Mechanical ventilator flow and pressure sensors: Does location matter?

Shane Toma, Mia Shokry, Ehab G. Daoud

Abstract

Introduction

Accurate measurements of ventilatory outputs are crucial during mechanical ventilation support. These measurements are achieved through sensors that monitor parameters such as flow/volumes and pressures. External and internal flow sensors are both commonly used in mechanical ventilation systems to measure the flow of air entering and leaving the patient’s lungs. The sensors could be located outside the ventilator (external or proximal) or inside the ventilator (internal or distal), each of which have their own respective advantages and disadvantages. There are differences in the way they function and the information they provide, which can affect their accuracy and usefulness in different clinical situations. A few clinical studies have compared the use of external to internal sensors in mechanical ventilation, showing mixed results. The intent of this study is to reexamine the differences between two critical care ventilators utilizing external sensors to two other ventilators utilizing internal sensors.

Methods

A bench study using a lung simulator was conducted. We constructed three passive, single compartment models: 1) compliance of 40 ml/cmH₂O, resistance of 10 cmH₂O, 2) compliance of 40 ml/cmH₂O, resistance of 20 cmH₂O, 3) compliance of 20 ml/cmH₂O, resistance of 10 cmH₂O. In each experiment we used two different modes of ventilation, volume controlled (tidal volume 400 ml, respiratory rate 20, PEEP 5 cmH₂O, inspiratory time 0.7 seconds) and pressure controlled (inspiratory pressure 15 cmH₂O, respiratory rate 20, PEEP 5cmH₂O, inspiratory time 0.7 seconds). We compared the inspiratory flow, inspiratory tidal volume, peak inspiratory pressures and PEEP in four commercially available critical care ventilators. Two use external flow sensors: G5 (Hamilton Medical), Bellavista 1000e (Vyaire Medical), and two use internal flow sensors: Evita Infinity 500 (Drager), and PB 980 (Medtronic). We also compared these parameters to a mathematical model. Statistics to compare the parameters in all four ventilators were done with Kruskal Wallis test followed by post-hoc Dunn’s test. A t-test was used to compare the parameters between the ventilator-measured and simulator-measured parameters in each ventilator. A P value of < 0.05 was considered significant.

Results

There were statistically significant differences (P < 0.001) in all four measured parameters: inspiratory flow, tidal volume, PIP and PEEP between all four ventilators, and between the mathematical model and all four ventilators in both modes, in all three clinical scenarios. The post-hoc Dunn test showed significant differences between each ventilator, except for a few parameters in PIP and PEEP. but not in flow or volume. There were variable but significant differences between some of the four parameters measured from the ventilator compared to those measured from the simulator of all four ventilators in both modes. The two ventilators using external sensors had more accurate differences between the delivered and measured tidal volumes (P < 0.001) and inspiratory flow (P < 0.001), however, the other two ventilators with internal sensors had more accurate differences between the delivered and measured PIP (P < 0.001) and PEEP (P < 0.001) levels.

Conclusions

All four ventilators performed differently from each other and from the mathematical model. The two ventilators using external sensors had more accurate differences between the delivered and measured tidal volumes and inspiratory flow, the two ventilators with internal sensors had more accurate differences between the delivered and measured PIP and PEEP levels. Differences between the ventilators depends on multiple factors including location, type of sensor, and respiratory mechanics.

Keywords

Flow sensor, Pressure sensor, PIP, PEEP, Tidal volume, Flow

References

1. Govoni L, Dellaca’ RL, Peñuelas O, et al. Actual performance of mechanical ventilators in ICU: a multicentric quality control study. Med Devices (Auckl). 2012;5:111-119. https://doi.org/10.2147/MDER.S35864 PMid:23293543 PMCid:PMC3534536

2. Chatburn RL, Mireles-Cabodevila E, Sasidhar M. Tidal volume measurement error in pressure control modes of mechanical ventilation: A model study. Comput Biol Med 2016; 75:235-242. https://doi.org/10.1016/j.compbiomed.2016.06.011 PMid:27318572

3. Schena E, Massaroni C, Saccomandi P, Cecchini S. Flow measurement in mechanical ventilation: a review. Med Eng Phys 2015; 37(3):257-264. https://doi.org/10.1016/j.medengphy.2015.01.010 PMid:25659299

4. Tardi G, Massaroni C, Saccomandi P, Schena E. Experimental assessment of a variable orifice flowmeter for respiratory monitoring. Journal of Sensors 2015; 2015:1-7. https://doi.org/10.1155/2015/752540

5. Sanborn WG. Monitoring respiratory mechanics during mechanical ventilation: where do the signals come from? Respir Care 2005; 50(1):28-54.

6. Sirohi J, Chopra I. Fundamental understanding of piezoelectric strain sensors. Journal of Intelligent Material Systems and Structures. 2000;11(4):246-257. https://doi.org/10.1106/8BFB-GC8P-XQ47-YCQ0

7. Bakhoum EG, Cheng MHM. High-sensitivity inductive pressure sensor. IEEE transactions on instrumentation and measurement 2011; 60(8):2960-2966. https://doi.org/10.1109/TIM.2011.2118910

8. Cannon ML, Cornell J, Tripp-Hamel DS, et al. Tidal volumes for ventilated infants should be determined with a pneumotachometer placed at the endotracheal tube. Am J Respir Crit Care Med 2000; 162(6):2109-2112. https://doi.org/10.1164/ajrccm.162.6.9906112 PMid:11112123

9. Castle RA, Dunne CJ, Mok Q, et al. Accuracy of displayed values of tidal volume in the pediatric intensive care unit. Crit Care Med. 2002;30(11):2566-2574. https://doi.org/10.1097/00003246-200211000-00027 PMid:12441771

10. Motta-Ribeiro GC, Giannella-Neto A, Wrigge H, et al. Mechanical Ventilators – Comparison of Proximal and Distal Respiratory Mechanics XXIV Brazilian Congress on Biomedical Engineering – CBEB 2014: 859-862

11. Hess DR. Respiratory mechanics in mechanically ventilated patients. Respir Care 2014; 59(11):1773-1794. https://doi.org/10.4187/respcare.03410 PMid:25336536

12. Yamaguchi Y, Miyashita T, Matsuda Y, et al. The difference between set and delivered tidal volume: A lung simulation study. Med Devices; 2020;13:205-211. https://doi.org/10.2147/MDER.S259760 PMid:32765126 PMCid:PMC7367738

13. Koyama Y, Uchiyama A, Yoshida J, et al. A comparison of the adjustable ranges of inspiratory pressurization during pressure controlled continuous mandatory ventilation of 5 ICU ventilators. Respir Care 2018; 63(7):849-858. https://doi.org/10.4187/respcare.05286 PMid:29765004

14. Jammes T, Auran Y, Couvernet J, et al. The ventilatory pattern of conscious man according to age and morphology. Bull Eur Physiopathol Respir 1979; 15:527-540.

15. ISO 80601-2-12:2020. Medical electrical equipment – Part 2-12: Particular requirements for basic safety and essential performance of critical care ventilators. https://www.iso.org/standard/72069.html accessed January 2022

16. Gammage GW, Banner MJ, Blanch PB, et al. Ventilator displayed tidal volume: what you see may not be what you get. Crit Care Med 1988; 16:454. https://doi.org/10.1097/00003246-198804000-00188

17. Chatburn RL. Simulation-based evaluation of mechanical ventilators (editorial). Respir Care 2018; 63(7):936-940. https://doi.org/10.4187/respcare.06267 PMid:29941669

18. Lyazidi A, Thille AW, Carteaux G, et al. Bench test evaluation of volume delivered by modern ICU ventilators during volume-controlled ventilation. Intensive care medicine. 2010; 36(12):2074-2080. https://doi.org/10.1007/s00134-010-2044-9 PMid:20862452

19. Sharma H. Statistical significance or clinical significance? A researcher’s dilemma for appropriate interpretation of research results. Saudi J Anaesth 2021;15(4):431-434. https://doi.org/10.4103/sja.sja_158_21 PMid:34658732 PMCid:PMC8477766

20. Daoud EG, Chatburn RL. Comparing surrogates of oxygenation and ventilation between airway pressure release ventilation and biphasic airway pressure in a mechanical model of adult respiratory distress syndrome. Respir Investig 2014; 52(4):236-241. https://doi.org/10.1016/j.resinv.2014.03.002 PMid:24998370