A mechanical Ventilator is a positive or negative pressure breathing device, that can maintain Ventilation and oxygen delivery for a prolonged period.
Classification of Mechanical Ventilators
They are mainly two types
Negative ventilators and positive ventilators.
Negative pressures are the first generation ventilators.
Now positive pressure ventilators are using commonly.
Important normal values which are used to assess the efficiency of gas exchange
P.a.O2 = 80 to 100 mm of Hg.
P.a. CO2= 35 to 45 mm of Hg.
P.h= 7.35 to 7.45.
S.p.o2 = 95 to 100%.
S.a.O2 = 95 to 100%.
E.t.CO2= 35 to 45 mm of Hg.
Negative Pressure Ventilators
1.Iron Lung or Drinker Respiratory Tank
It is used widely during the polio epidemics.
2.Body wrap or Pneumowrap and chest cuirass or tortoiseshell
It is a portable device with a rigid shell.
Positive Pressure ventilators
Based on the use of the invasive artificial airway to deliver ventilation, it is 2 types.
Non-Invasive and invasive Ventilators.
Non Invasive ventilators.
This is mainly used in home-based set up to assist breathing. Examples are CPAP and BiPAP machines.
Invasive Ventilators.
They are 3 Types
1. Pressure cycled.
2. Volume cycled.
3. Time Cycled.
Here we are discussing Pressure and Volume cycled ventilators because time cycled ventilators are not used widely or used mainly in infants only.
Indications for Positive Pressure Ventilators
1.based on arterial blood gas report and clinical symptoms.
PaO2 <50 mm of Hg with FiO2 > 60%.
PaO2 >50 mm of Hg with p.H < 7.25.
Respiratory rate > 35/min.
2. To decrease the work of breathing.
Leading factors are.
a.Airway obstruction.
b.Reduced Respiratory compliance.
Lung compliance is the elasticity of the lungs to expand, it will reduce due to pathological conditions like A.R.D.S.
c. High CO2 Production.
2. To Reverse life-threatening hypoxia.
Leading factors are.
a. V/Q mismatch: There is a mismatch in ventilation and perfusion ratio, normally the value is one.
b.Shunt.
c. Gas exchange limitation.
3. To Support Acute Ventilatory Failure.
Leading factors are.
a. Respiratory Center failure.
b. Mechanical Disruption.
c. Neuromuscular disorders.
d. Reduced Alveolar Ventilation.
e. Pulmonary vascular disruption.
some conditions also lead to respiratory failure, such conditions are thoracic and abdominal surgery, drug overdose, inhalation injury, C.O.P.D, Multiple trauma, Shock, Multi-organ failure, etc.
The physiological effect of Positive pressure ventilators
1.Decreased Cardiac Output.
2.Increased incidence of barotrauma.
3. Decreased urine output. It is due to decreased cardiac output. So adequate intravenous fluid therapy to generate normal stroke volume is essential.
Basic modes of Positive Pressure ventilator
1.Controlled Mechanical Ventilation or C.M.V.
It may be pressure controlled or volume-controlled ventilation.
In pressure-controlled ventilation, a preset inspiratory pressure used to deliver required tidal volume. It may range from 5 to 35 cm of H2O to get a target Tidal volume of 6 to 8 ml / Kg of Bodyweight. For example, a patient with 70 kg required a tidal volume of 420 to 560 ml.
In Volume controlled ventilation, a preset tidal volume is used, irrespective of pressure, it will deliver a constant tidal volume in each ventilation.
The important point is that there is no patient’s triggering factor required to start ventilation. The ventilator will deliver a set rate of breath per minute irrespective of the patient’s effort of breathing if present.
The ventilator will respond to the patient triggering of breath with either a pre-set tidal volume or a pre-set level of the pressure support level.
The pressure Trigger may set from -1 to -10 cm of H2O. Initially, it may set to generate minimum negative pressure that is -1, gradually it can increase with the prognosis of the patient’s condition.
The volume trigger may set from 1 to 7 liter/minute. The patient should generate a minimum amount of tidal volume to initiate a ventilator breath.
In addition, a pre-set back up the rate of breaths will occur. if the patient does not trigger at the required rate. So it is safe to use when compared to Controlled mandatory ventilation mode.
In this ventilation, the ventilator will assist all patient’s effort of breathing, if the patient is not taking the breath, the ventilator will deliver the breath.
Pressure Support Ventilation or PSV mode
A pre-set level of inspiratory pressure support is delivered when the patient triggers a breath. The tidal volume of each breath depends upon the lung compliance and respiratory rate and can be adjusted to the level of pressure support to maintain normal range.
Pressure Support Ventilation
Synchronized Intermittent Mandatory Ventilation or SIMV
It is the support mode of the volume-cycled ventilator.
The ventilator will deliver a pre-set frequency of breaths but allows spontaneous breaths to be taken in between.
Ventilator breaths are synchronized with these spontaneous breaths.
The main difference from assist control ventilation is, it will not assist all breath initiated by the patient but synchronize with the patient’s breath at a regular flow rate or deliver the breath if the patient’s breath is inadequate in number.
These are basic modes of ventilators, but nowadays the variety of ventilator modes are available according to patient’s respiratory needs. For example, a combination of pressure cycled and volume cycled mode and inverse ratio ventilation etc. The primary reason is to improve oxygenation and remove excess CO2.
How to choose a ventilator mode
●Controlled Mechanical Ventilation or C.M.V is used to provide full ventilator support(when the patient is apnoeic).
