What Is Mechanical Ventilation?
Mechanical ventilation is a method to assist or replace spontaneous breathing. It helps patients breathe by assisting the inhalation of oxygen into the lungs and the exhalation of carbon dioxide. Depending on the patient’s condition, mechanical ventilation can help support or completely control breathing.
How is mechanical ventilation performed?
It may be provided either by a machine called a ventilator or by a person compressing a bag or set of bellows via a breathing circuit or Ambu bag.
Broadly mechanical ventilation can be classified as:
1 . Negative pressure mechanical ventilation
2 . Positive pressure mechanical ventilation
Negative pressure mechanical ventilation:
The Drinker and Shaw ventilator (1929) was one of the first negative-pressure ventilators (also called iron lung). The device consisted of a metal cylinder that completely engulfed the patient up to the neck. A vacuum pump created negative pressure in the chamber, which resulted in expansion of the patient's chest. This change in chest geometry reduced the intrapulmonary pressure and allowed ambient air to flow into the patient's lungs. When the vacuum was terminated, the negative pressure applied to the chest dropped to zero, and the elastic recoil of the chest and lungs led to expiration.
Drinker and Shaw negative-pressure ventilator (iron lung)
However, this mode of ventilation was cumbersome and led to patient discomfort and also limited access to the patient by health care providers.
Positive pressure mechanical ventilation:
History: Negative-pressure ventilation (NPV) was the primary mode of assisted ventilation for patients with acute respiratory failure until the Copenhagen polio epidemic in the 1950s. During the epidemic it was observed that a patient with polio and respiratory paralysis who was supported by manually forcing 50% oxygen through a tracheostomy had a reduced mortality rate. However, because of insufficient equipment, it was necessary to ventilate these patients continuously by hand. So the intervention required continuous activity of about 1400 medical students recruited from various universities!!!
The overwhelming manpower needed coupled with a decrease in mortality rate from 80% to 25% led to the adaptation of positive-pressure ventilators used in the operating room for use in the ICU. Thereafter, use of positive-pressure ventilators gained popularity and is now used routinely all over the world.
The design of the modern positive-pressure ventilators were based mainly on technical developments by the military during World War II to deliver oxygen and gas volume to fighter pilots operating at high altitude.
Basic principle of a positive pressure breath:
Positive-pressure is applied to the patient's airway through an endotracheal or tracheostomy tube. This positive air-pressure causes the gas to flow into the lungs until the ventilator breath is terminated. As the airway pressure drops to zero, elastic recoil of the chest causes passive exhalation to occur leading to pushing out of the tidal volume from the patient’s lungs.
Classify positive pressure ventilators.
· Invasive/ Non-invasive: Mechanical ventilation is termed invasive if it involves any instrument that enters the airway (such as an endotracheal tube or tracheostomy tube) and non-invasive when airway is not entered (BiPAP)
· Method of cycling from the inspiratory phase to the expiratory phase (most common classification): volume-cycled ventilator/ pressure-cycled ventilator/ time-cycled ventilator
· Based on clinical setting:
Ø Transport ventilators — These ventilators are smaller and more rugged, and can be powered pneumatically or via AC or DC power sources.
Ø Intensive-care ventilators — These ventilators are larger and usually run on AC power (but they have a battery to facilitate intra-facility transport and as a back-up in the event of a power failure). This type of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time). Many ICU ventilators also incorporate graphics to provide visual feedback of each breath.
Ø Neonatal ventilators — Designed with the preterm neonate in mind, these are a specialized subset of ICU ventilators that are designed to deliver the smaller, more precise volumes and pressures required to ventilate these patients.
Ø Positive airway pressure ventilators (PAP) — These ventilators are specifically designed for non-invasive ventilation. This includes ventilators for use at home for treatment of chronic conditions such as sleep apnea or COPD.
Ø Anaesthesia ventilators
Brief discussion on anesthesia ventilators:
Anaesthesia ventilators are an integral part of all modern anaesthesia workstations. Modern anaesthesia ventilators have either a double circuit, bellow design or a single circuit piston configuration. In the bellows ventilators, ascending bellows design is safer than descending bellows. Piston ventilators have the advantage of delivering accurate tidal volume. They work with electricity as their driving force and do not require a driving gas. In addition to the conventional volume control mode, modern anaesthesia ventilators also provide newer modes of ventilation such as synchronised intermittent mandatory ventilation, pressure-control ventilation and pressure-support ventilation (PSV).
Classification:
According to their mechanism of action ventilators can be classified as:
1. Mechanical thumbs.
2. Minute volume dividers.
3. Bag squeezers.
4. Intermittent blowers.
Mechanical thumbs:
This uses principle of T piece in providing ventilation. By rhythmical application of thumb to occlude the T piece, intermittent positive pressure ventilation is generated.
Minute volume dividers:
Here pressurised gas is fed into a ventilator system to be collected by a reservoir, which is continually pressurised by a spring, a weight or its own elastic recoil. It has one inspiratory valve and another expiratory valve, which are linked together and operated by a “bistable” mechanism. All driving gas that is supplied is delivered to the patient. For example if fresh gas flow delivered to patient is 10 L/min, this is delivered to patient as minute volume.
These ventilators are referred to as minute volume dividers since they just divide up intended minute volume supplied by the driving gas. Examples - East-Freeman® automatic vent, the Flomasta® and Manley MP3®.
