GUIDE 3-MECHANICAL VENTILATOR

The what and why of mechanical ventilation

GLOSSARY

Lungs: Are a pair of spongy, air-filled organs located on either side of the thorax. The windpipe conducts inhaled air into the lungs through its tubular branches, called bronchi. The bronchi then divide into smaller and smaller branches (bronchioles), finally becoming microscopic. The lungs are covered by a thin tissue layer called the pleura. The same kind of thin tissue lines the inside of the chest cavity -- also called pleura. A thin layer of fluid acts as a lubricant allowing the lungs to slip smoothly as they expand and contract with each breath.

Breathing: Is the process of moving air into and out of the lungs to facilitate gas exchange with the internal environment, mostly by bringing in oxygen and flushing out carbon dioxide. Breathing, or "external respiration", brings air into the lungs where gas exchange takes place in the alveoli through diffusion. The body's circulatory system transports these gasses to and from the cells, where "cellular respiration" takes place.

The breathing consists of repetitive cycles of inhalation and exhalation through a highly branched system of tubes or airways which lead from the nose to the alveoli. The number of respiratory cycles per minute is the breathing or respiratory rate, and is one of the four primary vital signs of life. Under normal conditions the breathing depth and rate is automatically, and unconsciously, controlled by several homeostatic mechanisms which keep the partial pressures of carbon dioxide and oxygen in the arterial blood constant. Keeping the partial pressure of carbon dioxide in the arterial blood unchanged under a wide variety of physiological circumstances, contributes significantly to tight control of the pH of the extracellular fluids (ECF).

Hyperventilation and hypoventilation, which decrease and increase the arterial partial pressure of carbon dioxide respectively, cause a rise in the pH of ECF in the first case, and a lowering of the pH in the second. Both cause distressing symptoms. Breathing has other important functions. It provides a mechanism for speech, laughter and similar expressions of the emotions. It is also used for reflexes such as yawning, coughing and sneezing. Animals that cannot thermoregulate by perspiration, because they lack sufficient sweat glands, may lose heat by evaporation through panting.

Volume: Is the quantity of three-dimensional space enclosed by a closed surface, for example, the space that a substance like are: solids, liquids, gases, or plasma or shape occupies or contains. Volume is often quantified numerically using the SI derived unit, the cubic meter. The volume of a container is generally understood to be the capacity of the container, the amount of fluid, gas or liquid that the container could hold, rather than the amount of space the container itself displaces.

Flow: Is defined as the quantity of fluid (gas, liquid or vapour) that passes a point per unit time. Flow is sometimes written as ∆Q that is rate of change of mass or volume. Flow can be divided into 2 different types, laminar and turbulent. A number of different physical characteristics determine whether a fluid obeys the principles of one or the other.

In laminar flow the molecules of the fluid can be imagined to be moving in numerous ‘layers’ or laminae as shown below. Not all fluid flow is laminar, but under certain physical conditions it becomes turbulent. When this happens, instead of the fluid moving in seemingly ordered layers, the molecules become more disorganised and begin to swirl with the formation of eddy currents, as shown below.

Pressure: Is defined as force per unit area. It is usually more convenient to use pressure rather than force to describe the influences upon fluid behavior. The standard unit for pressure is the Pascal, which is a Newton per square meter. When you deal with the pressure of a liquid at rest, the medium is treated as a continuous distribution of matter. But when you deal with a gas pressure, it must be approached as an average pressure from molecular collisions with the walls. Pressure in a fluid can be seen to be a measure of energy per unit volume by means of the definition of work. This energy is related to other forms of fluid energy by the Bernoulli equation.

WORKSHOP

Expiratory Valves:

• Close during inspiration to direct gas flow to patient.

• Open during exhalation allowing patient to exhale gas into atmosphere.

• PEEP is used to improve oxygenation.

• Peep is provided by threshold resistor or flow resistance.

• Both valves prevent complete exhalation of expired tidal volume.

Diaphragm Expiratory Valve

• Diaphragm over exhalation valve closes during inspiration.

• During exhalation, pressure on fiaphragm is released.

• When PEEP is used, preassure against diaphragm is maintaines thus preventing complete exhalation.

Spring-Loaded Valves:

• Can be used to provide PEED.

• Spring is used to adjust amount of preassure applied against exhalation valve to prevent complete exhalation.

Electromagnetic Valves:

• Amount of electrical current is adjusted to regulate amount of force applied to expiratory flow.

• The higher the level of current, the more force applied to exhaled gas.

• Increases PEEP level.

Flow-Control Valves

• Three types of flow controlling valves.

1. Proportional solenoid valve.

2. Stepper motors with valve.

3. Digital valves with on/off configuration.

• Current ICU ventilators use proportional selenoid valve.

