extrication. With this in mind, wilderness critical care POCUS may be better applied in remote clinics than in the backcountry. 

Cardiac Arrest

Some experts have advocated for the use of POCUS in cardiac arrest to look for pneumothorax or cardiac tamponade for promptly reversal (Gaspari et al 2016). They also note that cardiac standstill bears a terrible prognosis (near 0% survival in out-of-hospital cardiac arrest), and may warrant early termination of resuscitation. Chin et al (2013) designed the “PAUSE Protocol” (Prehospital Assessment with Ultrasound for Emergencies) — paramedics using it were able to recognize pneumothorax, pericardial effusion, and cardiac standstill in under 30 seconds with less than two hours of training. 

Trauma and Shock

POCUS is highly effective at discerning between different shock states using the RUSH (Rapid Ultrasound for Shock and Hypotension) exam (Keikha et al 2018). This study involves looking at the lung to look for pneumothorax; the heart to look for tamponade, effusion, and right and left ventricular movement; the IVC to assess volume status; and the abdomen to assess for intraabdominal bleeding and AAA. When performed by an experienced provider, this can take as little as five minutes. Poor left ventricular function with fluid backup in the lungs may indicate left ventricular failure and cardiogenic shock, whereas right ventricular strain with an overfull IVC may indicate a massive pulmonary embolism. In one 2012 study, ED physicians were able to calculate cardiac index using only bedside POCUS (Dinh et al). All patients in shock should be transported out of the wilderness—ultrasound should be used to guide treatment decisions, with attention paid to emergent evacuation from austere environments to more definitive care. 

*Figure 1. All images and clips courtesy of The POCUS Atlas, thepocusatlas.com


Lung ultrasound is surprisingly easy to perform at a basic level. The probe is placed between two ribs, allowing the provider to visualize the visceral and parietal pleura coming into contact with one another. These should “shimmer” during breathing, showing that the patient has “lung sliding.” If this is absent, pneumothorax is likely (91% sensitivity; Alrajhi et al 2012).

Normal lung ultrasound

Decreased lung slide with pneumothorax

Sound waves pass through air poorly, so ultrasound into the lungs doesn’t visualize actual tissue so much as make sense of artifact. B-lines, as pictured below, indicate pulmonary edema and look like beams of light shone through fog. Thinking clinically, the discovery of B-lines in one lung should prompt us to look at the other. B-lines in one lung may represent pulmonary contusion; B-lines in both lungs may represent cardiogenic pulmonary edema or ARDS. 

B-lines showing pulmonary edema


Lung ultrasound can also be used to diagnose pneumonia—which looks more or less like a bunch of white junk along the pleura. Looking carefully at multiple specific points in the lungs, lung US is over 85% sensitive for finding pneumonia (compared to just over 50% for chest x-ray; Hansell et al 2021). It can also show us pleural effusion, which shows up as dark fluid around the lung. While many pneumonias can be treated on an outpatient basis, they are a major source of hypoxemia and sepsis. An otherwise healthy patient may be able to remain in the wilderness if oral antibiotics are available, but should be evacuated to more advanced care at the first sign of respiratory distress or hypotension.

Pneumonia, with small effusion alongside consolidated lung

Massive pleural effusion.
Lung tissue flopping within a sea
of black fluid (“jellyfish sign”), with
the diaphragm and liver visible
 to the right of the image



Heart & IVC

Cardiac ultrasound, or echocardiography, is a field in and of itself. Mastery requires years of practice. That said, with even minimal experience, it is relatively easy to get an idea of overall ventricular function, and to be able to evaluate for pericardial effusion/tamponade. Assessing diastolic function and determining flow across valves (e.g. when considering aortic stenosis) is far more complicated. 

There are four main cardiac views: parasternal long (PSL), parasternal short (PSS), apical four chamber (A4C), and subxiphoid/subcostal (SC). The probe can be rotated from the subcostal view so that the operator may see the IVC pass into the right atrium (see Figure 1). 

Normal PSL

Normal PSS


Normal A4C

Normal SX


Normal IVC. See the IVC passing through the liver, crossing over the diaphragm, and entering the right atrium on the right side of the image.

