Lung Ultrasonography in the Acutely Dyspneic Hemodialysis Patient

Introduction

A 79-year-old woman with ESKD presented to the emergency department with dyspnea. Three days before presentation, she developed rhinorrhea, cough, and fatigue. The day before admission, her prehemodialysis (pre-HD) weight was 52.6 kg, and her post-HD weight was 51.2 kg, 2 kg above her target weight. In the emergency department, she was afebrile, BP was 178/93 mm Hg, heart rate was 93 beats per minute, respiratory rate was 26 breaths per minute, and oxygen saturation was 83% on room air. She had no jugular venous distention or lower extremity edema, and her lung examination revealed right-sided rales. Her chest x-ray showed a right middle- and lower-lobe consolidation. She was started on antibiotics and placed on bilevel positive airway pressure with no relief of symptoms.

This patient with ESKD on HD presented with hypoxia. Although she was above her target weight, she had a history and chest x-ray finding that indicated pneumonia. The chest x-ray did not show pulmonary edema or vascular congestion. To what extent can ultrafiltration improve the symptoms of this critically ill patient? Lung ultrasonography is useful, because it can detect extravascular lung water with high sensitivity and specificity compared with computed tomography scan (¹). Here, we will describe the procedure of lung ultrasonography and use this patient to highlight how it can be used to improve patient care.

Lung Ultrasound Procedure

Lung ultrasonography can be performed with an array of ultrasound probes but is typically performed with a 5-MHz microconvex probe. The probe is placed perpendicular to the skin at a rib interspace. Rib shadows border the image field, and the depth is adjusted to keep the pleura in the middle of the view field. Typically, eight intercostal spaces are scanned. In research, it takes 6–15 minutes to complete a full study of 28 intercostal spaces (²). The more focused clinical examination takes less time.

Understanding Lung Ultrasonography

Lung ultrasonography is dependent on analysis of reverberation artifacts generated by aeration patterns deep to the pleural surface (¹). When an ultrasound beam interfaces with the pleural surface, it appears as a white line bordered by two rib shadows. During respiration, the visceral and parietal pleura move past one another, and this “lung sliding” will appear as a glistening at the pleural surface. Characterizing the pleura is the first step in interpreting lung ultrasound. If there is doubt about the quality of the pleural line, one can switch to a vascular probe. These higher-frequency probes do not penetrate deeply but yield a high-quality image of superficial structures.

The next step is to look for reverberation artifacts. In a normal lung, the ultrasound beam generates artifacts that appear as parallel horizontal lines that are deep to the pleural surface and equidistance from one another. These are known as A lines (Figure 1). In the presence of extravascular lung water, the ultrasound beam generates a sharp reverberation artifact that appears as a bright line that juts away perpendicular to the pleural surface. These are called B lines (Figure 1). B lines have the following characteristics: (1) arise from the pleural line, (2) move as the pleura slides, (3) are hyperechoic, (4) are discrete and well defined, (5) are long and spread to the end of the view field, and (6) efface A lines (²).

Figure 1.: Interpreting lung ultrasonography requires a systematic approach. (A) Pleural surface characterization: visualize the pleural surface at high resolution to see if it is smooth. (B) A-line identification: note a smooth, sliding pleural surface with A lines indicating normal aeration pattern. (C) Homogenously distributed B lines: note several B lines emanating from a smooth, sliding pleural surface. Pulmonary edema is the likely diagnosis. (D) Focal B lines: note a focal B line and irregular sliding (or nonsliding) pleural surface. Pneumonia or interstitial lung disease is more likely.

Supplemental Video shows a real-time lung ultrasonography with detailed explanation. An international consensus committee agrees that the exact physiologic basis of B lines is unknown (³). One theory states that B lines result from an acoustic mismatch between water-thickened interlobular septa and surrounding air.

Lung Ultrasound in Clinical Practice

In the same way that you would use a stethoscope to listen in different lung regions, you should use the ultrasound probe to characterize different areas of the lung. All nephrologists that use lung ultrasonography must be able to characterize what they observe and then clinically correlate.

In a normal aerated lung, the pleural line will be smooth, and you will notice the line moving during the respiratory cycle (lung sliding). If a normal pleura is seen in conjunction with A lines, then the differential diagnosis moves toward pulmonary embolism or bronchospasm. If there is no lung sliding and A lines are noted, then a pneumothorax at that interspace should be considered (¹).

If the pleural surface is smooth with lung sliding and if B lines are homogenously distributed between the rib shadows, then pulmonary edema should be considered. Because pulmonary edema is a diffuse process, these B lines should be seen at multiple interspaces. If B lines are seen unilaterally and focally, then a process, like pneumonia, should be considered. If the pleural surface is irregular or is not sliding and if B lines are seen, then pneumonia, pneumonitis, and acute respiratory distress syndrome should be considered.

Understanding B Lines

There are data that B lines correlate with cardiac filling pressures and extravascular lung water. Lichtenstein et al. (⁴) characterized patients into those with A-line pattern (no B lines) and those with B-line pattern. They reported that A-line pattern was 90% specific for a pulmonary artery occlusion pressure <13 mm Hg. Pivetta et al. (⁵) evaluated >1000 patients evaluated in emergency departments with acute dyspnea. After discharge, charts were reviewed by experts, and it was found that lung ultrasound plus clinical findings were more sensitive and specific for diagnosing decompensated heart failure than chest x-ray alone. In addition, Enghard et al. (⁶) showed that, in critically ill patients, B-line score is a good predictor of extravascular lung water by thermodilution.

Back to the Patient

In our patient, a careful examination showed that, at the right lower and middle lobes, the patient had an irregular pleura with absent lung sliding and with B lines. The B lines seen at the location of the pneumonia were not homogenously distributed between the rib shadows. At the apices of the right lung and throughout the left lung, the patient had a normal straight pleura, with lung sliding and homogenously distributed B lines. Our interpretation was that the right middle and lower lobes were affected by pneumonia but that rest of her lungs showed extravascular lung water. We concluded that pulmonary edema was significantly contributing to her dyspnea independent of her pneumonia. The patient received ultrafiltration treatment, her respiratory status improved, and she was taken off of bilevel positive airway pressure. The following day, lung ultrasonography continued to show similar findings at the right lung base, but the apices no longer showed B lines. Although it is possible that her pneumonia was improved, the repeat lung ultrasound clearly showed resolution of pulmonary edema.

Conclusions

The traditional physical examination is changing. Medical students are learning point of care ultrasonography in their preclinical curriculum and incorporating these findings into their decisions (⁷). Recently, there has been a growing interest in point of care ultrasonography in the field of nephrology, with half- and full-day practical courses offered at National Kidney Foundation Spring Clinical Meetings and the meetings at KidneyCon (⁸,⁹). In our institution, we routinely use ultraportable handheld devices to assess extravascular lung water in patients with ESKD, CKD, and AKI. Here, we described an instance where point of care lung ultrasonography provided timely, crucial information that resulted in improved patient care.

Disclosures

None.