Ventilator a Biomedical Point Of View

Photo by Robina Weermeijer

Ventilators are crucial machines that are one of the most important medical devices in a hospital not because of its complexity. But because it is one of the machines that are considered life-saving for patients who have trouble breathing normally.


In this article, we are not going to mention how the ventilator's preventive maintenance is done as we already covered that in a different article. But we will be shedding light on the various modes of operation in addition to explaining the various parts of a ventilator in a simplified way.


Ventilators or respirators are devices used to assist patients in ventilating their lungs and they are divided into two categories:

•         Controller ventilator
•         Assisted ventilator

For controlled ventilation, the respiratory ventilation of the patient is determined by the machine, and it sets the ventilator cycle. If the patient tried to ventilate, this will affect the machine, and it can oppose it too.

As for the "assister" mode ventilator, the respiratory ventilation is determined or controlled by the patient himself. So it assists rather than controls the patient in ventilation.

Another classification of ventilation is:

-        Negative pressure ventilation.

-        Positive pressure ventilation.

For negative pressure ventilators, which are more physiological than positive ones. A pressure, varying between atmospheric pressure and negative pressure is applied to the thorax of the patient to force him to ventilate.


For positive pressure ventilators, pressure, varying between atmospheric pressure and positive pressure, is applied and the air is blown into the lungs. Thus an exchange of gases is done in the patient's lungs.

Machine Components

•     Breathing circuit (inspiration – expiration)
•     A controlling system that defines the characteristics of the cycle used 
•     A gas supply 
•     Monitor
•     Valves, pressure sensors, and alarm systems. 

In this article, we are going to give an example of the Puritan-Bennett ventilator that is 760, Puritan Bennett is now part of Medtronic and is a specialized line for producing ventilators

Mode/breath type availability on 760 Ventilators:

•     VCV (volume control ventilation) breathe type.
•     PCV (pressure control ventilation) breathe type.
•     PSV ( pressure support ventilation) breath type (support pressure setting)
•     PSV (rise time factor and exhalation sensitivity setting)
•     SIMV (Synchronous Intermittent Mandatory Ventilation) mode
•     Apnea ventilation (choice of VCV or PCV breath type)

Functional description

By pressing keys and turning the knob on the ventilator keyboard, the operator gives initial instructions and data to the ventilator. The user interface micro-controller processes this information and stores it in the ventilator’s memory. The breath delivery micro-controller uses this stored information to control and monitor the flow of gas to and from the patient.

The 700 series ventilator uses a flow trigger to recognize patient effort. The trigger monitors flow from the piston during exhalation. When the patient inhales, patient circuit pressure drops very slightly below end-expiratory pressure. At the same time, the piston moves forward to deliver flow to the ventilator breathing circuit and maintain the preset PEEP/CPAP level. The level of flow depends on the patient’s effort. If this flow exceeds the user-set level, the ventilator triggers. By design, the ventilator attempts to maintain PEEP even with the presence of a circuit leak. Since a leak drives the piston to deliver more flow to make up for pressure losses. A circuit leak can require an increase in the flow trigger level to avoid auto cycling.

Puritan-Bennett Ventilator 760 Series 

During exhalation, the ventilator’s piston retracts and draws air and oxygen into the cylinder. The ventilator uses room air, which means the ventilator can operate without a compressor or wall air source. Room air enters the ventilator through a protected user-replaceable air intake filter just inside the ventilator cabinet. This filter captures airborne particles.


The oxygen from a cylinder or wall outlet enters the ventilator through a hose and oxygen fitting. The fitting is available in several versions. Once inside the ventilator, the oxygen is regulated to a pressure the ventilator can use and then mixed with air, according to the selected %O2.


The flow-triggered piston/cylinder system and motor controller circuit control the flow of gas to the patient. On the 760 Ventilator in PCV or PSV, the rate of flow is also determined by the preset rise time factor. This system is designed with a minute gap (about the size of a thin sheet of paper) between the piston and the cylinder wall. This design eliminates friction between the piston and cylinder, allowing it to respond more rapidly than a “sealed” system.


The piston delivers the mixed air and oxygen through the inspiratory manifold system, and out to the patient. The oxygen concentration and temperature of the delivered gas are monitored here. This is done using a galvanic oxygen sensor and a thermistor. The galvanic sensor generates a voltage proportional to the partial pressure of oxygen. From which the oxygen concentration is more than ten percentage points above or below the %O2 setting.


