Reading Time: 4 minutes

The Pre-brief

The phenomenon of pendelluft has several implications in mechanically ventilated and spontaneously breathing patients.  While largely theoretical in nature, advancements in technology have recently made pendelluft more tangible and importantly, measurable.  However, before we get into some of the recent applications and areas of research, let’s start by covering some of the basics.

What is a time constant?

The time constant is a borrowed term from mechanical engineering used to describe the decay of a variable over time.  In lung mechanics, it is the time it takes to deflate or inflate the lungs by 63% after a step change in pressure (Figure 1). Take note from Figure 1 that 95% of the equilibration value is achieved after 3 time constants. The time constant is a reflection of both the resistance and compliance of the lungs. Mathematically, time constant is the product of resistance and compliance (R.C). Clinically, a short time constant implies low compliance; a long time constant suggests high resistance. In normal, healthy lungs, the expiratory time constant is around ~0.5 seconds, meaning nearly all volume is expired by 1.5 seconds.

Lungs with different time constants will fill at different rates and to different volumes over a given time.  In a healthy state, the lungs are assumed to be largely homogenous and will have a single time constant.  However, it is well established that in disease states such as ARDS, there are many areas of heterogeneity.  Due to heterogeneity of the lungs and lung units, different portions of the lung may have different time constants.

What is pendelluft?

The most fundamental definition of pendelluft is the movement of gas within the lung without a change in tidal volume.  Pendelluft can also be conceptualized as the redistribution of air from units of short time constants to areas of long time constants.  This phenomenon can sometimes be seen on the ventilator as a slow drift during an inspiratory hold in a paralyzed patient however is largely an unobservable effect. 

Reasons/mechanisms of Pendelluft

Pendelluft is assumed to not occur in normal, healthy lungs. In diseased lungs, such as ARDS, pendelluft can occur due to two main reasons:

  1. Vigorous spontaneous efforts in patients with ARDS: In patients with dense basilar consolidates, vigorous diaphragmatic contraction leads to disproportionately negative pleural pressure in the dependent lung regions. This is because the peri-diaphragmatic regions manifest a solid-like behavior such that the pleural pressure swings are not transmitted to the non-dependent regions. This pleural pressure gradient leads to transfer of gas from non-dependent regions to the dependent regions. This occurs during the initial part of the breath, before any significant proportion of the tidal volume has been delivered.
  2. Post-inspiratory pause in patients with heterogenous lung units: Patients with ARDS often have lung units with heterogeneous time constants. If the set inspiratory flow rate is high, this scenario may result in preferential inflation of units with low time constants. A prolonged post-inspiratory pause may then result in redistribution of gas from areas with low time constant to those with high time constant. This, by definition, is pendelluft

Effects of Pendelluft 

The major concern with pendelluft is in patients with ARDS who have vigorous inspiratory efforts. As noted above, this leads to transfer of gas from non-dependent to dependent regions, thereby resulting in selective overinflation of dependent regions. In the seminal study by Yoshida et al, it was noted in patients exhibiting significant pendelluft, passive mechanical ventilation with a tidal volume of 14.8 cc/kg resulted in a similar degree of strain in the dependent lung regions as compared to 5.8 cc/kg tidal volume when spontaneously breathing.

Furthermore, gas subject to pendelluft comes from within the lung and contains higher CO2 and lower O2 as compared to fresh gas.  Thus, this gas does not contribute as efficiently to gas exchange leading to increased work and wasted energy by the pulmonary system and ventilator. 

Pendelluft in Action

Several studies have attempted to correlate the phenomenon of pendelluft to clinical applications however due to difficulty with measuring or quantifying pendelluft, studies are fairly limited and are mostly exploratory.

Electrical impedance mapping is one technology that has been shown to help identify pendelluft in action.  One of the first applications of this technology identified pendelluft in a spontaneously breathing ARDS patient while on a ventilator.  This mapping demonstrated movement of air from non-dependent areas to dependent areas without a change in tidal volume, ie pendelluft. This redistribution has several important implications.  First, even with lung protective volumes, local redistributions occur in diseased lungs, leading to possibly overdistension, barotrauma or atelectrauma.  In this ARDS patient, the amount of pendelluft appeared to be exacerbated by spontaneous breathing.  In comparison, these findings were not replicated in a patient with healthy, normal lungs. 

Another study measured an actual volume of pendeluff in 20 patients undergoing spontaneous breathing trials with varying degrees of mechanical support by using electrical impedance.  As pressure support was weaned from 12cmH20 to 2cmH20, pendelluft volume was proportionally increased in what was found to be a “pendeluff prone” group.  In this group, as pendeluff increased, so did heart rate, respiratory rate as well as EtCO2. Also using electrical impedance mapping, it was found that this pendeluff prone group was largely redistributing volume from ventral regions towards dorsal regions. While a preliminary study, these findings support the theory that pendelluft contributes to work of breathing and wasted energy. 

The phenomenon of pendelluft has a growing body of evidence to support its role in lung mechanics.  However, how to mitigate pendelluft and decrease wasted energy continues to be a topic of research and debate.

The Debrief

  • A time constant is a reflection of both the resistance and compliance of a lung unit
  • Air redistributes from different units of different time constants when there is no inhalation or exhalation.
  • Pendelluft is the movement of air between different lung units without a change in tidal volume.
  • Pendelluft appears to add to lung inefficiency and may play a role in additional barotrauma and atelectrauma.


  1. Yoshida T, Torsani V, Gomes S, De Santis RR, Beraldo MA, Costa EL, Tucci MR, Zin WA, Kavanagh BP, Amato MB. Spontaneous effort causes occult pendelluft during mechanical ventilation. Am J Respir Crit Care Med. 2013 Dec 15;188(12):1420-7. doi: 10.1164/rccm.201303-0539OC. PMID: 24199628.
  2. Coppadoro A, Grassi A, Giovannoni C, Rabboni F, Eronia N, Bronco A, Foti G, Fumagalli R, Bellani G. Occurrence of pendelluft under pressure support ventilation in patients who failed a spontaneous breathing trial: an observational study. Ann Intensive Care. 2020 Apr 7;10(1):39. doi: 10.1186/s13613-020-00654-y. PMID: 32266600; PMCID: PMC7138895.
  3. Enokidani Y, Uchiyama A, Yoshida T, Abe R, Yamashita T, Koyama Y, Fujino Y. Effects of Ventilatory Settings on Pendelluft Phenomenon During Mechanical Ventilation. Respir Care. 2021 Jan;66(1):1-10. doi: 10.4187/respcare.07880. Epub 2020 Sep 8. PMID: 32900913.


More Posts

Related Posts

Would love your thoughts, please comment.x