●Synchronized Intermittent Mandatory ventilation or SIMV is used when the patient is able to initiate some breaths but still requires ventilator assistance at a constant level; to maintain CO2 removal and oxygenation.
●Pressure Support Ventilation or P.S.V mode is used to support the patient’s own respiratory efforts; allowing increased patient comfort; reduced requirement for sedation; ongoing use of respiratory muscles; and the opportunity to gradually reduce the level of support to facilitate weaning.
Mechanical Ventilator settings
The ventilator settings are used to achieve the required tidal volume. and minute volume along with the normal range of P.a.O2; and PaCO2.
Respiratory rate (breaths/min, f )
●It is usually set at 10 to 15 breaths/min. but may be altered to manipulate the minute volume, P.O2, and P.CO2.
Tidal Volume
It is set at 6 to 8ml / kg of body weight.
Altered if there is difficulty in optimizing PO2 and PCO2.
Minute Volume
Target range from 2.5 to 12 Liters/min.
It is the product of tidal volume and Respiratory rate.
Pressure support (cm of H2O)
It ranges from 5 to 35 cm of H2O.
It is adjusted according to the target tidal volume required.
Inspiratory: Expiratory Ratio
The normal is 1:2, but may vary from 2:1 to 1:4 in order to increase the time for inspiration in severe airflow limitation.
Trigger or Sensitivity
Volume-based trigger ranging from 1 to 7 Liters/minute.
Pressure based trigger ranging from -1 to -10 cm of H2O.
Initial weaning stage; a minimum trigger needed to set to initiate the ventilator-assisted breath.
Positive end-expiratory pressure or PEEP ( cm of H2O)
Usually set between 5 to 10 cm of H2O.
The aim of PEEP is to reduce the alveolar collapse during each exhalation and to increase the area of gas exchange with minimum FiO2.
This is the example of settings in pressure support ventilation.
Here FiO2 is 50 %, peep is 7 cmH2O. And pressure support is 20 cmH2O. The right side of the monitor shows the patient is getting the minute volume of 3.5 liters per minute and tidal volume is 288 ml.
How to connect the mechanical ventilator to a patient
Following things to be ready before connecting with the ventilator.
1.Power supply/battery backup.
2. Ventilator.
3. O2 and air supply.
4. Humidifier.
Humidifier
5. Ventilator Circuit.
Ventilator Circuit
6. Catheter Mount.
Catheter Mount
7. E.T. Tube or Tracheostomy tube.
8. AMBU bag- for giving manual breath, if needed.
Before connecting to the ventilator, a self-test run is mandatory to assure the working status of the ventilator.
Also, select the required mode and settings for the patient’s condition.
First make sure uninterrupted power supply, oxygen, and air supply to the ventilator.
Connect the ventilator circuit with the ventilator, humidifier to be attached to the inspiratory limb or tube.
Catheter mount to be connected to the Y end tip of the ventilator circuit. Then it connects to the endotracheal or tracheostomy tube. Ensure that the cuff of the endotracheal tube is inflated.
The diagram showing the basic connection of a ventilator to the patient.
There may be slight changes in some kind of ventilator
Mechanical Ventilator Troubleshooting
1). High Airway pressure.
Causes.
E.T. Tube obstruction.
Pneumothorax.
Severe Bronchospasm.
A buildup of secretion.
Patient coughing.
Increased peak airway pressure resulting from increased tidal volume or inspiratory time is too short.
Displacement of the tube. Either upward or downward.
Nursing Interventions
If severely compromised; remove from ventilators and manually ventilate.
Perform suction to clear secretion.
If the cause is complete obstruction of E.T, then re-intubate.
Auscultation of lungs for wheezing; reduction in air entry and altered breath sound.
If there is a pneumothorax; immediate insertion of a chest drain or it will cause cardiovascular compromise.
Check blood gas to assess ventilation status.
If necessary administer sedatives as per order.
Review ventilator settings.
2. Low airway pressure
Manifested by sounds of leakage, decreased minute volume, low airway pressure.
Causes
Disconnection.
The major leak from the ventilator.
Burst cuff from E.T. Tube.
A leak from the circuit.
Broncho-Pleural fistula with a massive air leak from the chest drain.
Nursing Interventions
Check the patient’s attachment to the ventilator.
Check tubing’s connections for leakage.
Check the cuff pressure.
Check inspiratory tidal volume; to assess ventilator delivering adequate volume.
Check levels set for the alarm.
If still persists manually ventilate.
3.Low minute volume
Manifested by.
Low Minute volume alarm.
Audible cuff leak.
Desaturation alarm.
Causes
Disconnection from the ventilator.
Ventilator tube leakage.
Broncho-pleural fistula with a chest drain in situ.
Nursing Intervention
Unless the cause of low minute volume is immediately traced out; manually ventilate the patient.
Check ventilator tubings for leakage.
Review ventilator settings.
Check cuff leakage, auscultate the trachea if necessary.
Monitor air leakage through the chest drain.
4.High minute volume
Manifested by.
High M.V alarm.
Causes.
Patient making the respiratory effort.
Possible ventilator malfunction.
Nursingintervention
Check the cause of tachypnoea such as possible hypoxia or hypercapnia.
Review Ventilator settings.
So that is all about the basics of mechanical ventilation.
The important point is “ Follow doctor’s order always”.
Citation: Sheila K.Adam, & Sue Osborne (2009)”Oxford Handbook of Critical Care Nursing”, Respiratory Support: Mechanical Ventilation, Chapter -10, Page No.161 to 195.