The inspiratory manifold system also includes a safety valve to relieve patient pressure if necessary. For example, if the ventilator breathing circuit is kinked or occluded.


The patient includes the components external to the ventilator that route gas between the ventilator and the patient. These components include the inspiratory filter. Which protects against contamination between the ventilator and patient. A humidification device, ventilator breathing circuit, the tubing through which the gas travels, collector vial (which protects the exhalation system from moisture in the exhaled gas, and can be emptied without losing circuit PEEP), and an expiratory filter (which limits the bacteria in the patients' exhaled gas from escaping to room air or contaminating the ventilator).

Keyboard or Keypad

The 740 keyboard is divided into three sections

-  VENTILATOR SETTING: Where you set breath delivery variables.

-  PATIENT DATA: Where you set alarm limits and view the monitored pressures, breath timing, and volumes.

- VENTILATOR STATUS: Where you see the alarm status and operating condition of the ventilator.

User Interface Block Diagram (740 Keyboard)

Ventilator Setup 

Ventilator Settings

The ventilator setting section of the keyboard allows you to select the ventilation mode, breath type, and settings. To change the mode and setting, select the mode, then the breath type, and then the ventilator setting. The keys flash during setup and mode changes to ensure that you review all pertinent settings. The keyboard is designed to minimize accidental or unintentional changes.

The table summarizes the functions of the keys, knobs, and indicators in the ventilator setting section of the keyboard. Ventilator settings are also limited by these breaths delivery boundaries:

-        I: E ratio ≤ 4:1 for PCV, ≤ 3:1 for all other breath types

-        Inspiratory time = 0.2 to 8 seconds (excluding plateau)

     -         Expiratory time ≥0.2 seconds

     -         PEEP/CPAP + SUPPORT PRSSURE or INSPIRTORY PRESSURE ≤ 80 cmH2O  (80 hpa)

Pneumatic schematic

Pneumatic Schematic

The Pneumatic Schematic diagram shows the pneumatic circuit and how the ventilator controls the flow of air to be supplied to the patient.

Composition

1- 4 lamps that have the following functions:

- AC power supply indicator lamp.

- Internal battery indicator lamp.

- Alarm indicator lamp.

- Synchronous triggering indicator lamp.

2- LCD and control

3-pressure meter: it is dedicated to measuring the pressure of air that is coming out of the machine.

4-Air way: it connects to an inlet port of humidifier with a tube of 22 mm during inhalation phase, compressed gas in ventilator will be supplied to the breathing circuit through this port.

5-Oxygen supply: we can limit the flow as we want using oxygen supply.

6- Humidifier: can heat air and add humidity to the air.

7-Expiration valve: Takes CO₂ from the patient to outside.

8-Temperature sensor: Measures air temperature 

9-Blue line or airway expiration valve: it connects to pressure sensor to measure out pressure for the patient.

At the backside of the machine we have:

- 2 cables for the ventilator and humidifier.

- Timers.

- 2 fusses for protection from AC and one for DC.

Simplified Ventilator Block Diagram

Ventilator Simplified Block Diagram

Ventilator Modes and How they Work? 

There are 3 main modes that the ventilator can operate on 

1. A/C Mode
2. SPONT Mode 
3. SIMV Mode 

A/C Mode 

During this mode, the ventilator will be delivering mandatory breaths ONLY! Using VCV or PCV (Volume Controlled Ventilation or Pressure Control Ventilation).


During VCV mode the ventilator will deliver breaths that are based on the set Tidal Volume and Peak Flow Settings


During PCV mode the ventilator will deliver breaths that are based on the Inspiratory Pressure. I: E ratio (inspiratory: Expiratory Ratio) or inspiratory time and Rise Time Factor


In A/C Mode, the Respiratory Rate setting determines when the VIM (Ventilator-Initiated Mandatory) breath is delivered. However, Patient-Initiated Mandatory PIM breath or Operator-Initiated Mandatory OIM breath can also be performed in this mode. OIM and PIM will increase the respiratory rate and minute volume.


Patient- Initiated Mandatory PIM breath is delivered when the ventilator detects an effort made by the patient, Operator-Initiated Mandatory OIM breath is delivered when the operator usually the nurse  presses Manual Insp button, 

SPONT Mode

The ventilator delivers spontaneous breaths during SPONT mode. This is done by the Pressure Support ventilation PSV.


During SPONT mode the ventilator helps the patient inspiratory effort to sustain the set level of support pressure. The patient triggers all spontaneous breaths and influences the inspiratory flow, tidal volume and inspiratory time.