For the beginner, it’s best to look at a bunch of hearts with US and see how they move. Muscle should move together, and concentrically. The mitral valve should almost tap the ventricular septum at the end of systole (best seen in PSL view). We can even estimate the left ventricular ejection fraction by measuring this distance (see EPSS, end point septal separation). The right ventricle should be about 2/3 the size of the left ventricle. Check out the phenomenal sample clips on The POCUS Atlas. Compared to the normal hearts seen above, take a look at this fairly obviously unhealthy one.

Cardiogenic shock

Cardiogenic shock

Here’s another slam dunk. That beating thing is the heart. All that fluid around it is pericardial effusion. It’s not hard to see that this heart isn’t generating a lot of output. 


Cardiac tamponade

IVC ultrasound is a little easier to interpret. If you were to look at the IVC in the above patient with the cardiac tamponade, what would you expect to see? Since the heart is obstructed, the IVC would be enlarged, or “plethoric.” To assess volume status, look at the IVC about 3 cm below the diaphragm. It should be about 2 cm in diameter and should collapse about 1/3 of its diameter with each negative pressure inspiration. If it is larger than this and doesn’t collapse at all, the patient likely has obstructive shock (pulmonary embolism, cardiac tamponade) or ventricular failure causing backup of venous return. If the IVC collapses almost completely with inspiration and is narrower than 2 cm, the patient is probably hypovolemic. This is an excellent guide to fluid resuscitation in remote clinics, although experts state its use in hospitals is at least somewhat controversial (see Scott Weingart's 2017 podcast).   

Plethoric IVC.
See the fat blood vessel passing through
the liver, then crossing the diaphragm and
leading into the right atrium at the
right of the image: 


IVC in hypovolemia. See the narrow IVC collapsing
completely with inspiration


The FAST exam (Focused Assessment with Sonography for Trauma) is designed to show us blood in the peritoneum. It also includes a subcostal view of the heart, as described above, to rule out cardiac tamponade; the eFAST (extended FAST) adds a single view of each lung to assess for pneumothorax. The abdominal views of the FAST exam include RUQ, LUQ, and suprapubic. The FAST is approximately 75% sensitive for hemoperitoneum in blunt trauma, although this number may be higher for hemoperitoneum significant enough to require the OR (Nishijima et al 2012), and requires about 200 ml of blood for a positive test. Blood shows up as solid black between organs. In the ED, a “positive” FAST will generally get an unstable patient straight to the OR for an exploratory laparotomy, and in the wilderness, accelerate the urgency of evacuation.

Of the three abdominal views of a FAST exam, the RUQ is the most important, as it itself provides the positive view in approximately 80% of cases (Ultrasound Course in a Box). Blood settles first in the hepatorenal junction (Morrison’s pouch) or along the tip of the liver. Blood in the LUQ usually settles between the spleen and the diaphragm. Blood in the pelvis generally settles behind the bladder. In females this is known as the pouch of Douglas, which is also where blood from a ruptured ectopic pregnancy usually accumulates. 

RUQ, positive FAST. Blood along the liver tip


Aorta ultrasound is relatively simple. The aorta sits just anterior to the left edge of the vertebral bodies. It should be <2 cm in diameter. A measurement >3 cm is diagnostic of a AAA, and a measurement >5 cm generally warrants urgent surgery. The aorta is a retroperitoneal structure, so a leaky or ruptured aneurysm will usually not lead to a positive FAST exam. To fully assess for a AAA, start at the diaphragm, and measure the transverse diameter of the aorta at three points between the diaphragm and the iliac bifurcation. This bears a 99% sensitivity and specificity (Rubano et al 2013). A longitudinal view may also be obtained. AAA is often an incidental diagnosis, but any patient with AAA and abdominal pain or signs of shock should be emergently evacuated from the wilderness. 


Massive aortic aneurysm about 6 cm in diameter, with a large mural thrombus


Ultrasound is a highly portable technology that may be useful in the diagnosis of minor pathologies (covered in our last issue) or of potential life threats (covered here). We hope this column was useful to new adapters of ultrasound, and to those providers just looking for a brief review, we strongly recommend in-person training with ultrasound if available in your wilderness or EMS system. 

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