Biomedicalism: 2012

Mechanical Ventilation:

The Roman physician Galen may have been the first to describe mechanical ventilation: "If you take a dead animal and blow air through its larynx [through a reed], you will fill its bronchi and watch its lungs attain the greatest distention."

A mechanical ventilator is a machine that generates a controlled flow of gas into a patient’s airways. Oxygen and air are received from cylinders or wall outlets (internal Gases Network), the gas is pressure reduced and blended according to the prescribed inspired oxygen tension (FiO2), accumulated in a receptacle within the machine, and delivered to the patient using one of many available modes of ventilation.

But before going in depth of Ventilation modes and Ventilators we should know more about structure of our Respiratory System.

Respiratory System:

The respiratory system provides the means for gas exchange required in living cells. Examples: Carbon Dioxide and Oxygen. When you inhale you are bringing oxygen to the lungs. When you exhale you are getting rid of carbon dioxide. When you hold your breath, does your body start saying breath I need oxygen or does it say help the carbon dioxide levels that are out of whack? If you said carbon dioxide levels you are correct. Let's take a look now at what the respiratory system consists.

There are 2 tracts of the respiratory system:

- Upper respiratory system.

- Lower respiratory system

It also can be divided into a conducting portion and respiratory function.

Conducting portion: Nose, Nasal cavity, pharynx, larynx, trachea and the smaller progressively airways (primary bronchi to terminal bronchioles).

Respiratory portion: is composed of small airways called respiratory branchioles, alveolar ducts and alveoli (air sacs).

Respiratory System

Respiratory System Anatomy

Some of Respiratory System main Functions:

Pulmonary Ventilation:

Inhalation: bringing gas into the lungs.

Exhalation: letting gas flow out of the lungs.

Gas exchange: Oxygen is drawn in by inhalation and is transported to the body cells from the lungs by blood circulation. The body uses the oxygen to generate carbon dioxide as a waste product which is then transported to the lungs and is then exhaled.

Gas Conditioning: Gases entering the body are "modified" before reaching the gas exchange surfaces. These gases are warmed to body temperature, filtered of any harmful particles and humidified by contact of the respiratory epithelium and the sticky mucus covering in the winding pathways in the nasal cavity and the paranasal sinuses.

Some Ventilation Concepts:

Negative-pressure ventilation: where air is essentially sucked into the lungs.

Positive pressure ventilation: where air (or another gas mix) is pushed into the trachea.

When physicians use a ventilator?

Medical Ventilator is a machine designed to mechanically assist in moving breathable air into and out of the lungs, to provide the mechanism of breathing for a patient who is physically unable to breathe, or breathing insufficiently.

Ventilator mainly in its simplest form, a modern positive pressure ventilator consists of:

- A compressible air reservoir or turbine,

- Air and oxygen supplies,

- A set of valves and tubes,

- A disposable or reusable "patient circuit",

The air reservoir is pneumatically compressed several times a minute to deliver room-air, or in most cases, an air/oxygen mixture to the patient. If a turbine is used, the turbine pushes air through the ventilator, with a flow valve adjusting pressure to meet patient-specific parameters. When overpressure is released, the patient will exhale passively due to the lungs' elasticity, the exhaled air being released usually through a one-way valve within the patient circuit called the patient manifold. The oxygen content of the inspired gas can be set from 21 percent (ambient air) to 100 percent (pure oxygen). Pressure and flow characteristics can be set mechanically or electronically.

Ventilators may also be equipped with monitoring and alarm systems for patient-related parameters (e.g. pressure, volume, and flow) and ventilator function (e.g. air leakage, power failure, and mechanical failure), backup batteries, oxygen tanks, and remote control. The pneumatic system is nowadays often replaced by a computer-controlled turbo pump.

Modern ventilators are electronically controlled by a small embedded system to allow exact adaptation of pressure and flow characteristics to an individual patient's needs. Fine-tuned ventilator settings also serve to make ventilation more tolerable and comfortable for the patient. In Germany, Canada, and the United States, respiratory therapists are responsible for tuning these settings while biomedical technologists are responsible for the maintenance.

The patient circuit usually consists of a set of three durable, yet lightweight plastic tubes, separated by function (e.g. inhaled air, patient pressure, exhaled air). Determined by the type of ventilation needed, the patient-end of the circuit may be either noninvasive or invasive.

Noninvasive methods, which are adequate for patients who require a ventilator only while sleeping and resting, mainly employ a nasal mask. Invasive methods require intubation, which for long-term ventilator dependence will normally be a tracheotomy cannula, as this is much more comfortable and practical for long-term care than is larynx or nasal intubation.

Setup of Ventilator in hospital ICU room:

The illustration shows a standard setup for a ventilator in a hospital room. The ventilator pushes warm, moist air (or air with increased oxygen) to the patient. Exhaled air flows away from the patient.

