How to recognize increases in expiratory filter resistance

Article

Author: Ken Hargett

Date of first publication: 07.09.2020

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

How to recognize increases in expiratory filter resistance

Alarm Exhalation obstructed

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.

Compliance and resistance are not enough

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.

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 in Figure 1 below.

Table showing flow graphics for different resistance — 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.

Screenshot showing exp flow value — Figure 1

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. The table below shows typical values for RCexp for the different lung conditions (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(6):1117-1122. doi:10.1097/00003246-199506000-000192).

Lung condition	RCexp (sec)
Normal	0.5–0.7
ARDS	0.4–0.6
COPD	0.7–2.1

RCexp changes with additional resistance in expiratory filter

Hamilton Medical ventilators measure the RCexp at 75% of the exhaled volume, which has been shown to improve accuracy (Lourens MS, van den Berg B, Aerts JG, Verbraak AF, Hoogsteden HC, Bogaard JM. Expiratory time constants in mechanically ventilated patients with and without COPD. Intensive Care Med. 2000;26(11):1612-1618. doi:10.1007/s0013400006323). 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 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.

Screenshots showing dynamic lung and monitoring parameters — Panel 1: Changes in the RCexp for a near-normal compliance lung at four different filter conditions

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.

Full citations below: (Tonnelier A, Lellouche F, Bouchard PA, L'Her E. Impact of humidification and nebulization during expiratory limb protection: an experimental bench study. Respir Care. 2013;58(8):1315-1322. doi:10.4187/respcare.017851)

Footnotes

References

1. Tonnelier A, Lellouche F, Bouchard PA, L'Her E. Impact of humidification and nebulization during expiratory limb protection: an experimental bench study. Respir Care. 2013;58(8):1315-1322. doi:10.4187/respcare.01785
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(6):1117-1122. doi:10.1097/00003246-199506000-00019
3. Lourens MS, van den Berg B, Aerts JG, Verbraak AF, Hoogsteden HC, Bogaard JM. Expiratory time constants in mechanically ventilated patients with and without COPD. Intensive Care Med. 2000;26(11):1612-1618. doi:10.1007/s001340000632

Impact of humidification and nebulization during expiratory limb protection: an experimental bench study.

Tonnelier A, Lellouche F, Bouchard PA, L'Her E. Impact of humidification and nebulization during expiratory limb protection: an experimental bench study. Respir Care. 2013;58(8):1315-1322. doi:10.4187/respcare.01785

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BACKGROUND

Different filtering devices are used during mechanical ventilation to avoid dysfunction of flow and pressure transducers or for airborne microorganisms containment. Water condensates, resulting from the use of humidifiers, but also residual nebulization particles may have a major influence on expiratory limb resistance.

OBJECTIVES

To evaluate the influence of nebulization and active humidification on the resistance of expiratory filters.

METHODS

A respiratory system analog was constructed using a test lung, an ICU ventilator, heated humidifiers, and a piezoelectric nebulizer. Humidifiers were connected to different types of circuits (unheated, mono-heated, new-generation and old-generation bi-heated). Five filter types were evaluated: electrostatic, heat-and-moisture exchanger, standard, specific, and internal heated high-efficiency particulate air [HEPA] filter. Baseline characteristics were obtained from each dry filter. Differential pressure measurements were carried out after 24 hours of continuous in vitro use for each condition, and after 24 hours of use with an old-generation bi-heated circuit without nebulization.

RESULTS

While using unheated circuits, measurements had to be interrupted before 24 hours for all the filtering devices except the internal heated HEPA filter. The heat-and-moisture exchangers occluded before 24 hours with the unheated and mono-heated circuits. The circuit type, nebulization practice, and duration of use did not influence the internal heated HEPA filter resistance.

CONCLUSION

Expiratory limb filtration is likely to induce several major adverse events. Expiratory filter resistance increase is due mainly to the humidification circuit type, rather than to nebulization. If filtration is mandatory while using an unheated circuit, a dedicated filter should be used for ≤ 24 hours, or a heated HEPA for a longer duration.

Simple method to measure total expiratory time constant based on the passive expiratory flow-volume curve.

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(6):1117-1122. doi:10.1097/00003246-199506000-00019

Link to article

OBJECTIVE

In intubated, mechanically ventilated patients, inspiration is forced by externally applied positive pressure. In contrast, exhalation is passive and depends on the time constant of the total respiratory system. The expiratory time constant is thus an important determinant of mechanical ventilation. The aim of this study was to evaluate a simple method for measuring the expiratory time constant in ventilated subjects.

DESIGN

Prospective study using a lung simulator and ten dogs.

SETTING

University hospital.

SUBJECTS

Commercially available lung simulator and ten greyhound dogs.

INTERVENTIONS

Different expiratory time constants were set on the lung simulator. In the dogs, the endotracheal tube was clamped to increase airways resistance by 22.5 cm H2O/(L/sec) and the lungs were injured with hydrochloric acid to decrease total respiratory compliance by 16 mL/cm H2O. This procedure resulted in a wide range of expiratory time constants.

MEASUREMENTS AND MAIN RESULTS

Pneumotachography was used to measure flow and volume. The ratio of exhaled volume and peak flow was calculated from these signals, corrected for the limited exhalation time yielding the "calculated expiratory time constant" and compared with the actual expiratory time constant. The typical error was +/- 0.19 sec for the lung simulator and +/- 0.15 sec for the dogs.

CONCLUSIONS

The volume and peak flow corrected for limited exhalation time is a good estimate of the total expiratory time constant in passive subjects and may be useful for the titration of mechanical ventilation.

Expiratory time constants in mechanically ventilated patients with and without COPD.

Lourens MS, van den Berg B, Aerts JG, Verbraak AF, Hoogsteden HC, Bogaard JM. Expiratory time constants in mechanically ventilated patients with and without COPD. Intensive Care Med. 2000;26(11):1612-1618. doi:10.1007/s001340000632

Link to article

OBJECTIVE

In mechanically ventilated patients, the expiratory time constant provides information about the respiratory mechanics and the actual time needed for complete expiration. As an easy method to determine the time constant, the ratio of exhaled tidal volume to peak expiratory flow has been proposed. This assumes a single compartment model for the whole expiration. Since the latter has to be questioned in patients with chronic obstructive pulmonary disease (COPD), we compared time constants calculated from various parts of expiration and related these to time constants assessed with the interrupter method.

DESIGN

Prospective study.

SETTING

A medical intensive care unit in a university hospital.

PATIENTS

Thirty-eight patients (18 severe COPD, eight mild COPD, 12 other pathologies) were studied during mechanical ventilation under sedation and paralysis.

MEASUREMENTS AND RESULTS

Time constants determined from flow-volume curves at 100%, the last 75, 50, and 25% of expired tidal volume, were compared to time constants obtained from interrupter measurements. Furthermore, the time constants were related to the actual time needed for complete expiration and to the patient's pulmonary condition. The time constant determined from the last 75% of the expiratory flow-volume curve (RCfv75) was in closest agreement with the time constant obtained from the interrupter measurement, gave an accurate estimation of the actual time needed for complete expiration, and was discriminative for the severity of COPD.

CONCLUSIONS

In mechanically ventilated patients with and without COPD, a time constant can well be calculated from the expiratory flow-volume curve for the last 75% of tidal volume, gives a good estimation of respiratory mechanics, and is easy to obtain at the bedside.