Is pneumoperitoneum the terra ignota in ultrasonography?

In most cases, pneumoperitoneum is caused by gastrointestinal perforation, which usually requires surgical treatment. Many authors believe that ultrasound imaging of pneumoperitoneum is at least as effective as conventional radiography, or even that its efficacy is superior. In such a situation, it is imperative to make this modality one of the main tools in the diagnostic arsenal of emergency medicine. This is the main aim of this paper. First, ultrasound anatomy of so-called thoracic-abdominal border is discussed. The equipment requirements emphasize that the diagnostic process can be conducted with the simplest portable US scanner, even without the Doppler mode. The technique of a US examination, the aim of which is to detect, free air in the peritoneal cavity is also simple and conducted with the patients lying down, either in the supine or lateral position. A convex transducer with the frequency of 3.5-5 MHz is applied above the lower intercostal spaces on the right and left side, to the epigastric region below the xiphoid process and in various sites of the abdominal wall. The most effective examination, however, is conducted in the left lateral position via the right intercostal spaces. The differential diagnosis on the right side under the diaphragm should include the presence of a subdiaphragmatic abscess with gas and a hepatic abscess with a similar content as well as transposition of the colon in between the diaphragm and the liver (Chilaiditi syndrome). It seems that the inclusion of a US examination to the E-FAST method in order to detect free gas in the peritoneal cavity is justified since it is a sign of gastrointestinal perforation in numerous cases, and is clinically as relevant as the presence of free fluid.

It is hard to imagine contemporary medicine without ultrasonography. Thanks to its comprehensive advantages, this method has been adapted to all medical specializations. Since, in contrast with other imaging techniques, ultrasonography can be conducted in extreme external conditions (e.g. at a site of an accident, in a mine, at the frontline, at a spacecraft, in a submarine, in a helicopter, at the bedside etc.) and in internal conditions (endoscopic, laparoscopic, intraoperative ultrasound), its strong role in anesthesiology, intensive care and emergency medicine is not surprising(. Although gas has physical properties that considerably limit ultrasound propagation, it has become an important element of ultrasound imaging(. Having conducted the analysis of indications for this examination in emergency situations, the authors concluded that conditions such as pneumoperitoneum, pneumomediastinum, pneumopericardium and sternum injuries are rarely included(. Taking into account 20 years of experience, we wish to remind the readers about the role of ultrasonography in the diagnosis of pneumoperitoneum( in order to widen the diagnostic spectrum of this method, particularly in emergency medicine.

To begin with, the fundamental significance of the paper by Seitz and Reising from 1982, which is frequently omitted in references, must be emphasized(. These authors were the first to demonstrate the US ability to detect even 1 ml of air in patients with ascites. Currently, such an amount of this gas in the peritoneal cavity can be detected without the presence of ascites (Fig. 1). In the subsequent years, a range of other publications confirmed Seitz and Reising's observations concerning the usefulness of ultrasonography in pneumoperitoneum diagnosis(. Since the basic way to show this sign is to search for air under the diaphragm, the first thing to do is to become familiar with the normal topography of this region. This is the right place to refer to our previous observations(. The thoracic-abdominal border is characterized by the presence of the diaphragm, which on the right side separates the aerated lung from the liver (Fig. 2), and on the left side – the aerated lung from the spleen (Fig. 3). In normal conditions, the diaphragm demonstrates a typical behavior: it gets thicker on inspiration and thinner on expiration (Fig. 2 and 3). However, one must be familiar with the anatomic structures that compose what we call “the diaphragm.” These elements can be observed from the side of the lungs in the following order: two parallel hyperechoic lines representing two portions of the parietal pleura (costal and diaphragmatic) focused in the phrenicocostal sinus, the diaphragmatic muscle of lower echogenicity, peritoneal fat (hypoechoic) and diaphragmatic peritoneum (hyperechoic line) (Fig. 4). The peritoneal fat is the most changeable element in terms of thickness. In obese patients, its deposition might be impressive. Generally, however, the diaphragm in this region creates a hypoechoic band that separates the pleural cavity from the peritoneal cavity. The thoracic-abdominal border is characterized by a tissue “step” caused by the presence of the barrier created by the diaphragm. In such a situation, both the liver and spleen will always be situated below the diaphragm and gas in pneumoperitoneum will always be found in these localizations (Fig. 5 and 6). It must be added that the assessment of this sign on the left side can be difficult in elderly patients due to the involution of spleen which is replaced by the colon. Such a situation can lead to a false positive diagnosis of pneumoperitoneum.

