A ventilator is a machine that helps you breathe or so to say, it breathes for you. It does this, through a breathing circuit (a combination of inhalation and exhalation tubing). One end of the inhalation tube is connected to the device (ventilator) and the other end to the patient’s airways, down up to the lungs, inserted through a process called intubation. From the lungs, the circuit draws exhaled CO2 and other waste gases and brings it back to the ventilator vide exhalation tube. It is then filtered (HEPA) and let out. The latest international regulation guideline requires this exhaust from the ventilator to be further sucked into a central pipeline, again filtered and let out.   

On inspiration, we have gas coming from the ventilator at a positive pressure. This is drawn into a HME (Heat and Moisture Exchanger). Here it humidifies and warms the air, prior to entering the patient’s lungs. This step is required because we are bypassing the patient’s upper airways, where normally these functions are carried out.

How are we bypassing? We normally use an Endo-tracheal tube (ET tube) to reach the patient’s lungs, in invasive ventilation. Invasive ventilation is tried only if, noninvasive methods don’t work. If it is for long-term then another method called tracheostomy is done. We can also nebulize medications and give those into the circuits, just before air reaches the lungs.

One important thing beginners should take note of is; ventilator in itself is not a treatment. By using a ventilator we are just assisting a patient in breathing, till the underlying cause of the actual disease/ trauma is being treated. The underlying disease or trauma may be affecting lung compliance. The word compliance here means the ease or ability of the lungs to expand and contract.

Some basic physiologic impairments leading to mechanical ventilation:

1.  Apnea – cessation of breathing

2. Hemodynamic instability or cardiovascular collapse – abnormal heart rate owing to abnormal blood pressure

3.  Acute ventilatory failure – inability to oxygenate arterial blood and eliminate CO2.

4.  Impending ventilatory failure – patients with continuously declining Arterial Blood Gas (ABG) and increasing Work of Breathing (WOB)

5.  Refractory hypoxemic respiratory failure – inadequate arterial oxygenation despite the optimal level of inspired oxygen

6.  Respiratory muscular fatigue – instability of a muscle to continue to generate or maintain the required force.

7.  Obstruction (could be due to pulmonary edema, trauma, etc)

Above impairments can alter the acid-base balance (pH) of our body.

Now don’t get scared by the above high-sounding scary medical terms. That is just for your information. The understanding of a patient’s ventilation requirement is judged by an Intensivist or a Pulmonologist. The settings of the ventilator is based on every patient’s individual needs, and are done by an Intensivist or a Respiratory Therapist. 

Ninety–nine percent of the invasive ventilation is now Positive pressure ventilation. For academic knowledge, one should know there is a thing called negative pressure invasive ventilation as well.

Therapeutic benefits of positive pressure ventilation are:

1.  Guaranteed delivery of high Oxygen levels

2.  By providing positive pressure, we can reduce intrapulmonary shunt. This helps to keep alveoli open, esp. in pulmonary edema

3.  We can provide the Work of Breathing (WOB), till the patient is able to breathe on his own.

Within this gamut of positive pressure ventilation, there are number of basic and advanced intelligent modes in which a critical care ventilator can be operated.

Broadly these are classified under Volume Control mode or Pressure Control mode.

As defined earlier lung compliance is the ability of the lungs to expand and contract. Normal adult lung compliance ranges from 0.1 to 0.4 L/cm H2O. Under static condition (no resistance) it is approximately 0.2 L/cm H2O.

Therefore formula for Compliance can be ratio of change in volume to change in pressure.

Compliance (C) = Change in Volume/Change in pressure

If we can control one of these variables, volume or pressure, then the other one is determined by what the patient’s compliance is. In Volume Control mode, we are going to set the volume of air, which is to be delivered, and this is going to be constant and therefore depending upon patient’s lung compliance, the pressure that is required to reach that volume is adjusted, as pressure is the variable here. Vice versa happens for Pressure Control.

In Volume Control, we need to ensure the patient receives a set volume of breath each minute. This can be accomplished by various modes.

Before we enter into this, we need to know certain terms used here:

1.  Tidal Volume (Vt) – Volume of air that is set to be delivered with each breath. It is also defined as amount of air that moves in and out of the lungs with each respiratory cycle. This is approx. 500 ml in an average healthy adult.

2.  Frequency/ Respiratory (f/ RR) rate: Number of breaths per minute, that are being delivered to the patient

Set Respiratory rate/ 60 = amount of time between each individual breath

3.  Spontaneous: This means natural breathing

4.  Oxygen Concentration (FiO2): means a fraction of inspired oxygen.

We generally say the patient is on 50%/ 80% or say 100% FiO2 expressed in decimals or fractions. For e.g. 50% FiO2 means setting is on 0.5, 100% means FiO2 of 1.00. The normal range can be anywhere between 0.35 to 1.00

5.  Positive End Expiratory Pressure (PEEP): This is the pressure that exists in the lungs at the end of expiration. Similar to CPAP, BiPAP, EPAP. This is a constant pressure that is being applied throughout expiration to really help open and keeps open alveoli. The range of setting for PEEP is 5 – 20.

