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The Use Of Mechanical Ventilators Engineering Essay

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Published: Mon, 5 Dec 2016

The respiratory system, composed of different structures, is involved in ventilation and gas exchange. Its main function is to provide a surface for gaseous exchange of oxygen and carbon dioxide [1]. Gas exchange is performed at the alveoli, specialised cells which are part of the lung parenchyma. It provides oxygen to the blood and removes the carbon dioxide produced in the body as a product of cellular metabolism; for the oxygen to reach the lungs there must be a series of tubular structures that communicate with the outside. The diagram below shows a block diagram of the anatomic structure for the respiratory system (Fig. 1).

Figure Block diagram of the Respiratory System anatomic structure

Air diffusion into these channels is conducted by the respiratory muscles (intercostals and diaphragm) which increase and decrease rhythmically the size of the thoracic cavity (inspiration and expiration). The pleural cavity contributes on this phenomenon when its negative pressure opposes the elastic recoil of the lung; this action gives place to a conductive portion of the system, whose function is to allow air penetration. In addition the respiratory portion composed bronchioles, alveolar ducts, alveolar sacs and alveoli; establishes homeostasis.

Figure Muscles and Pressures involved during breathing

The ventilation of the lungs can be measured by studying a gas volume and its variations in the lungs [2]. Boyle’s, Charles’, Dalton’s and Henry’s law of gasses are critical in the understanding of gas exchange, measurement of gas movement take an important part in mechanical ventilation. During breathing movements are cyclic, and volume in the thoracic cavity is changed by the muscles mention before. During inspiration the pressure within the thoracic cavity and lungs is decreased and the same time the volume is increased, allowing air flow in. On the other hand during expiration the elastic lungs and the thoracic wall recoils producing an increase of pressure but a decrease in volume; allow letting air flow out (Fig. 1).

Figure Respiratory performance and volume relationships

Figure 3

At the same time, inhalation and exhalation allow the mobilization of the volume of gas which can vary depending on the type of respiratory movement and lung elastic forces. Lung capacities are defined by the sum of different volumes. Figure 2 shows a graphic representation of the respiratory performance volume relationships. FCR (Functional Residual Capacity) represents the remaining air after a cycle. VT (Tidal Volume) is the flux of air in a normal inspiration and expiration. IRV (Inspiratory Reserve Volume) is the amount of air moved during a maximum and forced inspiration and results over the Tidal Volume. Similar to IVR, ERV (Expiratory Reserve Volume) is the amount of air mobilised during a maximum and force expiration resulting below the Tidal Volume. The vital capacity is the sum of IRV, VT, and ERV. VR (Residual Volume) as its name says, is the amount of air remained in lungs after a maximum exhalation. The IC (Inspiratory Capacity) is the flux of air after a quiet cycle. And finally the TLC (Total Lung Capacity) corresponds to the total volume of gas remaining in the lung after a maximal and forced inspiration. Volumes and lung capacities may be altered in different diseases; its measurement is a critical element for diagnosis, performed by pulmonary function tests.

The respiratory parameters: compliance, lung elasticity, intrathoracic pressure, airway resistance, intra-alveolar pressure; help measure the strength in muscles when breathing.

Airway resistance is determined by the Poiseuille Law (eq. 1):

{1}

Where η represents the viscosity of the fluid, l is the longitude in the airways, and r is the radio on the airways. Resistance has a great significance in pulmonary physiology; and it is analysed by the ratio of the pressure differential flow. The airway resistance can be increased significantly in the presence of disease such as Bronchitis, Asthma, and Emphysema among others. In addition a great amount of patients admitted to intensive care have need of some form of respiratory support; due primarily to hypoxaemia or ventilatory failure. Respiratory support ranges from oxygen therapy by face mask, through non-invasive techniques such as continuous positive airways pressure, to full ventilatory support with endotracheal intubation[3].

Figure Block Diagram of a basic mechanical ventilator

A mechanical ventilator is an automatic machine, designed to provide all or part of the work the body must produce to move air (gas) from the inside to the outside and vice versa. Furthermore mechanical ventilators are designed to transmit energy applied in a predetermined manner to perform a specific task. Interface between machine and patient stable, energy source, control system (for timing and size of the breaths regulations) and monitoring (device performance and patient’s condition) are the general requirements for ventilators (Fig. 4).

A further analysis as well as comparison of this equipment will be made along the paper, with the purpose of a better understanding of its designed and future developments.