Some expressions:

Minute Volume: is the volume of air (oxygen) inhaled (inhaled minute volume) or exhaled (exhaled minute volume) from a person’s lungs in one minute (5-40 L/min).

Tidal Volume (Vt): the volume of air (oxygen) what is given to patient in one breathe (0.1-2 L).

Flow: volume/time (5-35 L/min).

BPM: Breathe Per Minute (2-60)

Ti: Inspiration time.

Te: Expiration time.

I:E: Inspiration to Expiration ratio.

Respiratory Cycle:

Relation between Tidal Volume, Inspiration time and Flow:

How to work on ventilator? The classification of ventilators refers to the following elements:

1- Control: How the ventilator knows how much flow to deliver

Volume Controlled (volume limited, volume targeted) and Pressure Variable.
Pressure Controlled (pressure limited, pressure targeted) and Volume Variable.
Dual Controlled (volume targeted (guaranteed) pressure limited).

2- Cycling: how the ventilator switches from inspiration to expiration: the flow has been delivered to the volume or pressure target - how long does it stay there?

Time cycled such in pressure controlled ventilation.

Flow cycled such as in pressure support.

Volume cycled the ventilator cycles to expiration once a set tidal volume has been delivered: this occurs in volume controlled ventilation. If an inspiratory pause is added, then the breath is both volume and time cycled.

3- Triggering: what causes the ventilator to cycle to inspiration. Ventilators may be time triggered, pressure triggered or flow triggered.

Time: the ventilator cycles at a set frequency as determined by the controlled rate.

Pressure: the ventilator senses the patient's inspiratory effort by way of a decrease in the baseline pressure.

Flow: modern ventilators deliver a constant flow around the circuit throughout the respiratory cycle (flow-by). A deflection in this flow by patient inspiration is monitored by the ventilator and it delivers a breath. This mechanism requires less work by the patient than pressure triggering.

4- Breaths are either: what causes the ventilator to cycle from inspiration

Mandatory (controlled) - which is determined by the respiratory rate.

Assisted (as in assist control, synchronized intermittent mandatory ventilation, pressure support)

Spontaneous (no additional assistance in inspiration, as in CPAP)

5- Flow pattern:

Sinusoidal: this is the flow pattern seen in spontaneous breathing and CPAP

Decelerating: the flow pattern seen in pressure targeted ventilation: inspiration slows down as alveolar pressure increases (there is a high initial flow). Most intensives and respiratory therapists use this pattern in volume targeted ventilation also, as it results in a lower peak airway pressure than constant and accelerating flow, and better distribution characteristics.

Constant: flow continues at a constant rate until the set tidal volume is delivered.

Accelerating: flow increases progressively as the breath is delivered. This should not be used in clinical practice.

6- Mode or Breath Pattern:

Continuous Mandatory Ventilation (CMV): In which ventilator provides a mechanical breath on a preset timing, without allowances for spontaneous breathing from patient side. This is usually only used in an unconscious patient. It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing.

Assist-Control: That minimizes patient effort by providing full mechanical support with every breath. This is often the initial mode chosen for adults because it provides the greatest degree of support.

Intermittent Mandatory Ventilation (IMV): It mixes controlled breaths and spontaneous breaths, as the ventilator provides a preset mechanical breath (volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 seconds, thus a respiratory rate of 12 results in a 5 second cycle time). Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor. When the ventilator senses the first patient breathing attempt within the cycle, it delivers the preset ventilator breath. If the patient fails to initiate a breath, the ventilator delivers a mechanical breath at the end of the breath cycle. Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath. However.

SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate, which requires the patient to take additional breaths beyond the SIMV triggered breath.

Pressure Support: Where the patient has control over all aspects of his breath except the pressure limit.

Continuous Positive Airway Pressure (CPAP): A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation, decrease the work of breathing, and decrease the work of the heart (such as in left-sided heart failure CHF). Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths. In addition, no additional pressure above the CPAP pressure is provided during those breaths

Synchronized Intermittent Mandatory Ventilation (SIMV): In which ventilator provides a preset pressure limited mechanical breath every specified number of seconds SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate, which requires the patient to take additional breaths beyond the SIMV triggered breath.

Positive End Expiratory Pressure (PEEP): may or may not be employed to prevent atelectasis in adult patients. It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis.

Dräger Company:

As an international leader in medical and safety technology, Dräger develops innovative equipment and solutions people the world over trust when livesare on the line and “Technology for Life” is their guiding principle and mission.

And we invited "Dräger International Co." agent company in Egypt "Life care Technology Co." and their representative Eng. Shawkat Ahmed (Service Supervisor of Co.) who welcomed the invitation and spent about three hours illustrating the Medical Ventilation Machines to SBME's and bring one of their most advanced models of their Ventilators Evita XL (which is shown among their products below).

Dräger’s range of Ventilators Products: