Flow Resistance of Expiratory Positive-Pressure Valve Systems

doi:10.1378/chest.90.2.212

Chest

Volume 90, Issue 2, August 1986, Pages 212-217

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The flow-resistive characteristics of a variety of commercially available expiratory positive-pressure valve systems used to provide continuous positive airway pressure (CPAP) and positive end-expiratory pressure were evaluated. One flow-resistor and seven threshold-resistor expiratory pressure valve systems were set at 5, 10,15, 20, and 25 cm H₂O of expiratory pressure, and sinusoidal exhaled flows peaking at 50, 100, and 200 L/min were directed through each valve at each level of expiratory pressure. The Siemens flow-resistor valve demonstrated the greatest deviation in pressure above set CPAP levels at peak flow rates of 100 and 200 L/min, which suggests high resistance to exhaled flow. The Vital Signs threshold-resistor valve demonstrated the least deviation in pressure from set CPAP levels at all rates of exhaled flow, which suggests low flow resistance. The Emerson and IMV Bird threshold-resistor systems resisted flow less than the BEAR-2 and the Puritan-Bennett MA-2 and 7200 inflatable-balloon threshold-resistor-like valve systems. These data suggest that threshold resistors may be classified as low-resistance or high-resistance types. Using only low-resistance threshold resistors for CPAP may minimize the incidence of barotrauma and other deleterious effects related to airway pressure.

Section snippets

Materials and Methods

The ventilator systems used were the Siemens 900C (n=3), BEAR-2 (n=3), Puritan-Bennett MA-2 (n=3) and 7200 microprocessor types (n=3), IMV Bird (Bird-Products Corp.) (n=3), and IMV Emerson (J. H. Emerson Co.) (n=6; three newer and three older expiratory pressure valve models). For all ventilators the stock permanent-type expiratory pressure valve recommended by the manufacturer was used. These ventilators all generate CPAP by means of a demand valve or demand valve-like system.⁷ Three sets of

Results

At all exhaled flow rates, all expiratory pressure valve systems increased expiratory positive pressure above the set level, some systems offering significantly more resistance to flow than others. The Siemens flow-resistor expiratory pressure valve caused the greatest deviations in pressure above most levels of CPAP at 50 L/min and for all levels of CPAP at 100 and 200 L/min of exhaled flow rate (p<0.05) (Table 1). The Vital Signs multiple spring-actuated valves demonstrated the lowest

Discussion

With CPAP during exhalation, expiratory pressure valves of many breathing circuits used today resist flow, which substantially increases pressure during exhalation.¹⁰ Consider an expiratory pressure valve exerting 10 cm H₂O of expiratory positive pressure with a flow resistance of 15 cm H₂O/L/sec (assumed linear over a given range of flow rates); such a valve would increase airway pressure to 25 cm H₂O at an exhaled flow rate of 1 L/sec (60 L/min). Should the patient cough or exhale rapidly,

Appendix

The equation of motion for a rectilinear mechanical system provides a mathematical description of the mode of operation of a “passive” threshold resistor:¹⁶ $F = Kl + cv + ma$ F is the force applied against the expiratory valve face (in newtons), K is the elastic modulus (spring constant) of the valve system (in newtons per centimeter), 1 is the distance of compression or extension of the valve face from its natural resting position (in centimeters), c is the viscous resistance opposing valvular

Acknowledgment

Lynn M. Carroll provided editorial assistance, and Ms. Joy Kuck provided the grapnics.

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Cited by (33)