Current State of the Art

Since the invention of artificial respiratory supply, mechanical ventilators have evolved in the past 40 years. There are five generations of mechanical ventilators where changes have been made in order to present a better apparatus.

The first generation consisted of only one mode of ventilation, and the electronics used was primitive compared to the one used nowadays. The equipment was no safe since the control with the user was not precise and it did not count with any alarm system.

The second generation provided basic alarms, containing electronic circuitry as well as an analogue control of fluid.

A major evolution took place on the third generation; digital electronics, microprocessors, were employed for most of the functions.

The fourth generation included modern displays such as CRT or LCD ensuring a better patient care.

The generation currently used is the fifth generation which features a better onscreen display control. It is also made by advanced logarithms that permit graphic display, calculation of lung’s mechanical properties, and system diagnostics.

Principles of Operation

Mechanical ventilation is all different types of procedures that provide artificial respiration employing machinery to meet the respiratory function of a person who cannot perform it by itself [4]. In addition Mechanical Ventilation (MV) is the product of interaction between a ventilator and a patient, and through this equipment parameters of volume, flow, pressure and time are controlled. Considered as a generator of positive pressure that supplies active phase of the respiratory cycle; there are basically four types of MV: controlled by pressure, time, volume and flow.

Mechanical ventilation systems create an intermittent positive pressure where air or a gas mixture enriched in oxygen is insufflated in the patient’s airway. Pressure in the airway at the end of passive expiration and that at the same time goes beyond atmospheric pressure is known as positive end-expiratory pressure (PEEP) [5]. PEEP is extremely important in effects and mechanisms of the respiratory system. It plays major roles in gas exchange, lung mechanics, and hemodynamic effects. Some effects in lung mechanics it prevents the lung from collapsing, increases FRC among others.[5] In order to provide respiratory support, a MV such as the one in figure is used.

Figure Flow and control of gas exchange during artificial ventilation

Input of this system provides O2 as a medicinal gas; and in the case of portable ventilators the medicinal gas can be supplied by a dry air compressor. Ventilatory gas is passed through a pressure regulator; which is in charge of preserving the preset pressure for the inspiratory gas and ensures the integrity of the airway. The gas flown to the patient is allocated by an electro-valve (Fig. 5); this remains energised until the end of the inspiratory time previously programmed, when de-energised the gas flow is ceased. Finally the expiratory electro-valve is activated causing the air exhaled by the patient to be expelled to the environment by a biological filter that prevents the contamination of this.

Models of Ventilator-Patient Interaction

Figure Model representing breathing, were a rigid flow conducting tube is connected to an elastic compartment

The Respiratory System can be modelled to illustrate the relations amongst the variables of interest; providing a better understanding of patient-equipment interaction. The model most frequently used is shown in figure where a rigid flow conducting tube is connected to an elastic compartment [6].

When airway pressure goes higher than the base line, the inspiration is assisted (Fig. 6)The Transrespiratory pressure (eq.2) leads inspiration, and is the pressure at the airway opening, ,minus the pressure at the body surface:

{2}

At the same time has two components, transairway pressure (eq. 3) and transthoracic pressure (eq. 4):

{3}

{4}

A mathematical model that represents volume, pressure and flow during ventilation is known as the equation of motion for the respiratory system [6] (eq. 5) :

{5}

Where is the pressure generated by the ventilator, is the pressure generated by the ventilator muscles, is the respiratory system elastance, is the respiratory system resistance, and is lung volume where the derivate of volume with respect of time is the flow in the system. Table (1) compares typical values against values during mechanical ventilation [6].

Table Pressures and Volumes during Mechanical ventilation

Typical Values

Mechanical Ventilation

The model provides the basis for monitoring the patient’s current condition, and it is done in terms of R and E which are mechanical properties.

Figure Electrical model representing breathing composed of a RC circuit

Another model used for representation is the electrical model (Fig. 7); this model is analogous to an electrical circuit consisting of a resistor and a capacitor (RC circuit), a power supply, which in this case represents the pressure generated by a mechanical ventilator. The electric current stands for the flow of air in the system. In this model, pressure, volume and flow are variables (functions of time) while the resistance and compliance are constant [4]. Second Law of Kirchooff can be used to analyse the electrical model and the following equation(eq. 6) can be derived:

{6}

Current and charge can be related by , the electrical parameters of the circuit can be now represented by the ventilator variables. When applying a pressure to the input of the system (output pressure of the ventilator), the volume varies according to the following differential equation (eq. 7) the total pressure applied is equal to the sum of the differences in pressure due to the compliance of the system and to the resistance of the airway:

{7}

According to this system is the output pressure of the ventilator, the inspiratory volume, and is the compliance of the lung [4].

