Mechanical Ventilators

Mechanical Ventilators

PKGhatak,MD

 

Mechanical Ventilators:

Every person today has heard "Ventilators”. The ventilator has become the most sought after breathing machine by hospitals around the world inundated with COVID-19 patients requiring assistance in breathing just to keep them alive.

Today's Intensive Care Unit of hospitals is a reminder of 1930s hospitals filled with Polio patients on Iron Lung – a rigid box supporting breathing by applying Negative Pressure on the torso, only head and foot patients were sticking out of the box.

A leap forward:

In the early 1940s Jet ventilation was introduced and Bennett and Bird. A mechanical device produced "Intermittent Positive pressure Breathing". (IPPB) 

The introduction of ICU (intensive care units) in hospitals virtually eliminated Iron Lung machines and a new era of Non-Invasive IPPB came into medical practice. In 1954 Salk's polio vaccine was given to school children and eventually wiped out polio and the need for ventilators fell way down.

Then came the Pressure control to Volume control ventilators and Non-invasive to Invasive ventilation. 


The chest wall moves outwards during inspiration. The lungs follow the chest wall outward. This causes the pressure inside of the lungs to fall below the atmospheric pressure and the air rushes into the lungs from outside. The recoil of the elastic tissues lungs and chest wall brings the lungs back to their initial state in expiration. In mechanical ventilation the reverse pressure difference takes place. The chest wall and lungs remain in a passive state. Air is forced into the lungs by a positive pressure (more than the atmospheric pressure) that makes the lungs and chest wall move outwards. That is not something the delicate lung structures are able to withstand without damage for just a few hours. Damage to the lung by mechanical ventilation is called barotrauma.

The progressive forward march of innovations in mechanical ventilators continued. From an Open Circuit to Closed Circuit to Double Circuits ventilators came next. Manual control of airway pressure, breath volume (tidal volume), adjustments of the duration of inspiration and expiration ratio, and Positive End Exploratory Pressure (PEEP) controls were induced by Puritan Bennett in their MA 1 ventilators.

Patient trigger ventilators came next. Intermittent Mandatory Ventilation (IMV), then Synchronized IMV (SIMV) were possible in new ventilators.

3^rd Generation of ICU ventilators:

Ventilators with Microprocessors were available. All aspects of ventilation came in Puritan Bennett 7200 ventilators with a display of pressure loop which eased operating complexities.

4^th Generation of Ventilators:

Much smaller size models were easier to operate during the transport of patients on ventilators, home use, and use in stable patients.

Non-Invasive ventilation came back in popularity.

Mechanical ventilation in ICU and emergency room (ER) meant the placement of an Endotracheal tube in patients. It is a traumatic experience for patients.

The endotracheal tube has to be secured in place by tapes to the mouth or nose. Even then, the tube has a tendency to slip down into the right bronchus, particularly when patients needed to be turned on their sides or out of bed for any reason.

The tube can be left in the trachea for only 5 days without causing local damage to soft tissues. A tracheotomy (an opening in the trachea) is needed to keep the endotracheal tube for a longer time.

All of these can be avoided if Non-Invasive-Ventilation provides as good an outcome obtained by Invasive ventilation. Recent developments in basic science made non-invasive ventilation gain its rightful place.

Common problems with ventilators:

It is a complex machine. It has several dials like the tidal volume, minute volume, pressures (in, out, peep, end-exp), oxygen concentration, carbon dioxide in expired breath, humidity, temperature, etc. There are as many alarms and lights as adjustment dials.

One should not expect a nurse or a doctor, who has not previously operated on a complex and very sensitive machine, to handle it without adequate training. 

A more advanced ventilator " Adaptive Support Ventilator was introduced" to ease operation. When the patient's height, weight and desired tidal volume, maximum airway pressure, etc. are entered, then the machine automatically delivers the correct volume. 

But new does not always mean better. More automation implies more complexities and may not translate to better performance.

What are the adverse effects of mechanical ventilation:

1. Accidental disconnection of tubes from the machine may produce catastrophic events.

2. There are so many ventilators going on at the same time in the ICU, that there is hardly any moment without an alarm bell ringing. Nurses get immune to warnings and mistakes happen.

3, Under ventilation:

Under ventilation and loss of volume (atelectasis) of the left lung from the endotracheal tube sliding down into the right bronchus.

4. Rupture of lung and pneumothorax.

When one lung has near normal elasticity the other is not (due to the more involvement of one lung over the other), the more compliant lung gets more volume due to lower resistance, causing rupture of the lung from excess volume or pressure.

5. Injuries to delicate structures of the lung. 

When the lungs become very stiff, from accumulated products of inflammation a higher pressure is required to ventilate, producing damage to the alveoli.

6. Aspiration Pneumonia:

An inflatable balloon is positioned below the vocal cord in order to retain the tube in the correct place. But accumulated secretion trickles down the tube into the lungs and causes aspiration pneumonia.

7. Infection:

It is an unfortunately common problem in ICU. It is known as pneumonia associated with ventilation.

8. Damage to the vocal cord:

This should not happen but does happen.

Today's ventilators are far cry from earlier days, these machines now can perform amazing maneuvers - not only can breathe for patients but also sigh, change the rate of breathing, hold breath for a specific period of time, and warm or cool temperature as required. Also, it can change oxygen concentration, and increase airway resistance during exhalation.

The mechanical ventilator is a life saving device for patients with respiratory failure. It can keep the patients alive for a short or a long period of time and even permanently with expert help and suitable machines.

It is a complex machine and expensive. Ventilators can't be turned out of factories on short notice. To properly operate a ventilator, one has to be trained properly and it takes time.

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