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# 20210228 Bag-Valve-Mask (BVM) Performance Evaluation Report

Project repository: https://github.com/tgh-apil/BVM-Evaluation

Report (Source File): https://hackmd.io/@bag-valve-test/BJU82dW2P

Data collection: Monday February 4, 2021 at Toronto General Hospital, 200 Elizabeth St, Toronto, ON M5G 2C4
Report: Monday February 22, 2021.
Revised: February 28, 2021.

Kate Kazlovic, Vahid Anwari, Azad Mashari MD
Department of Anesthesia and Pain Management, Toronto General Hospital

[TOC]

## Abstract
**Background** Bag-valve-mask (BVM) devices are critical equipment in acute care, and are used routinely during resuscitation, in operating rooms and intensive care units and during patient transport among numerous other settings. Unanticipated findings during an unrelated experiment involving BVMs suggested significant variations in the inspiratory resistance across different models and, more concerning, repeated failure of the valves in one brand in use at our hospital.

**Methods** We evaluated inspiratory flow and resistance with open or blocked expiratory ports in 3 different models of BVM devices available at our hospital (3 samples of each model). Testing was consistent with procedure outlined in CSA-Z10651-4-08 (R2018): Lung ventilators — Part 4: Particular requirements for operator-powered resuscitators, section A.4.8 (CSA revision of ISO 10651-4:2002).

**Results** All samples from two of the three models (Laerdal LSR and Ambu Spur II) showed anticipated behaviour with no effect of expiratory port blockage on inspiratory flow and resistance. All three samples of the third model, CARE-BVM (CAREstream Medical, Oakville, ON) showed consistent and significant increases in inspiratory flow resistance when the expiratory port was blocked. This suggested significant entrainment of outside air during inspiration, via a leaking expiratory valve. To confirm this we directly measured entrained air flow from the expiratory valve with simulated inspiratory flows of 25 and 50 L/min. Results showed that between 43-74% of inspiratory flow was entrained from the expiratory valve instead of flowing through the self-inflating reservoir of the BVM.

**Conclusion** Our results raise a significant safety concern regarding the performance of CARE-BVM devices suggesting that spontaneously breathing patients may often be receiving a significantly lower concentration of oxygen than assumed by care providers.

## Background
Bag-valve-mask (BVM) devices are critical pieces in the respiratory support armament of modern health care, providing high concentration oxygen and potentially ventilatory support across a wide range of acute and intensive care settings. The ingenious combination of valves and reservoir chambers in BVMs allows them to 1) function with or without a pressurized gas source and 2) efficiently provide concentrations of inspired oxygen approaching 100% using low to moderate gas flows.

During the COVID-19 pandemic and the acute shortages of ventilators during the first waves, the wide availability and versatility of BVMs led them to become the cornerstone of numerous strategies to provide invasive and non-invasive respiratory support for patients unable to access regular ventilators. The majority of emergency use ventilator initiatives relied on the use of reservoir and valve assemblies from BVMs to greatly simplify the design of emergency ventilators.

We set-out to evaluate the function of the inspiratory-expiratory control valves in three common models of bag-valve-mask (BVM) devices: Ambu Bag Spur II, CAREstream CARE-BVM and Laerdal LSR. This study was motivated by the unexpected finding during testing of another device, that the inspiratory resistance of CAREstream BVMs appeared to change significantly with blockage of the expiratory port.

In addition to providing a means of assessing the valve function of BVMs, the inspiratory flow resistance is clinically relevant in situations were the BVM is used to provide high oxygen concentrations to spontaneously breathing patients. This typically occurs during procedures (for example electroconvulsive therapy) or during management of patients in respiratory distress, where the BVM is often used for pre-oxygenation prior to induction of general anesthesia and endotracheal intubation. Higher inspiratory resistance in such situations can potentially lead to significant reductions in tidal volume and inspiratory flows, especially in patients in respiratory distress with limited ability to generate the negative pressures required to obtain adequate flows. This can be mitigated by manually supporting the patient by squeezing the bag during the patients spontaneous inspiratory effort.

The inspiratory flow resistance of a BVM device can be calculated by measuring the negative pressure required at the inspiratory port to generate various flow rates. If the valves in the device are competent these values will be independent of any blockage of the expiratory port (by a PEEP valve for example).