Operating Modes

Mechanical ventilators count with different operating modes, which are the manner the ventilator ensures that the patient is provided by the appropriate minute ventilation; satisfying the respiratory needs without damaging any pulmonary tissue. Operating modes can be identified by: breathing pattern, Control type, Control Strategy [6].

When specifying just the breath control variable (Primary Breath Control), there are three approaches: pressure control, volume control and dual control modes. Pressure control (PC) is used when patients can initiate respiration; pressure in the airway is increased during inspiration. Volume control (VC) employs a control system to guarantee that a set tidal volume is distributed during the inspiratory cycle. The Dual Control (DC) is simply a combination of both, used in order to provide minute ventilation while maximizing patient synchrony[6].

Breath sequence is the other component of breathing pattern operating mode. There are two ways airflow can be delivered using this mode, mandatory or spontaneous. The difference between the two of these is that on mandatory breath the ventilator initiates and establishes the tidal volume, Vt. Contrary to mandatory breath on spontaneous breath the patient establishes and starts its own breathing. From these, three different modes of breath sequence can be delivered: Continuous Mandatory Ventilation (CMV), Continuous Spontaneous Ventilation (CSV), and Intermittent Mandatory Ventilation (IMV). CVM and CSV, all breaths are mandatory or spontaneous respectively; however in IMV breaths can be either mandatory or spontaneous [6].

Controls

In order to select breathing mode and ventilation pattern parameters, controls are used. There are two different ways on which breathing can be controlled, and at the same time there are control strategies which depend on the variables and parameters set to obtain this. A system can be controlled by an open loop or closed loop (Fig. 8). Like any open loop system, there is no feedback, and the system could be affected by mechanical changes in the lungs, patient’s ventilatory efforts and leaks [6].

Figure Control systems used for mechanical ventilation

Closed loop sense breathing variables such as pressure, volume, and flow to provide a feedback signal which is compared to the desired value set at the input. There are different types of closed loop systems depending on the number of variables used.

The instruments used to measure volume-flow rate are referred to as volume flowmeters; they may be classified as rotameters, penumotachographs, hot-wire anemometers, time-of-flight flowmeters, ultrasonic flowmeters, and vortex flowmeters [2]. Depending on their principle of operation, flowmeters can be classified in four main categories: rotating-vane, ultrasonic, thermal-convection, and differential pressure flowmeters.

Rotating-vane Flowmeters

These types of sensors contain a small motor or turbine which rotates with airflow, and then flow rate is related to the revolution of the rotor. This type of flowmeter is commonly used in ventilator machines and respiratory monitoring [2]. The spins are detected optically and converted into voltage to be recorded or displayed.

Ultrasonic Flowmeters

Ultrasonic flowmeters can measure instantaneous flow and the effect of the flowing gas on the transit time of the ultrasonic signal [7]. A crystal is used for transmitting and receiving and it is placed externally and obliquely to the axis of the tube through which the gas flows [7]. The time elapsed will depend not only on the velocity, but on the temperature as well as composition of the gas analysed. One main advantage of this type of transducer is that unidirectional flow can be measured, which is applicable for clinical monitoring.

Thermal-Convection Flowmeters

Thermal sensing technologies are usually made of hot wires, metal film, and thermistol which all use heat to sense gas flow. The wires are heated by an electric current and the heat transfer is used to measure the gas flow [2]. The wire is heated above flow gas temperature, to associate temperature differences; a metal mesh is placed at both ends of the tube. This type of sensing is limited to only one flow direction, more sensors can be located in the tube for multiple directions and for breathing a calibration factor must be considered. [7].

Differential Pressure Flowmeters

Flowmeters that use the relationship of pressure drop with airflow through a system. There include elements such flow resistors.

Common Failures

Figure Closed system during mechanical ventilation

The most common failures presented in MV are mainly as a consequence of poor maintenance and user error. Leaks in the circuit due to bad connections or due by perforations in tube are a frequent dysfunction. Leaks stop the proper delivery of tidal volume as well as an accurate sensing flow from the ventilator. PEEP can also be affected by this interfering with O2 saturation (Fig. 9).