Bag squeezers:
It is usually used in conjunction with a circle or Mapleson D system. Bellows may be squeezed pneumatically by placing it in a canister and feeding a driver gas in the space between canister and bellows or squeezed mechanically by means of a motor. e.g., Manley servovent®, Penlon Nuffield 400® series ventilator, Ohmeda® 7800, Servo® 900 series.
Intermittent blowers:
These ventilators are driven by a source of gas or air, at a pressure of 45-60 psi. The driving gas is normally delivered to patient undiluted, but it may be passed through a venturi device so that air, oxygen or anaesthetic gases may be added to it. e.g. Penlon Nuffield® 200 series ventilator.
Alternative classification: Modern anaesthesia ventilators can also be classified based on the basis of power source, drive mechanism, circuit designation, cycling mechanism and type of bellows.
Power source: Power source can be a compressed gas, electricity or a combination of both electricity and compressed gas.
Drive mechanism and circuit designation:
§ Double circuit: Bellows ventilator: Double circuit ventilators are most commonly used in modern anaesthesia workstations. These ventilators have bellows in box design. In these ventilators, a pressurised driving gas compresses bellows and bellows in turn deliver ventilation to patients. Driving gas will be outside bellows and the inside of bellows is connected to breathing system gas, thus forming a dual circuit.
§ Single circuit: Piston ventilators: They use a computer controlled motor instead of compressed gas to deliver gas in the breathing system. In these systems, instead of dual circuit with patient gas in one and driving gas in other, a single patient gas circuit is present.
Cycling mechanism: Most anaesthesia machine ventilators are time cycled and provide ventilator support in the control mode. Inspiratory phase is initiated by a timing device.
Type of bellows: The direction of bellows movement during the expiratory phase determines this classification.
Ascending (standing) bellows ascend during the expiratory phase, whereas descending (hanging) bellows descend during the expiratory phase.
Most contemporary anaesthesia ventilators have ascending bellows design and are safer. Ascending bellows do not fill and tend to collapse when disconnection occurs. The descending bellows continue their upwards and downwards movement even after disconnection.
Describe the terminology commonly used during mechanical ventilation.
· Independent and Dependent variables:
Ø Parameters set by clinicians which determine the ventilator output are called Independent variables.
Ø Parameters measured by the ventilator are called Dependent variables
· Tidal Volume (VT): The amount of air delivered to the patient per breath. It is customarily expressed in millilitres.
· Respiratory rate/frequency (f): The number of breaths per minute. This can be from the ventilator, the patient, or both. It has a unit of breaths/ minute.
· Minute ventilation (VE): The product of VT and respiratory frequency (VT * f). It is usually expressed in litres/ minute.
· Peak airway pressure (Paw): The pressure that is required to deliver the VT to the patient. It has a unit of centimetres of water (cm 1-120).
· Plateau pressure (Pplat): The pressure that is needed to distend the lung. This pressure can only be obtained by applying an end-inspiratory pause. It also has a unit of cm H2O.
· Peak inspiratory flow: The highest flow that is used to deliver VT to the patient during inspiratory phase. It is usually measured in litres/ minute.
· Mean airway pressure: The time-weighted average pressure during the respiratory cycle. It is expressed in cm H2O.
· Inspiratory time: The amount of time (in seconds) it takes to deliver VT.
· Positive end-expiratory pressure (PEEP): The amount of positive pressure that is maintained at end-expiration. It is expressed in cm H2O.
· Fraction of inspired oxygen (Fio2): The concentration of O2 in the inspired gas, usually between 0.21 (room air) and 1.0 (100% O2).
Describe a ventilatory cycle.
There are four phases during a ventilatory cycle:
· trigger phase (breath initiation)
· flow delivery phase
· cycle phase (breath termination)
· expiratory phase
Breaths can be initiated during the trigger phase by three mechanisms:
1. Machine timer, in which breaths are initiated by a timer in the machine set by the clinician
2. Pressure change (pressure trigger), in which patient’s effort pulls the airway/circuit pressure negative, and machine breaths are initiated when this pressure drop exceeds the set negative pressure threshold (pressure sensitivity)
3. Flow change (flow trigger), in which patient’s effort draws flow from the circuit, and machine breaths are initiated when flow into the patient exceeds the set flow threshold (flow sensitivity).
Once a breath is triggered, the inspiratory valve in the ventilator opens, and the flow is delivered. The flow delivery is governed by a target or limit set by the clinician. These are of two types:
1. Flow target: here flow rate and pattern are set by the clinician. Therefore, airway pressure varies
2. Pressure target: here inspiratory pressure limit is set by the clinician and so, flow and volume vary.
The flow delivery phase is followed by the cycle phase, during which the machine terminates the breath. Cycling can be of four types:
1. Volume Cycled: breath is terminated when a target volume is achieved
2. Time Cycled: breath is terminated when a set inspiratory time is achieved
3. Flow Cycled: breath is terminated when inspiratory flow has fallen to a pre-set level
4. Pressure Cycled: breath is terminated when a set inspiratory pressure is achieved
The cycle phase is followed by expiration. It is mostly passive and depends on lung recoil pressure (compliance) and airway/circuit resistance. The product of compliance and airway resistance is called the time constant (Tc). Patients with a long Tc (e.g., COPD and asthma) will need a longer expiratory time to empty the lung completely, whereas patients with a short Tc (e.g., ARDS, pulmonary fibrosis) can empty the lung quickly.
