Post by Detajin Junhasavasdikul MD.
Division of Pulmonary and Critical Care Medicine. Department of Medicine, Faculty of Medicine Ramathibodi Hospital, Mahidol University. Thailand.
(Formerly a Post-doctoral Research Fellow in the Interdepartmental Division of Critical Care Medicine, University of Toronto and the Department of Critical Care, St. Michael’s Hospital, Toronto, Canada)
The characteristics of the flow limitation waveforms
The flow-time curve from the ventilator display showed typical characteristics of “expiratory flow limitation (EFL)”. The expiratory flow initially begins with a “spike” of high expiratory flow which is followed by a flat part with a very low flow rate. This pattern is comparable to a flow-volume loop of the spirometry in COPD patients (Figure 1). Such a pattern is probably produced by the “collapsed” airway or “choke point / equal pressure point” which is a widely-believed mechanism causing EFL. Please note that the EFL in this ventilator waveform occurred during tidal breathing, not at a maximal expiratory force exerted during a spirometry examination. Thus, this condition is specifically called: “tidal EFL (EFLT)”. Additionally, patients will commonly display characteristics of high resistance which can be measured by assessing the difference between PIP and Plateau (Resistance = PIP-Plateau/L/sec) (Figure 1a).
Figure 1: A comparison of the flow-time waveform from the ventilator (left) to a spirometry flow-volume loop of a COPD patient (right).
Figure 1a: The difference between peak-inspiratory pressure (PIP) and plateau pressure (Pplat) represents the pressure required to overcome resistance (resistive pressure).
What does it actually mean and in which patients will we find this?
The exact definition of EFL is “a dynamic condition in which expiratory flow at a given lung volume has already reached its maximal value despite an increasing expiratory driving pressure (or force expiration)”. The EFL is dynamic and could appear or disappear within the same patient under different conditions e.g. at different lung volumes, fluid status, and the patients’ position. By common sense, we would expect to find the EFLT in COPD patients or patients with airway instability (tracheobronchomalacia). However, the EFLT is also found in obese patients, patients with ARDS ventilated at ZEEP, or patients with pulmonary edema – this is probably due to the compression of small airways, causing limited expiratory flow in some part of the lungs, especially at lower lung volume.
Why care about it?
The EFLT is a major determinant of intrinsic PEEP (PEEPi). Patients with EFLT frequently have higher PEEPi, and patients with higher PEEPi are more likely to have EFLT. The hyperinflation caused by the trapped air can lead to bad physiologic effects. The EFLT is also reported to cause more post-operative complications, weaning failure, or probably causing airway injuries in ARDS. Conscious patients might get a dyspneic sensation and an attempt at forced expiration would fail to shorten the expiratory duration instead resulting in a waste of energy.
How to detect the EFL?
We can suspect the EFLT by the typical characteristics of the waveform as already discussed, and by the presence of high PEEPi. However, these findings are not specific to EFLT and a specific test is needed for the definitive diagnosis. There are some methods available for the diagnosis of the EFLT – either by a direct change of the expiratory driving pressure (mimicking the definition of the EFL) or by (indirectly) adjusting the resistance of the airway or using an interrupter technic.
The most practical method to be performed at the bedside is probably the “PEEP reduction” maneuver. This would immediately change the expiratory driving pressure and thus make detection of EFLT possible (see Video1 for the physiologic reasons behind this).
To perform the PEEP reduction maneuver, in short, follow these steps:
- Checking that the patient is in a monotonous respiratory pattern (probably with a constant tidal volume with less than 10% variation) – putting the patient in VCV mode is somewhat helpful.
- From the current PEEP settings, immediately lowers the PEEP by 5-10 cmH2O (or to ZEEP). Compared the flow-volume loops of this “test breath” and the “index breath” just preceding it. If the test breath fails to gain higher expiratory flow than the index breath, then the patient is defined to have EFLT (Figure 2 and 3).
- For practical use without any additional special recording equipment, set the ventilator to display the flow-volume loop with an overlapping feature if available (Figure 4).
- If such function is not available, we may also try to look at the flow-time tracings that the expiratory-flow spike is minimally increased during the PEEP reduction and that the tidal volume is not much increased, implying a patient has EFLT (Video2).
Figure 2: The pressure-time and the flow-time curve (left), and the flow-volume loops of the two breath (the index breath – in blue AND the test breath – in orange) plotted together in the same axis (right). In a normal subject without EFLT, PEEP reduction maneuver will result in an increase of expiratory flow (red arrowheads) and a larger exhaled tidal volume (blue arrowheads).
Figure 3: In a subject with EFLT, PEEP reduction maneuver will result in unchanged (or minimally increased) expiratory flow compared to the index breath (red arrowheads) and virtually the same exhaled tidal volume (blue arrowheads).
Figure 4: Using a ventilator’s built-in flow-volume loop function with the overlapping features. This screenshot shows the flow-volume of a breath with the set PEEP of 10 cmH2O overlapping with the 2 following breaths with an immediate reduction of the set PEEP to 5 cmH2O. The loops are almost completely overlapped with each other – indicating the presence of EFLT.
How to correct the EFL?
The main aim is to address the correctible causes (such as pulmonary edema, high pleural pressure, or bad patient positioning). Other methods that may alleviate EFLT include: PEEP titration (to 80-100% of PEEPi), diuretics, bronchodilators etc. Please refer to the review article in the “further reading” section for additional information.
The EFLT is a condition that may result in bad clinical consequences but can often be reversed once detected. An awareness of EFLT and knowing how to diagnose it are keys to successful treatment of these patients.
Video 1 (narrated by Thomas Piraino)
Expiratory Flow Limitation During Mechanical Ventilation.
Junhasavasdikul D, Telias I, Grieco DL, Chen L, Gutierrez CM, Piraino T, Brochard L.
Chest. 2018 Oct;154(4):948-962. doi: 10.1016/j.chest.2018.01.046.