CIRCUIT DIAGRAM ANALYSIS

Pneumatic Circuit

• Consists of two series tubing:

1. Internal Circuit: Directs flow within the ventilator. It can be single or double circuit.

2. The External Circuit, Patient Circuit: Directs the flow from the ventilator to the patient.

Internal Circuit, Single-cicuit design:

• Single circuit: Gas moves through ventilator and is delivered to patient circuit. Most commonly used.

Internal Circuit, Double-circuit desing:

• Doble ciircuit includes bellows or a bag containing desired tidal volume.

• Bag is compressed and desired tidal volume is delivered to lungs.

The External Circuit, Patient Circuit

Primary Ventilator Control Variables

• Primary variable ventilator controls to cause inspiration.

• 3 possible explicit primary variable

1. Pressure controlled.

2. Volume controlled.

3. Flow controlled.

• Only one can be controlled; other two become dependent variables.

Pressure Controller

• Ventilator controls pressure (P), but volume and flow varu with changes in compliance (c) and resistance (Raw).

• Pressure waveform will be squre during inspiration.

• Positive or negative pressure controlled.

• i.e.m iron lung controls with negative P.

Volume and Flow Controllers

Volume controller:

• Ventilator controls volume so will be constant.

• Flow is volume/time, so flow is also contant.

• Pressure will vary wuth changes in C and Raw.

Flow Controller:

• As above, flow and thus volume constant.

• Pressure varies with changes in C and Raw.

• Old neonatal ventilators used flow interruption to deliver volume during inspiration.

DEVELOPMENT

1. Self-test:

The self-test is carried out, in which the mechanical fan starts working, with all the details you have, measuring if leaks appear, that all electrical circuits work correctly and that your mechanical site does not have errors.

1. Ventilation modes:

a. Pressure controlled: Controlled pressure, where the inspiratory pressure is programmed, can be constant or a verifiable pressure is established; On the contrary, the volume and the flow had to vary. This controlled pressure must have a time of inspiration while the glume decreases and the alveolar pressure approaches the pressure delivered.

b. CPAP: CPAP ventilation mode is the time of positive pressure after expiration in the airways, air must be administered at pressure avoiding obstructions in the airways.

Questionnaire : What kind of ventilation mode did you use? Why?

The ventilation mode controlled and CPAP were used, because the in the first:

Each breath is either an assist or control breath, but they are all of the same volume. The larger the volume, the more expiratory time required. If the I:E ratio is less than 1:2, progressive hyperinflation may result. ACV is particularly undesirable for patients who breathe rapidly – they may induce both hyperinflation and respiratory alkalosis. Note that mechanical ventilation does not eliminate the work of breathing, because the diaphragm may still be very active.

And the second was positive pressure given throughout the cycle. It can be delivered through a mask and is can be used in obstructive sleep apnea, to postpone intubation, or to treat acute exacerbations of COPD.

These modes were chosen because in controlled ventilation the equipment helps or fully performs the breathing process to the patient, on the other hand the CPAP mode is a therapy because it acts by sending a continuous positive pressure to the airways to keep them open and avoid its collapse, in this mode the behavior of the patient's breathing controls the device.

How can you make the control by Volume and what is the difference with the Pressure control?

The difference is that consistent minute ventilation is only possible in volume ventilation, it is not possible in pressure control ventilation, because respiratory rate and although pressure ventilation can help protec the lung from being overinflated it cannot ensure that the patient is receiving adequate and consistent minute ventilation.

REFERENCES

[1]"The Lungs (Human Anatomy): Picture, Function, Definition, Conditions", WebMD, 2018. [Online]. Available: https://www.webmd.com/lung/picture-of-the-lungs#1.

[2]Hall, John (2011). Guyton and Hall textbook of medical physiology (12th ed.). Philadelphia, Pa.: Saunders/Elsevier. p. 5.

[3]Pocock, Gillian; Richards, Christopher D. (2006). Human physiology : the basis of medicine (3rd ed.). Oxford: Oxford University Press. p. 311,320.

[4]"Johns Hopkins Medicine, based in Baltimore, Maryland", Hopkinsmedicine.org, 2018. [Online]. Available: https://www.hopkinsmedicine.org/.

[5]P. Clements and C. Gwinnutt, The Physics of Flow. Salford, UK, pp. 1-5.

[6]"Pressure", Hyperphysics.phy-astr.gsu.edu, 2018. Available: http://hyperphysics.phy-astr.gsu.edu/hbase/press.html.

[7]Pilbeam's mechanical ventilation: physiological and clinical aplications/ J.M. Cairo-6th edition, 2016.

[8]Ega's fundamentals of respiratory care / Robert M. Kacmarek, James K, Stoller and Al Heuer. - 10th edition.2013.

[9] C.P.A.P Manual para el paciente. Barcelona: ESTEVE TEIJIN HEALTHCARE, S.L., pp. 1-5.