The Rise Time Factor setting determines how fast the target pressure is achieved. Exh Sensitivity setting will determine the point where the breath cycles to exhalation


During SPONT Mode Operator-Initiated Mandatory breath that can be triggered by pressing on the Manual Insp button.

SIMV Mode 

A combination of mandatory and spontaneous breaths can be delivered during SIMV mode. Mandatory breaths are triggered by the ventilator, patient or operator while spontaneous breaths are triggered by the patient. 

Ventilator Alarm Testing 

Testing the ventilator alarms is an important step to make prior to attaching the ventilator to a patient

Types of Alarms that can be tested

• Low Insp Pressure 
• Low Exhalation Tidal Volume 
• Disconnect 
• High Pressure 
• Continuous Hi-Pressure 
• Loss AC Power 
• APNEA 
• % O2 low 

Settings to Apply before starting 

Mode                                  A/C 

Respiratory Rate                5/min 

Tidal Volume                     500 ml 

Peak Flow                          60L/min

Plateau                               0 s 

High Rate                          100/min 

Low Inpiration Pressure    3 cmH2O (3 hPa)

High Pressure                    50 cm cmH2O (50 hPa)

High Tidal Volume            1500 ml 

Low Tidal Volume             5 ml

low Minute volume           0 L

PEEP/CAP                         5 cm cmH2O (5 hPa)

Trigger Sensitivity             3L/min 

Connect oxygen source and set the ventilator to operation, also attach an adult or pediatric breathing circuit then attach a test lung* to the wye.

* If you don't have a test lung what can you do? for suggestion please post in the comments below:  

Testing Low Inspiration Pressure, Low Ex Tidal Volume, and Disconnect Alarms

First, you need to allow the ventilator to perform at least 10 breaths, during the inspiratory phase of a breath disconnect the test lung. 

You should get the Low Insp Pressure alarm during the first breath after you disconnect the test lung. The Disconnect and Lo Ex Tidal Volume Alarms are triggered after the fourth breath. 

Testing the High-Pressure alarm 

Change the High-Pressure alarm setting to 11 cm cmH2O (11 hPa) then press the reset key to reset the Disconnect alarm. After that block the wye, the ventilator should terminate inspiration without alarming on first breath, then the ventilator announces a High-Pressure alarm during the second alarm. 

Testing the Continuous Hi-Pressure alarm  

Reset the high-pressure alarm setting to 20 cm cmH2O (20 hPa) then block the exhaust port and press the reset alarm key to reset all alarms. 

Now press the Manual Insp, one breath after the ventilator must announce a continuous Hi Pres Alarm. In case the alarm was not triggered make sure that the breathing circuit is mot leaking)

Now return everything to the first settings by unblocking the Exhaust port, unblock the wye and reconnect the test lung. Also, change the alarm setting 50 cm cmH2O (50 hPa). Finally, press the reset alarm key to rest all alarms. 

Testing AC Power Alarm Test 

Make sure that the battery is fully charged before starting this test. 

Allow the ventilator to deliver 3 or more breaths, now disconnect the power cord from AC power. Then the ventilator will alarm Loss AC Power alarm, once you reconnect the power cord the alarm should stop. 

Testing Apnea Alarm 

Press the Menue Key then turn the knob to display user settings, now press accept. Turn the knob to display Apnea Interval (Ta) then press accept. Now turn the knob to select the Apnea interval of 20 seconds. Then select the following settings: 

Mode                        SPONT 
Apnea Alarms: 

Respiratory rate       5/min 
Tidal Volume           400L
Peak Flow               10L/min 
Support Pressure     0 cm cmH2O (0 hPa)

The ventilator should alarm you of apnea after 20 seconds after. To simulate 2 patient-initiated breaths squeeze the test lung twice, once this happens the apnea alarm should reset. 

Testing %O2 alarm 

Mode                                  A/C 

Respiratory Rate                20/min 

Tidal Volume                     1000 ml 

Peak Flow                          60L/min

Plateau                               0 s 

High Pressure                    90 cm cmH2O (90 hPa)

%O2                                   100%

Allow for around 30 seconds for the oxygen settings to stabilize, now disconnect the oxygen supply. The ventilator should announce a LOW O2 Supply alarm within 2 breaths. And a % O2 Low alarm should start after around 30 seconds. Once you reconnect the Oxygen supply the alarms should stop.


I wish that you liked this article! There are many other things that we can explain about ventilators, please comment below of suggestions of what topics you want us to write about?

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