Vol. 20 • Issue 10 • Page 12
Lung recruitment strategies can help prevent lung collapse and improve oxygenation in patients on mechanical ventilation, but their practice is highly varied across the critical care setting. When used incorrectly, a ventilator can be a weapon of mass lung destruction.
This article will review the pros and cons of several techniques. But before clinicians decide on an approach to lung recruitment, they first must agree on the difference between recruitable and non-recruitable lung diseases. For example, pneumonia is typically a non-recruitable lung disease, whereas postoperative atlectasis will respond to lung recruitment strategies.
These vastly important distinctions must be established due to the fact that gas will take the path of least resistance. As a result, healthy areas of the lung will become over-inflated, while the collapsed areas of the lung will not benefit. When attempting to recruit the lungs, remember that the oxygen delivered by the ventilator inflates each lung, lobe, acinus, and alveoli at different time intervals. (See Sidebar.)
APRV pros and cons
Airway pressure release ventilation (APRV) uses spontaneous ventilation to mimic our normal, healthy respirations. It permits the diaphragm to pull down on the lung instead of being forced down by positive pressure. When gas is forced into the lung, it preferentially goes to areas with better compliance and the lowest bronchial resistance (RAW) creating a shift in the distribution of West Zones. Gas spontaneously pulled into the lung by the diaphragm enters the lung’s dependent regions, allowing for a more natural, sinusoidal respiratory pattern that may decrease sedation requirements.
Early application of this mode is critical for the patient to benefit. Historically, modes such as APRV are regarded as non-traditional or salvage modes and are not applied until the lung process is so severe that little benefit can be realized. The resulting effect is a feeling that these modes do not work. Early application of APRV to reduce the atelectatic lung and potential subsequent pneumonia may well render the description of the non-recruitable lung and time constants a moot point. Simply put, maintaining the lungs open prophylactically could mitigate the risk of additional injury to compliant sections of the lungs while re-recruiting collapsed or consolidated areas.
Due to its infrequent use, the most common issue with APRV is therapist and physician unfamiliarity with the mode. When using APRV with a low pressure (P-low) of zero, the expiratory time constants are difficult to measure and monitor. They change on a minute-to-minute basis, and the patient could forcefully exhale below the closing pressure of the lung.
To mitigate these concerns, some medical facilities have incorporated higher P-lows (up to 5 cm H2O) to reduce end expiratory collapse in the face of the auto-PEEP generated by the inverse ratio of APRV. Lung de-recruitment occurs faster and easier with restrictive lung diseases caused by obesity and abdominal distention because of an imbalance in the normal transpleural pressure relationships. The expiratory time constants measured at the ventilator represent the whole respiratory system and not each individual alveolar unit, which makes the EEF/PEF ratio determination difficult to follow and prevents atelecta-trauma and lung de-recruitment. Increased experience with the mode’s initiation and management will lead to appropriate management as well as successful patient outcomes.
HFOV pros and cons
High frequency oscillatory ventilation (HFOV) can best be described as continuous positive airway pressure with a “wiggle” because the mode works by oscillating around a constant mean airway pressure. This prevents the lung from deflating and eliminates the need to focus on ventilation over a defined time constant. Early initiation of HFOV in specific subsets of patients can maintain alveoli recruitment and reduce continuous opening/closing pressure issues (shear injury) that contribute to the body’s bio-trauma response. It can minimize lung flooding in lung contusion patients receiving large amounts of fluid, which can help them to maintain a functional lymphatic system.
While HFOV may lead to lung recruitment, the cost of its use and ICU practitioners’ unfamiliarity with it may undermine its overall efficacy. Often an attending physician or senior respiratory therapist successfully institutes HFOV but then struggles with best practices regarding its application. It also requires a high level of sedation
and/or paralytic management which can increase ventilator days and the length of hospital stays.
40/40 maneuver pros and cons
The theory behind using 40 cm H2O of PEEP for 40 seconds after a course of traditional ventilator mode application is to recruit as much lung as possible prior to the initiation of HFOV. However, any disconnection from the ventilator may cause massive lung de-recruitment. Clamping the endotracheal tube while switching to HFOV may help, but unless the set PEEP is increased to a level that maintains as much of the newly recruited lung as possible, any lung recruitment achieved by the maneuver is completely lost within the next few minutes. Lung deflation occurs for a variety of reasons, and simply forcing open the lung does not stop the forces that caused the initial collapse to occur again.
Incremental PEEP pros and cons
Incremental PEEP may be the easiest and best understood method, which makes it one of the safer approaches for lung recruitment. It is performed by titrating up PEEP in steps of 2 cm H2O while monitoring oxygen delivery, lung compliance, and VCO2. The goal of this approach is to appreciate oxygenation improvement as the lung recruits.
Incremental PEEP must be closely monitored by a well-trained respiratory therapist because forcing the lungs open with too much pressure can damage the lung and heart. It’s crucial to allow time for the patient to equilibrate between each settings change and to constantly monitor the process to avoid loss of oxygenation from overdistention of the lung.
Pressure volume loop pros and cons
Using a conventional mechanical ventilator’s pressure volume waveform can help identify the critical opening and closing pressures during volume ventilation. If PEEP is set to this level, it can prevent the lung from collapsing.
However, these numbers are more accurate when the flow is slower. The dynamic nature of breaths at full speed creates airway resistance and will cause these numbers to be falsely higher. Some institutions use sedation and paralytics to obtain lower, more accurate numbers.
Improve your chances of success
With whatever strategy you employ to recruit the lung, communication and teamwork within the interdisciplinary team are absolutely critical to success. All too often, one observes a single therapist turning knobs every which way trying to recruit the lung with little or no systematic thinking or approach. This lack of established practice and goals can not only be detrimental to the patient but also can erode the confidence level of our physicians and interdisciplinary partners regarding our practice. A collaborative team approach will make a difference in applying any of the methods described above and will help improve your chance of success.
Matthew Davis, RRT, is clinical education coordinator, respiratory care services, at University of Maryland Medical Center in Baltimore. Chris Kircher, MS, RRT, is associate director for respiratory care at the same facility. ADVANCE board member Michael J. Hewitt, RRT-NPS, FAARC, FCCM, is director of respiratory care services at St. Joseph’s Hospitals in Tampa, Fla.
Varying Time Constants
A time constant is the relationship between compliance and resistance, which sets the different filling times for each lung unit. The value for a normal time constant (Tc) is approximately 0.2 seconds for the average healthy adult human. One Tc is the amount of time it takes in seconds for the respiratory system to equilibrate to the change in ventilator pressure.
Each Tc continues to fill at a rate of 63 percent of the available volume left. One Tc reaches 63 percent equilibration, two Tc reaches 87 percent equilibration, three Tc reaches 95 percent equilibration, and therefore at the fifth Tc, we have about 99 percent equilibration. Thus, a normal adult inspiration time is at the fifth Tc or five times 0.2 seconds, which equals one second.
To permit gas to enter the slower filling alveolar units, pressure must be applied for greater than five times the time constant to create enough lateral wall pressure to move gas from areas with short time constants to areas with longer time constants via pendelluft gas movement. This lateral wall pressure creates lung recruitment and collateral ventilation via the pores of Khon, canals of Lambert, and channels of Martin.