Lung ultrasound in mechanically ventilated patients

OWEN MOONEY, ANDREW W. KIRKPATRICK and LAWRENCE M. GILLMAN

Overview

Mechanically ventilated patients often have contraindications to transport outside a monitored setting. Ultrasound is an alternative to traditional imaging techniques in the critical care environment because of its portability, absence of radiation, and real-time image acquisition. This chapter will highlight the utility of bedside ultrasound in the day-to-day management of mechanically ventilated patients.

Recruitment/positive end-expiratory pressure (PEEP)

Recruitment maneuvers play an integral role in the management of patients with severe lung injury. Key to the success of a recruitment maneuver is identification of patients with potentially recruitable lung units. However, the amount of potentially recruitable lung varies widely within this patient population, from negligible to upward of 50% of lung weight.¹ Identifying who will and will not benefit from such maneuvers is crucial because there are risks to maintaining these high airway pressures, even if done transiently, including hemodynamic effects, pneumothoraces, and unintended airway trauma.

High-resolution computed tomography (CT) of the chest and evaluation of static pressure-volume loops on modern ventilators have been used to assess the effectiveness of lung recruitment. However, each of these has potential limitations because the improvements in oxygenation may not be immediately evident.² Point-of-care lung ultrasound offers a noninvasive, portable, easily reproducible method of identifying potential recruitable lung parenchyma. The normal and abnormal ultrasonographic appearance of lung parenchyma has been well described in previous chapters. The degree of lung aeration can easily be identified at the bedside. This bedside assessment can be categorized into four discrete ultrasonographic patterns.³ These patterns include (1) horizontal A-lines, representing normal lung aeration; (2) multiple vertical B-lines that are regularly spaced, representing a moderate loss of lung aeration; (3) coalescence of closely spaced B-lines, representing severe loss of lung aeration; and finally, (4) gross collapse. With this simple classification, researchers have developed a prediction model to better define how much an individual patient may benefit from attempts at lung recruitment (Table 23-1). The lung re-aeration score is simply a tool that helps quantify the amount of lung recruited (in milliliters) postrecruitment. This scoring system has been validated against traditional pressure-volume loop analysis.³ Each lung is systematically examined in six lung regions (upper anterior, posterior, and lateral; lower anterior, posterior, and lateral). The prerecruitment degree of lung aeration is then categorized as normal (N), moderate loss of aeration (B1), severe loss of aeration (B2), or completely consolidated (C) in each of these regions. A score of 1, 3, or 5 is then assigned to the change in the appearance of the lung postrecruitment. For example, an area of lung that went from being completely consolidated (C) to normal (N) would be assigned 5 points, whereas an area of lung that went from completely consolidated (C) to moderate loss of aeration (B1) would be assigned 3 points. The lung aeration score is calculated by the sum of scores from each of the six lung regions. A lung aeration score of +8 predicts a recruitable lung volume of greater than 600 mL, with the application of moderate amounts of PEEP.⁴ Identification of patients who will minimally benefit (lung aeration score <8) from such maneuvers is tantamount in avoiding unnecessary and potential harmful attempts at future recruitment.

TABLE 23-1

Ultrasound Lung Re-aeration Score

B1, Multiple, well-defined, either regularly spaced 7 mm apart or irregularly spaced B-lines (moderate loss of lung aeration); B2, multiple coalescent B-lines (severe loss of lung aeration); C, lung consolidation; N, normal pattern (normal lung aeration).

Note: The ultrasound re-aeration score is calculated as follows: In a first step, ultrasound lung aeration (N, B1, B2, and C) is assessed in each of 12 lung regions, examined before and after application of positive end-expiratory pressure of 15 cm H₂O. In a second step, the ultrasound lung re-aeration score is calculated as the sum of each score characterizing each lung region and examined according to the scale shown in the table above.

Modified from Bouhemad B, Brisson H, Le-Guen M, et al: Bedside ultrasound assessment of positive end-expiratory pressure-induced lung recruitment, Am J Respir Crit Care Med 183(3):341-347, 2001.

The “lung pulse” has also been advocated to identify potentially recruitable lung units.⁵ Sliding of the pleura with respiration in normal lungs prevents ultrasonographic perception of cardiac oscillation. When visualized lung is atelectatic, the motion of the patient’s beating heart is transmitted through the collapsed lung and perceived as cyclic motion, termed the lung pulse. Abolishment of this sign and the resumption of normal lung sliding postrecruitment maneuver can be an early sign of successful reexpansion of collapsed lung.⁵

The duration and level of peak airway pressure attained during recruitment is variable between studies and institutions. This may be because each collapsed lung unit has a unique time constant, and a “one-size-fits-all protocol” may not be appropriate to all patients. The use of real-time lung ultrasound allows the operator direct visualization of the lung during recruitment, thus yielding real-time feedback and allowing customization of both the peak pressure and duration of the maneuver (Figure 23-1, Video 23-1).³^,⁴^,⁶

Figure 23-1 Point-of-care lung ultrasonogram during a recruitment maneuver. Lung tissue can be seen to progress from initial lung atelectasis (A) to improving atelectasis (B) and eventually visualization of the pleural interface and comet-tail artifacts (C) as the atelectatic lung expands.

After successful recruitment of atelectatic lung, it is important to ensure that the recruited lung does not derecruit after the sustained high airway pressure is removed. Arbitrarily increasing the PEEP postrecruitment maneuver without confirmation that reexpansion of collapsed lung was successful can have detrimental effects on other areas of the lung. When the increased PEEP is not transmitted to the atelectatic lung, the parts of the lung that were nonatelectatic are now exposed to higher end-expiratory pressures, potentially resulting in overdistention and worsening V/Q mismatch. In theory, direct visualization of the lung would be of benefit for the setting of “best” PEEP, but further research is needed to validate its use routinely.

Screening for complications of mechanical ventilation

As well described in Chapter 21, lung ultrasound is both a sensitive and specific method of identifying pneumothoraces. Critically ill patients undergoing mechanical ventilation are at high risk of developing a pneumothorax from invasive procedures (e.g., central line placement, thoracentesis) or simply from the mode of ventilation.⁷ This is a very dangerous occurrence because mortality has been reported as being up to 91% if a tension pneumothorax occurs on a ventilator. In patients with a high clinical suspicion of pneumothorax, awaiting a chest radiograph or CT scan to confirm or refute its presence can delay intervention, which may have detrimental effects on the patient. However, blind needle decompression or chest tube placement in these critically ill patients with tenuous respiratory status, although sometimes lifesaving, can have severe consequences if the diagnosis is incorrect. As well, even the complication rate of tube thoracostomy tubes is much greater than clinicians typically perceive. The speed and accuracy of diagnosis without having to transport the patient outside a monitored setting makes ultrasound an attractive alternative to other methods used to identify a pneumothorax.