We report bench-top measurements of the inspiratory resistance of three BVM models from different manufacturers with, and without blockage of the expiratory ports, in order to test the competence of the valves. We also measured flow through the expiratory port during continuous inspiratory flows of 25 and 50 L/min for BVMs that showed increases changes in inspiratory resistance with blockage of the expiratory port.

## Procedure
### Devices Tested
Three samples of each of the following devices were tested.
1. [Ambu Spur II Adult BVM Disposable Resuscitator (Ambu A/S Copenhagen, Denmark)](https://www.emsstuff.com/Spur2-adult-BVM-ambu/)
* Sample 1 Lot#: 1000269921
* Sample 2 Lot#: N/A
* Sample 3 Lot#: N/A
2. [CAREstream CARE-BVM CS-100-A100-F-Univ Disposable Resuscitator (CAREstream Medical, Oakville, ON - Eastern Canada, Surrey, BC - Western Canada)](http://carestreammedical.com/product/care-bvm/)
* Sample 1 Lot#: 220070
* Sample 2 Lot#: 230070
* Sample 3 Lot#: 140059
3. [Laerdal Silicone Resuscitator (LSR; Laerdal Medical, Toronto, CA)](https://laerdal.com/products/medical-devices/airway-management/laerdal-silicone-resuscitator/)
* Sample 1 Lot#: 4512
* Sample 2 Lot#: 3509
* Sample 3 Lot#: 0915

The Spur II and CARE-BVM devices are disposable models. The Laerdal LSR is a reusable device. For the disposable device three previously unused samples were selected from three different lots. The LSR devices were reprocessed units used clinically at our hospital. Flow and pressure values were measured using two [FluxMed (R) GrH](https://mbmed.com/fluxmed-grh/) Respiratory mechanics monitor ([MBMed, Buenos Aires, Argentina](https://mbmed.com)) (S/N: 2802020022; 28002019032)

***Figure 1: Test set-up.** Suction was connected directly to the patient port of the BVM device (without a face mask) and titrated to achieved target flow rates of 25 and 50 L/min. The negative pressure required to generate each target flow value was recorded in two conditions: expiratory port open or sealed. The CAREstream devices underwent additional testing with the secondary Flow/Pressure meter at the Expiratory port to quantify entrained flow through the faulty expiratory valves.*

Inspiratory flow resistance was measured in accordance with CSA-Z10651-4-08 (R2018): Lung ventilators — Part 4: Particular requirements for operator-powered resuscitators, section A.4.8 (CSA revision of ISO 10651-4:2002). The test set-up is shown in Figure 1. The BVM was mounted on a retort stand attached to a table. The oxygen inflow of the BVM was connected to a standard wall oxygen flow meter connection at 15 L/min. The inspiratory port of the BVM device was connected to the in-line respiratory monitor which was then connected to wall suction.

For each device and test condition (expiratory port open vs. blocked) suction pressure was titrated to achieve flows of approximately 25 and 50 l/min. The pressure and flow values were recorded at 256 Hz sampling rate and averaged over 120 seconds. Inspiratory resistance was calculated by dividing the pressure in cm H~2~O by the flow in L/min. For the sealed measurements, the expiratory port was fully sealed with polyethylene plastic food wrapping. The respiratory monitor was calibrated prior to the testing of each device and condition.

## Results
All results in format mean(standard deviation/error) unless otherwise indicated. Units as follows: flow (L/min), pressure (cm H~2~O), resistance (cm H~2~O.min/L).

Table 1 presents the measured inspiratory flows and corresponding inspiratory pressures with the expiratory ports open and blocked, averaged across all 3 samples of each BVM model. Results are separated by target inspiratory flow rate (25 or 50 L/min). The absolute and percentage change in resistance with blockage of the expiratory port are presented the right-most columns and in figure 2. The expected values is 0, that is blockage of the expiratory port should have no effect on the inspiratory flow resistance. While the Spur II and LSR devices behaved as expected the CARE-BVM devices showed an average resistance increase of over 300% at 25 L/min and almost 200% at 50 L/min. This behaviour was consistent in all three samples of the CARE-BVM tested. Results for individual samples are presented in Supplemental Table 1.