At times, when an patient with intubation is not able to trigger the ventilator, or the ventilator senses by mistake a patient’s effort and delivers breaths, is known as patient-ventilator dyssynchrony. As a result the machine delivers an unsuitable breath to the rate of the patient’s inspiratory efforts. This type of error is also identified as trigger failure or desynchronisation, mismatching, and “fighting the ventilator” [8]. One cause for patient-ventilator dyssynchrony is fixing the trigger sensitivity improperly. When a desynchronisation with the patient’s efforts to initiate a breath exists, work of breathing can occur which can be accompanied with respiratory distress preventing pulmonary gas exchange..

Another usual failure is due to user error with the interface. MVs are complex equipments, and the need of the clinician to be familiar with the machine is crucial. It is important that Mechanical Ventilators count with an audible and visual alarm when detecting a leakage or disconnection.

Possible hazards to humans

Problems may occur while using a mechanical ventilator, especially with patients that been required the use of a MV for a prolonged amount of time. The risks occasioned by the use of respiratory support can lead to severe harmful or even death. Common hazards that may occur due to the use of a ventilator are: infections, pneumothorax, and lung injury.

Infections

The most common risk reported is acquiring Ventilator-associated pneumonia, which is caused by an infection. The tube allows germ (bacteria) to penetrate more easily into the lungs. This can cause pneumonia. Pneumonia can be a serious problem and may mean that a person may not be able to initiate respiration leading to a longer use of a MV. In addition a recent study reported factors related such as the development of shock, and renal failure [9]. In order to prevent infections a number of control procedures can be performed, and these include maintaining the ventilator as well as the breathing circuit [8].

Pneumothorax

Occasionally when a part of the lung is weak, this may become over full of air and as a result an air leak may occur. The leak allows air into the space between the lung and chest wall. The air in this region occupies space in a manner that the lung begins to collapse. If there is air leakage, a chest tube into is used to drain the excess air; allowing the lung to re-expand and stop the leak.

Lung Injury

The pressure generated by introducing air into the lungs with a ventilator can damage the lungs. Furthermore, very high levels of oxygen can also be harmful to the lung. As a solution to try to keep this risk to a minimum the lowest pressure necessary as well as the only oxygen needed is supplied.

Prolonged intubation usually defined as a period longer than 48 hours [10] may lead to swallowing dysfunction. This is mainly caused by impairing glottic closure reflex, reducing subglottic pressure, limiting laryngeal elevation, desensitizing the larynx and hypopharynx, and causing disuse muscle atrophy of the larynx and pharynx [10].

Advantages and limitations of various techniques

Once analysed the principles of operations and risks of Mechanical Ventilators, for a better understanding of these is necessary to mention the advantages and disadvantages that they could bring. The impact MV have had over the past 40 years is massive due to the fact that mechanical ventilators provide vital support.

Nowadays ventilators found in the market present vast options in terms of modes, control and displays which in many cases can result complicated and the knowledge of these is required[8]. In addition features need to be evaluated in order to establish which configuration mode is suitable for each patient [8]. Ideally hospitals should acquire equipment that incorporates the latest development in ventilation; however as mention before this could lead to complications and misuse of the devices. Requesting companies training to all staff involved in the use, handling and care of the equipment helps to reduce the risk [11].The complicity of the equipment could be considered as a disadvantage of MV, nevertheless ventilators with good human factors design provide major advantage [8].

Mechanical ventilators, being devices that provide respiratory support the duration and need may vary from patient to patient; age is also an important factor as well as the condition. Ventilators are usually used in patients that are in Intensive Care Unit (ICU) and after remaining in intubation after 48 hours the risk to the patient increases. Weaning from mechanical ventilation (MV) permits patients to restart spontaneous breathing steadily; however some risks are involved [12] and are mentioned in section. Risks and hazards to patients should always be considered when dealing with medical devices; however the benefits that they bring play a major role. But still as an advantage mechanical ventilators as mentioned before, bring vital support where initiation of breathing or respiration cannot be performed by the patient.