6.  Minute Ventilation (VE): It is the volume of gas inhaled or exhaled from a person’s lungs per minute.

7.  CPAP – Continuous positive airway pressure

Now it’s adjusting PEEP along with FiO2 which is how we essentially maintain a patient’s oxygenation.

Various basic modes of ventilator:

VOLUME CONTROL MODE

1.  Assist Control (AC/ VC): Here a pre-determined volume is set for each breath (Vt). Breath/min (f) is also set. We also set FiO2 & PEEP. We are not going to use any pressure support here.

AC mode actually takes into account patients trying to take their own breath. If a patient tries to take their own breath (spontaneous breath), the vent is going to recognize this and what it does is, it’s going to deliver the full set of tidal volume (Vt) with time. It’s like, I know you want a breath, take the full breath. There is no pressure support here.

Problem with this is, it can lead to excessive ventilation in some cases. This can blow off too much CO2 and ultimately lead a patient into a respiratory alkalosis.

Because of the above problem another mode was developed known as SIMV (Synchronous Intermittent Mandatory Ventilation).  

2.  Synchronous Intermittent Mandatory Ventilation (SIMV): Here again we set Vt, f, FiO2 & PEEP. In addition to this we take Pressure support as well.

In SIMV instead of delivering the full set volume of air, the patient is allowed to take whatever size breath they can. The breath that is going to be delivered from vent is actually going to be synchronous with patient’s spontaneous breath, essentially to increase their lung compliance.

In other words, when patient triggers the vent in SIMV, we can add Pressure Support to that breath, to make it easier for the patient to achieve a good tidal volume.

If patient is not taking any spontaneous breath SIMV = AC

SIMV is good because they can gradually allow patients to take back the work of breathing over time. Less support we give on the mandatory breath. It was originally designed for weaning a patient out of ventilator. But off late this has become one of the primary modes of ventilation being used. This needs close monitoring. 

PRESSURE SUPPORT MODE

Here we set FiO2, PEEP and Pressure Support. We also set Inspiratory pressure and Inspiratory time here. It isn’t a volume control mode. So no Vt and f settings here.

In pressure support, minute ventilation and thus our patient’s work of breathing (WOB) will be almost dependent on the patient’s effort of breathing, with the assistance of whatever we set pressure support as. This is used as the last step before extubating a patient out of the ventilator.

The above mentioned are some of the basic modes.

There are advanced intelligent modes like the following below:

PRVC (Pressure Regulated Volume Control): This is a combination of both volume and pressure control. Also known as dual mode ventilation.

APRV (Airway Pressure Release Ventilation): Its form of CPAP with really high pressure.

HFOV (High-Frequency Oscillatory Ventilation): Here, high mean airway pressure is maintained and very high frequency is set. This is mainly for neonatal application. Very small Vt but at 240 – 420 breaths per minute.

Anyhow, advanced modes are beyond the scope of this article.

Parameters that we monitor in a ventilator:

1.  Minute Ventilation (Ve): Amount of air delivered per minute

Ve = Vt x f

Tidal Volume (Vt) is based on body weight, 6-8 ml/kg IBW

IBW Male = 50kg + 2.3 x (ht. in inches – 60)

IBW Female = 45.5kg + 2.3x (ht. in inches – 60)

More the Ve, we can have more CO2 clearance and vice versa.

Goal for patient’s minute ventilation is 5 – 10 liters/ min

2.  Peak Inspiratory Pressure (PIP): It is the maximum pressure that’s reached during an inspiration. Ideally should be < 35 (cmH2O) to prevent lung injury. In cases of ARDS it may go high.

3.  Plateau Pressure (Pplat): This is the most important indication of our lung compliance. Higher Pplat indicates a problem with lung compliance. It should be < 30 (cm H2O) to help prevent barotrauma. We measure this at the end of inspiration with an Inspiratory hold or pause maneuver.

Complications associated with mechanical ventilation:

1.  Hemodynamic Compromise: Since there is a change in intra-thoracic pressure compared to spontaneous breathing affecting cardiovascular response.

2.  Barotrauma – Alveoli rupture due to excessive pressure or over-distension of alveoli’s. Ultimately leading to cardiovascular collapse. Avoid setting large Vt, cautiously use PEEP and avoid high airway pressure

3.  Volutrauma – Another form of lung injury owing to overstretched alveoli’s. It causes fractures and flooding. Avoid higher pressure and set smaller Vt.

4.  Ventilator Associated Pneumonia (VAP). This can be prevented to an extent by implementing following methods:

i)         Head of the bed should be adjusted at 30 – 45 degree tilt (helps in aspiration.

ii)        Subgllotic suctioning

iii)       Minimize circuit disconnect

5.  Ventilator Malfunction: Mostly due to inappropriate use

Conclusion:

A ventilator is a machine used in an ICU on extremely critically ill patients. It’s highly sensitive equipment and one of the toughest machines to handle. People working in an ICU are highly stressed. In spite of that, they make every effort to continuously speak to these patients, even though they may not get a response. It is something that holds life & hope.

One has to see a patient on a ventilator to understand the value of air we breathe so easily, day in & day out.