Figure 3 illustrates the same finding in term of pressure-flow relationships in the expiratory port open and blocked conditions for each model. The expected behaviour is for the open and blocked conditions to be identical within margin of error. Again the LSR and Spur II display the expected behaviour while the CARE-BVM shows a significant difference between the two conditions. With any leak flow through the expiratory port blocked the CARE-BVM also has the highest inspiratory resistance of the three models tested.

### Table 1. Inspiratory flow resistance change with blockage of the BVM expiratory port.
***Table 1.** Inspiratory flow resistance chance with blockage of the BVM expiratory port. Results by model and target flow.*

### Figure 2. Percent change in inspiratory resistance with blockage expiratory port.
***Figure 2.** Percent change in inspiratory resistance with of blockage expiratory port in 3 models of BVM devices at 25 and 50 l/min of simulated inspiratory flow. The expected value is 0 (that is, the state of the expiratory port should have no influence on the inspiratory resistance). The CAREstream Device shows a marked effect or expiratory port closure.*

### Figure 3. Pressure drop vs. inspiratory flow with expiratory ports open or blocked
***Figure 3.** Pressure drop vs. inspiratory flow with expiratory ports open (dotted lines) or blocked (solid lines) in 3 models of BVM devices. Each colour represents a different model. The expected behaviour is for the two lines of each colour to be identical as they are for the Laerdal and Ambu models (that is, the state of the expiratory port should have no influence on the inspiratory resistance). The CAREstream Device shows a marked effect or expiratory port closure.*

The above results suggest a failure of the expiratory valve in the CARE-BVM. To verify this we repeated the experiment in the CARE-BVM devices with a second flow meter at the expiratory port in order to measure any leak flow through the expiratory port during continuous simulated inspiration. The results are shown in Table 2. Separated for 25 L/min and 50 L/min inspiratory flows. Across the 3 samples leak flow through the expiratory port accounted for 43-73% of the inspiratory flow.

***Table 2.** Entrainment of outside air during inspiration via leaking expiratory port in CAREstream CARE-BVM devices. Results shown for each of the 3 samples at target inspiratory flows of 25 and 50 L/min.*

Complete test data are available at the [project repository](https://github.com/tgh-apil/BVM-Evaluation).

## Conclusion
Our testing revealed consistent and significant failure of CARE-BVM devices consisting primarily of large leaks via the expiratory port with the likely consequence of dramatically lowering the oxygen concentration provided to patients. In addition the CARE-BVM devices had markedly higher inspiratory resistance (when expiratory leak was blocked) than the other models at 50 L/min.

Our testing did not reveal any failures in the samples of Spur II and LSR BVM devices tested.

Our testing is limited by the sample size, however our sample included 3 new devices from 3 different lots of devices. We did not explicitly test leak flow through the expiratory port of the LSR and Spur II since their stable inspiratory resistance despite blockage of the expiratory port did not raise any suspicion of a leak. In addition the geometry of these devices made it very challenging to directly measure expiratory flow with our set-up.

## Supplementary Data
### Table S1: Inspiratory flow resistance by individual BVM samples
***Supplementary Table 1a.** Inspiratory flow resistance change with blockage of the BVM expiratory port. Results for individual samples.*

### Set-up
***Figure S1.** Inspiratory Resistance Set-up*

***Figure S2.** Set-up for measuring expiratory leak flow in CARE-BVM devices*

## Authors
Azad Mashari MD FRCPC
Assistant Professor, Department of Anesthesia and Pain Medicine, University of Toronto; Staff Anesthesiologist and Director of Lynn and Arnold Irwin Advanced Perioperative Imaging Lab, Department of Anesthesiology and Pain Management, Toronto General Hospital, University Health Network, Toronto, ON

Vahid Anwari, MRT\(R\), MSc Student,
Rehabilitation Science Institute, University of Toronto; Research Assistant, Lynn and Arnold Irwin Advanced Perioperative Imaging Lab, Toronto General Hospital, University Health Network, Toronto, ON

Kate Kazlovich, PhD Candidate,
Institute of Biomaterials and Biomedical Engineering, University of Toronto; Research Assistant, Lynn and Arnold Irwin Advanced Perioperative Imaging Lab, Toronto General Hospital, University Health Network, Toronto, ON

## Competing Interests
None.