Bedside tip: How to recognize increases in expiratory filter resistance

Author: Ken Hargett, Reviewer: Süha Demirakca, Matthias Himmelstoss, Simon Franz, Munir Karjaghli, David Grooms

It is well known that expiratory filters can become clogged with humidity and particles from nebulized medications trapped in the filter media (1).

This results in increased resistance in the expiratory limb of the circuit and can affect the function of the ventilator. It is important for the caregivers to recognize the development of resistance over time. The most extreme case of filter resistance is when the filter is completely clogged, generating the Exhalation obstructed alarm on the ventilator.

The criteria for this alarm on a HAMILTON-G5 ventilator are as follows: Flow is < 300 ml/sec 220 milliseconds after the detection window is open or delta pressure does not drop by 33% of peak value.

Hamilton Medical ventilators utilizing a proximal flow sensor measure pressure and flow at the patient airway opening and display calculated values of airway compliance and resistance in the Dynamic Lung window. While these values reflect the condition of the patient, they do not track resistance changes in the expiratory filter. There are several ways to recognize increasing resistance that is building up over time.

Expiratory flow

Particle build up impedes expiratory flow out of the circuit. The expiratory flow rate decreases over time as the resistance increases. Monitoring of the expiratory flow is a way to determine increasing resistance in the expiratory filter. Expiratory flow is a function of the lung recoil and is passive in most instances and related to the compliance of the lung. Table 1 shows expiratory flow rates for two compliance conditions. Normal compliance has a lower initial expiratory flow than the reduced compliance lung. Over time as the expiratory filter resistance increases from new filter baseline to R2, R3 and finally R4, expiratory flow decreases in both compliance conditions. We can also observe that the shape of the expiratory flow curve changes as resistance increases. It takes longer for the flow to return to zero as resistance increases.

Table 1: Expiratory Flow Graphics for Normal and Reduced compliance at four different stages of expiratory filter resistance, starting with the new filter baseline on the left and continuously increasing expiratory filter resistance at stages R2, R3 and R4. Expiratory flow reduces as filter resistance increases.

The value of expiratory flow can be displayed in several ways. By freezing the flow graphic and using the controller to move the indicator to the maximum expiratory flow, the value is displayed as circled in the graphic below. Additionally, breath-to-breath values are displayed in panel 3/12 as shown below.

Expiratory time constant: RCexp

The expiratory time constant describes the speed of lung emptying after the pressure drop created by opening of the expiratory valve. The components of the RCexp are compliance and resistance. A stiff lung (e.g., ARDS) with reduced compliance and normal or near-normal resistance has a short RCexp, whereas a more compliant lung (e.g., COPD) with normal to high compliance, but increased resistance, has a longer RCexp. Table 2 shows typical values for RCexp for the different lung conditions (2).

  Normal ARDS COPD
RCexp (sec) 0.5 - 0.7 0.4  -  0.6 0.7 - 2.1

Hamilton Medical ventilators* measure the RCexp at 75% of the exhaled volume, which has been shown to improve accuracy (3). While the proximal flow sensor reflects patient characteristics, the RCexp reflects the entire patient and ventilator circuit. Additional resistance present in the expiratory filter creates changes in the RCexp without changes in compliance or inspiratory resistance measurements. Panel 1 below shows changes in the RCexp for a near-normal compliance lung (Cstat 47.5 ml/cmH2O) at four different filter conditions. The RCexp goes from 0.50 seconds with the new filter to 0.63 at R2, 0.78 at R3 and 1.01 at R4 with no change in the compliance or inspiratory resistance (Rinsp) values displayed.

Panel 1: Changes in the RCexp for a near-normal compliance lung at four different filter conditions

Panel 2 below shows changes in the RCexp with a low compliance lung (Cstat = 19.1 ml/cmH2O). RCexp goes from the expected value for ARDS of 0.29 to 0.69, which is indicative of a more compliant lung. No change in compliance or inspiratory resistance values displayed are seen with increases in expiratory filter resistance.

Panel 2: Changes in the RCexp with a low compliance lung at four different filter conditions


It should be a routine practice to change out expiratory filters on a routine basis. In the event of filter shortages, institutions have prolonged each filter use and often the filter may be neglected.

A good way to track increases in expiratory filter resistance over time is the use of trending. Plotting expiratory flow and RCexp will show the gradual increases in filter resistance. Here we see decreases in expiratory flow and increases in RCexp over the last hour.

Changing out expiratory filters

In order to prevent contamination of the surrounding area, it is advisable to maintain a closed ventilator circuit at all times. Changing out in-line expiratory filters requires briefly breaking the circuit. Trained staff should be wearing PPE including N-95 mask, gowns, gloves and face shields. Additional protective equipment might also be necessary as per hospital guidelines.

A direct disconnection of the ventilator circuit will result in a purge of gas that can spray the room with aerosol particles. To avoid contamination by aerosols while changing the filter, place the ventilator in standby before every disconnection of the circuit.. If you prefer not to put the ventilator in standby, the suctioning tool (Oxygen enrichment button) will lessen the flow during disconnection and reduce aerosols. When this feature is active, disconnection of the ventilator results in suppression of ventilation and not a purge of high flow gas. The ventilator will display the Ventilation suppressed banner during disconnect and resume ventilation after the circuit has been reconnected.

Review of Hamilton Medical’s document on safe use of Hamilton Medical ventilators with highly infectious diseases is recommended (4). More information regarding the use of filters can also be found on Hamilton Medical’s dedicated COVID-19 webpage.

* Not all features, modes or ventilators available in all markets


  1. Tonnelier A. Lellouche F. Bouchard PA, L’Her E. Impact of Humidification and Nebulization During Expiratory Limb Protection: An Experimental Bench Study. Respiratory Care 2013;58 (8) 1315-1322
  2. Brunner JX, Laubscher TP, Banner Mj, Iotti G, Braschi, A. Simple method to measure total expiratory time constant based on the passive expiratory flow volume curve. Crit Care Med 1995; 23:1117-1122
  3. Lourens MS van den Berg B Aerts JG, et al. Expiratory Time Constants in mechanically ventilated patients with and without COPD. Intensive Care Med 2000; 26(11):1612-1618
  4. Safe use of Hamilton Medical ventilators on patients with highly infectious diseases

Related Articles

RCexp, resistance, compliance, filter resistance, filter, occlusion, trending, expiratory flow, expiratory resistance, inspiratory resistance

Date of Printing: 27.10.2020
The content of this newsletter is for informational purposes only and is not intended to be a substitute for professional training or for standard treatment guidelines in your facility. Any recommendations made in this newsletter with respect to clinical practice or the use of specific products, technology or therapies represent the personal opinion of the author only, and may not be considered as official recommendations made by Hamilton Medical AG. Hamilton Medical AG provides no warranty with respect to the information contained in this newsletter and reliance on any part of this information is solely at your own risk.