Sechrist 500A Hyperbaric Ventilator

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The Sechrist Model 500A Hyperbaric Ventilator: Presented merely as an example of a hyperbaric ventilator

Operation

The 500A is a pneumatically powered, fluidically controlled, time-cycled ventilator designed to automatically compensate for pressure changes occurring within the hyperbaric chamber. Initiation and termination of inspiration are determined by the ventilator settings for Inhalation Time and Exhalation Time. A one-way valve (emergency intake valve) on the patient breathing circuit allows the patient to spontaneously inspire additional gas from the chamber should the patient's inspiratory flow rate exceed the flow rate delivered by the ventilator.

The 500A requires a 70 psig source gas pressure rather than the standard 50 psig wall outlet pressure. The oxygen regulator on an H tank is set to 70 psig and used to backfeed a wall outlet in the chamber room. The H tank is secured in another room away from the chamber. (The chamber requires a 50 psig wall outlet pressure).

A manifold system that connects to the gas inlet is available to provide a switch over to air. Both the oxygen and air tanks should have a minimum of 1500 psig at the start of therapy.

The ventilator system has two parts: 1) the ventilator control module which remains outside the chamber, and 2) the patient breathing circuit located within the hyperbaric chamber.

500A Control Module

The control module allows the operator to set inspiratory time, exhalation time, and flow rate. Inspiratory time and exhalation time determines the respiratory rate and I:E ratio. The flow rate control and inspiratory time determines the tidal volume.

The Master ON/OFF valve switches the control module on or off. In the OFF position, the patient can breathe spontaneously through the emergency intake valve in the circuit.

The Flow (vol.) control adjusts the drive pressure, and therefore the flow to the breathing circuit. The drive pressure automatically compensates for pressure changes inside the chamber.

The Inhalation Time control adjusts the time allowed for inspiration from 1.0 - 3.5 seconds.

The Exhalation Time control adjusts the exhalation time from 1.0 to 5.0 seconds during which time there is no flow delivered to the patient circuit.

The pressure gauge indicates drive pressure referenced to Chamber pressure in psi.

Above both the inhalation and exhalation time controls are Phase Indicators that provide visual indicators of the inspiratory and expiratory time periods.

The Emergency Manual Button allows manual delivery of gas flow to the patient by controlling inspiratory pressure. The pressure will increase according to how much the button is depressed. Inspiratory time continues for as long as the button is depressed. The Emergency Manual Button overrides all ventilator controls.

Patient Circuit

The two large hoses of the patient circuit attach to the Manifold Block which hooks onto two mounting buttons located on the Intercom Speaker inside the Chamber door.

A four-foot high pressure hose connects the ventilator Control Module to the bottom Pass-Thru fitting in the Chamber door.

On the inside of the Chamber door a small supply tube threaded to the Pass-Thru fitting tees off to supply the venturi, 500cc nebulizer, and exhalation valve.

Exhalation Valve of the Manifold Block

A Safety Pressure Relief Valve and Emergency Valve attach to the top of the manifold block. A pressure gauge on the front of the block measures circuit pressure.
All tubing connections must be tight. A HME is connected to the patient wye and the nebulizer is not filled. A closed suction system may also be attached to the HME.

Operation Check

Turn the Control Module ON.

Check that the Exhalation Time Control can adjust the exhalation period. Set it for 4 seconds.

Check that the Inhalation Time Control can be adjusted. Set it for 1 second.

Check that the Flow Control is adjustable from 0 to 20 psi. Set it for 7 psi.

Check the Safety Pressure Relief Valve by occluding the patient wye during inhalation and observing the pressure reached on the patient circuit pressure gauge (manifold block). The Relief Pressure should be set slightly higher than the maximum pressure needed to ventilate the patient. Adjust if necessary.

Patient Preparation

Provide ventilation as patient is transported via HBO cart. Move patient about half-way into Chamber.

Suction endotracheal/tracheostomy tube. Deliver manual ventilations as cuff is partially deflated to check for secretions above the cuff. Suction orally. Have a second syringe filled with 10 cc of saline ready. Completely remove air from the cuff and immediately refill the cuff with saline.

Thorough suctioning of the ETT/TT should be completed prior to HBO therapy since the patient will be inaccessible after the treatment begins.

Manual ventilations with the cuff slightly deflated move secretions that may be above the cuff up into the oropharynx for suctioning and helps prevent aspiration of those secretions when the cuff is deflated.

Connect the patient to the circuit and begin ventilation

Typically set IT and ET controls in the 12 o'clock position and the Flow Gauge between 6-8 psi.
Adjust the controls to obtain the desired tidal volume, respiratory rate, and I:E ratio.
Observe the patient's chest excursions and monitor clinical signs.
Measure tidal volume with a Wright respirometer connected to the Exhalation Valve outlet.
When ventilation is acceptable and stable, move the patient all the way into the chamber.
When closing the door, make sure the circuit is positioned appropriately to prevent kinking or excessive torque on the patient's airway.
Continuously monitor the patient's chest excursions and clinical signs throughout the entire procedure. Adjustment of the ventilator controls should not be necessary unless the patient's lung compliance or airway resistance changes significantly.
If the patient no longer requires ventilatory support during the course of the HBO treatment, the Control Module may be turned OFF and the patient may breathe in through the Emergency Intake Valve.
After use, disassemble and disinfect the circuit.