Fig. 1

Two small air bubbles (arrows) above the liver

Fig. 2

Right thoracic-abdominal border during expiration and inspiration. Arrows point to the diaphragm. P – aerated lung; L – liver

Fig. 3

Left thoracic-abdominal border during expiration and inspiration. Arrows point to the diaphragm. P – aerated lung; S – spleen

Fig. 4

Phrenicocostal sinus – a magnified image. The upper arrow indicates two parallel hyperechoic lines representing the parietal pleura (costal and diaphragmatic). The lower arrow points to the parietal peritoneum (hyperechoic line). The horizontal arrow is placed at the level of the diaphragmatic muscle (band of lower echogenicity). A layer of peritoneal fat (hypoechoic) is visible between the muscle and parietal peritoneum

Fig. 5

Under the diaphragm on the right side (arrows). P – aerated lung; G – air; L – liver

Fig. 6

Under the diaphragm on the left side (arrow). P – aerated lung; G – air; S – spleen

The simplest US scanner without the Doppler mode and outfitted with a convex probe with the frequency of 3.5–5 MHz, is sufficient to detect pneumoperitoneum. However a linear probe with the frequency of 7.5 MHz enables better visualization of small gas bubbles. The technique of the examination can be reduced to the simplest model, recommended by most authors(.

Namely, having positioned the patient on the left side, the transducer is applied along the lower intercostal spaces on the right side. When the aforementioned thoracicabdominal border has been found (the tissue “step” at the border of the lung and liver), air is looked for under the diaphragm in the subsequent intercostal spaces, as it can be present in slight amounts in the form of single bubbles (Fig. 1). Depending on the volume of accumulated gas, it will manifest itself with various types of reverberation. At larger amounts, it causes an identical phenomenon to that of the aerated lung, i.e. multiple reverberations (Fig. 5). At smaller amounts, dirty acoustic shadow is observed and gas bubbles present themselves as focal thickening of the parietal peritoneum band with subsequent gentle, bright glow (Fig. 1)(. When the presence of air is suspected, the patient should be positioned on their back, keeping the head in the previous position. The disappearance of air from this localization confirms its free character (free movability). This test is also conducted to rule out gas in subdiaphragmatic or hepatic abscesses localized under the capsule in this area. The examination is similar on the left side above the spleen, but the patient is then positioned on the right side. If patients can only assume the supine position, the transducer should be applied to the epigastric region under the xiphoid process. In such a situation, the left liver lobe is used as the background for visualizing air in the peritoneal cavity (Fig. 7)(. Problems are encountered in patients with hypoplasia of this part of the liver and in elderly men, in whom the left liver lobe may be small. In such cases, gas in the stomach or transverse colon may be interpreted as pneumoperitoneum. Imaging upon deep inspiration and directing the ultrasound beam at the left liver lobe may be helpful. Moreover, air can also be detected under the integuments, by assessing it in various places (Fig. 8)(. It must be added that in the right epigastric region, the colon can be interposed in between the diaphragm and liver (interpositio hepatodiaphragmatica coli – Chilaiditi syndrome). In such cases, there are no clinical signs of gastrointestinal perforation, and detailed examination reveals the typical image of the colon, i.e. haustra (Fig. 9). Moreover, a change in the body position does not cause gas movement(. In a typical clinical picture of so-called acute abdomen, pneumoperitoneum is usually a sign of gastric or duodenal perforation, or intestinal rupture after trauma. An experienced ultrasonographer can detect the site of perforation in 80% of cases (Fig. 10)(. Numerous studies comparing the efficacy of conventional radiography with ultrasonography in diagnosing air in the peritoneal cavity, have revealed at least similar diagnostic outcomes or demonstrated the superiority of ultrasonography(.

Fig. 7

Air (G) above the left liver lobe (L). The arrow points to the diaphragm

Fig. 8

Free air (G) under abdominal integuments. Distance indicators show the width of accumulated gas

Fig. 9

Right part of the colon, located between the diaphragm (arrow) and liver (L), mimics pneumoperitoneum

Fig. 10

Duodenal bulb perforation (arrows) presented in two sections

It must be added that X-ray in a standing position and in the left lateral position as well as chest X-ray usually takes approximately 30 minutes. This means that the patient must be still in a given position for several minutes to make gas move to the desired place(. Based on the data presented, it can be assumed that ultrasonography should be treated as the basic method in diagnosing pneumoperitoneum. In trauma patients, gas in the peritoneal cavity is of similar clinical importance as fluid since both these conditions may require surgical intervention(. However, pneumoperitoneum after a blunt trauma will mainly indicate bowel perforation, which is observed in 5–25% of all abdominal injuries(. In these cases, ultrasonography has proven its acceptable utility in diagnosing free gas in the peritoneal cavity, with the sensitivity at the level of 85.7% and specificity of 99.6%(. When, however, fluid in the abdominal cavity is considered as the only criterion of bowel damage, the sensitivity of the method decreases to 58%(. Therefore, the logical conclusion is that the inclusion of both these signs (fluid and gas) in an ultrasound examination should increase its diagnostic efficacy in patients after blunt abdominal trauma. This is why the simplified version of the examination to detect pneumoperitoneum should be included to the E-FAST method by using the site for heart imaging under the xiphoid process of the sternum to assess the situation above the left liver lobe. When it is possible to position the patient of his or her left side, the imaging of gas under the diaphragm on the right side becomes optimal.

To gain proficiency in this technique, one should examine patients after laparoscopies or laparotomies, in whom carbon dioxide or air may be detected in the peritoneal cavity for several days after surgery.

Conclusion

It seems that the inclusion of a US examination to the E-FAST method in order to detect free gas in the peritoneal cavity is justified since it is a sign of gastrointestinal perforation, and is clinically as relevant as the presence of free fluid.