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Citation Excerpt :
However, this mechanism cannot generate a PEEP. The regulation of the expiratory valve causes the desired PEEP by taking into account the resistance of the breathing circuit [25]. There are two basic modes for mechanical ventilation according to variables that are controlled during inspiration.
During this period of COVID-19 pandemic, the lack of medical equipment (like ventilators) leads to complications arising in the medical field. A low-cost ventilator seems to be an alternative substitute to fill the lacking. This paper presents a numerical analysis for predicting the delivered parameters of a low-cost mechanical ventilator. Based on several manufactured mechanical ventilators, two proposed designs are investigated in this study. Fluid-structure interaction (FSI) analysis is used for solving any problems with the first design, and computational fluid dynamic (CFD) analysis with moving boundary is used for solving any issues with the second design. For this purpose, ANSYS Workbench platform is used to solve the set of equations. The results showed that the Ambu-bag-based mechanical ventilator exhibited difficulties in controlling ventilation variables, which certainly will cause serious health problems such as barotrauma. The mechanical ventilator based on piston-cylinder is more satisfactory with regards to delivered parameters to the patient. The ways to obtain pressure control mode (PCM) and volume control mode (VCM) are identified. Finally, the ventilator output is highly affected by inlet flow, length of the cylinder, and piston diameter.
Changing gas flow during neonatal resuscitation: A manikin study
2011, Resuscitation
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With a Neopuff T-piece device, the pressure generated is directly related to the resistance that the flow resistor valve imposes on the gas flow through that valve. This rule only applies when the flow rate is constant.11 When the valve resistance is constant, it can be predicted that the positive end expiratory pressure will rise upon increasing the flow rate.
When using a T-piece device, resuscitators may try to improve airway pressures by increasing gas flow instead of correcting face mask position.
To measure the effects of changing gas flow during positive pressure ventilation (PPV) on peak inspiratory pressure (PIP), positive end expiratory pressure (PEEP), expiratory tidal volume (V_Te) and mask leak.
Using a Neopuff T-piece device, 20 neonatal staff members delivered PPV to a modified, leak-free manikin. Resuscitation parameters were recorded. Study A: PPV for 4 min at PIP 30 cm H₂O and PEEP 5 cm H₂O. Each minute gas flow was increased (5, 8, 10, and 15 L/min). PIP and PEEP settings were unchanged. Study B: same pressure settings; PPV for 1 min with 5, 8, 10, and 15 L/min in a random order, at a rate of ∼60/min. The pressures were adjusted to maintain the same PIP and PEEP after each flow change.
Study A: As gas flow increased (5, 8, 10 and 15 L/min) the median PEEP increased from 4.7 to 26.4 cm H₂O (p < 0.002). Median V_Te decreased from 10.0 to 0.8 mL (p < 0.001). PIP increased slightly from 30 cm H₂O to 36 cm H₂O at 15 L/min (p < 0.005). Mask leak increased from 14% to 98% (p < 0.001) because mask pressure increased. Study B: when PIP and PEEP were maintained there were no significant differences in V_Te (p = 0.42) or mask leak (p = 0.51) with changing gas flow.
During PPV increasing gas flow dramatically increased PEEP and mask leak and in consequence reduced V_Te. Gas flow should rarely be changed during T-piece resuscitation.
Improved flow and pressure capabilities of the Datex-Ohmeda SmartVent anesthesia ventilator
2000, Journal of Clinical Anesthesia
Study Objective: To compare the flow and pressure capabilities of the Datex-Ohmeda SmartVent (Ohmeda 7900, Datex-Ohmeda, Madison, WI) to previous Ohmeda (7810 and 7000, Datex-Ohmeda, Madison, WI) anesthesia ventilators. To determine airway pressure and minute ventilation thresholds for intraoperative use of a critical care ventilator.
Design: Three anesthesia ventilators and one critical care ventilator (Siemens Servo 900C, Siemens, Solna, Sweden) were studied in a lung model. Retrospective medical record review.
Setting: Research Laboratory and Critical Care Unit of a Level I Trauma Center.
Patients: 145 mechanically ventilated patients treated for acute respiratory failure who underwent 200 surgical procedures.
Interventions: The effect of increasing pressure on mean inspiratory flow was determined by cycling each ventilator through increasing restrictors. Maximum minute ventilation was measured at low compliance (10–30 mL/cm H₂O), positive end-expiratory pressure (PEEP) (0–20 cm H₂O), and increased airway resistance (∼19 and ∼36 cm H₂O/L/sec) in a mechanical lung model.
Measurements and Main Results: Flow, volume, and pressure were measured with a pulmonary mechanics monitor (BICORE CP-100, Thermo Respiratory Group, Yorba Linda, CA). Preoperative peak airway pressure and minute ventilation (V̇_E) were extracted from the medical record. Mean inspiratory flow declined with increasing pressure in all anesthesia ventilators. The SmartVent and the 7810 produced greater mean inspiratory flow than did the 7000 ventilator. As compliance progressively decreased, the Siemens, the SmartVent, and the 7810 ventilators maintained V̇_E compared to the 7000 ventilator. The Siemens and the SmartVent maintained V̇_E with PEEP, compared to the 7810 and 7000 ventilators. During increased airway resistance, maximal V̇_E was lower for all ventilators. The SmartVent met the ventilation requirements in 90% of the patients compared to 67% of patients with the 7000 ventilator.
Conclusion: The improved pressure and flow capabilities of the SmartVent increase the threshold for using a critical care ventilator intraoperatively to a peak airway pressure >65 cm H₂O and/or V̇_E >18 L/min.
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Disposable resuscitation valves appear as relatively simple devices used to ventilate patients during both emergent situations and transport from the operating room to the intensive care unit. We report a case of a nonfunctional disposable resuscitation valve that resulted in barotrauma and bilateral pneumothoraces. A routine check for proper function of these valves before use in critically ill patients may help to eliminate such cases.
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Ventilator modes: Old and new
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The "old" conventional positive pressure ventilation remains the mainstay of ventilatory support in patients with acute respiratory failure. Although some of the "new" ventilatory modes might have the potential to replace conventional positive pressure ventilation in the future, its superiority over the "old" ventilatory mode remains to be established. A well-controlled study comparing the "new" ventilatory modes with the conventional positive pressure ventilation is important.

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: Manuscript received September 13; revision accepted January 13.

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Chest

Clinical InvestigationsFlow Resistance of Expiratory Positive-Pressure Valve Systems

Section snippets

Materials and Methods

Results

Discussion

Appendix

Acknowledgment

Br J Anaesth

Design of mechanical ventilators

Technical aspects of positive end expiratory pressure (PEEP): 1. physics of PEEP devices

Respir Care

Positive airway pressure: system design and clinical application

PEEP devices: flow-dependent increases in airway pressure (abstract)

Crit Care Med

Increases in airway pressure with positive end-expiratory pressure valve systems during simulated cough (abstract)

Anesthesiology

Applied respiratory physiology, 2nd ed

Respiratory care

Clinical Investigations
Flow Resistance of Expiratory Positive-Pressure Valve Systems