Critical Comparison

Figure Piston pump in HFOV

Differences between each mechanical ventilator is defined by their operation mode which establish the flow pattern, pressure and volume delivered to the patient with the purpose of controlling alveolar ventilation and as a result achieve the goals of mechanical ventilation. Ventilation modes are determined by the combination of breathing pattern, type of ventilation and control. As for this MV operation mode is going to vary according to the age, and state of the patient, in a way that ventilation is provided and the risk is minimal. Requirements

As mentioned before, the continuous use of ventilators may induce injury to the lungs. Air strained outside the normal air spaces creates a swelling pressure that may injure alveoli. The name of this condition is Barotrauma, and malfunction to the mechanical ventilation may occur. High pressures or volumes during inspiration, or when extreme PEEP is used are causes of Barotrauma. There has not been found an association of clinical injury with the level of pressure used, the problem is estimated to be an over expedition of volume [13].

High frequency ventilation(HFV) is a ventilation strategy for patients with respiratory failure; providing a small source of tidal volumes (VT) which is in most of the cases less than the anatomic dead space volume, with respiratory rates above 150rpm. Modern Research studies have shown that HFV can help reduce barotraumas in normal and injured lungs [14].

Figure Flow during high frequency ventilation

HFV can be classified according to the source that generates their frequency and the type of exhalation phase; there are four types: High Frequency Jet Ventilation (HFJV), High Frequency Oscillatory Ventilation (HFOV), High Frequency Flow Interruption (HFFI), and High Frequency Positive Pressure Ventilation. The most commonly is used is the HFOV where in a continuous positive air pressure circuit the frequencies are oscillated by a piston pump (Fig. 10).

During inspiration, each high frequency pulse in the flow creates a profile shaped like a “bullet” (Fig. 11), with the central molecules moving on beyond the airway than those found in the periphery.

Table (2) describes and compares main differences between common ventilation and high frequency ventilation.

Table Comparison between HF ventilator and Conventional ventilator

HFV

Conventional Ventilator

Frequency is measured in Hertz (Hz)

Frequency is measure in rpm

Uses Displacement Volume (Vd)

Uses Tidal Volume (Vt)

Volume per minute is measured:

Volume per minute is measured:

Medical Devices Available on the Market

There is a wide range of medical ventilators currently available on the market, and they all offer variety of options ranging in modes, variables monitored and ways of control [8] . Specific requirements and recommendations can be separated according to the complexity based on their performance.

Some of the most common brands available nowadays are listed in table (3), and they all offer ventilators that may vary according to specific needs.

Table Brands currently available

Brand

Model

ACOMA

ART-100

ART-21EX

BIO-MED DEVICES

CV-3

CV-4

DRAEGER

Carina Home

Evita 2 dura

Evita 4

Evita XL

Oxylog 1000

Oxylog 2000

Oxylog 3000

Savina

EVENT MEDICAL

Inspiration

Inspiration LS

GE HEALTHCARE(DATEX-OHMEDA)

Centiva/5

Engstrom Carestation

HAMILTON

GALILEO GOLD

RAPHAEL COLOR

IMPACT

Unit-Vent 754

INTERMED

INTER5 PLUS/ GMX

INTER PLUS VAPS/ GMX

KIMURA

KV-3N

MAQUET

Servo-i(Adult:Infant)

Servo-S ( Adult: Pediatric)

NEWPORT

E100M

E150 Breeze

E360

E500 Wave

PULMONETIC SYSTEMS/VYASIS HEALTHCARE

LTV 900

LTVO 950

LTV 1000

RESPIRONICS

Esprit

SAIME

ELISEE

SIARE

Siaretron 1000 ICU

Siaretron 1000 IPER

Siaretron 3000 ICU

TAEMA

eXtreria

Horus 4

NEFTIS icu

TECME

Neumovent Graph

TYCO HEALTHCARE PURITAN BENNETT

740

760

840

VERSAMED

iVent201

VYASYS HEALTHCARE

AVEA

Vela

Vela +

Vela Comprehensive

Future Stage of Development

Advanced features like recording and accurate and advanced predictions will come in a future state of development. In addition the ability to link multiple devices on one is now available, where the ventilator monitor can display lectures from other devices.

Portable devices are starting to become more common, they are light and compact devices. Important upgrades have been made to portable devices, where advanced features are now presented. Current portable ventilators present various modes of ventilation and longer power supply.

In order to avoid complexity, it is important when developing new features to consider the principal use of ventilators, which is respiratory supply. In addition for longer term care many features may not be used and costs can increase.

Conclusions

Mechanical ventilators are vital equipments that provide vital support to a patient. They provide artificial respiration to patients that cannot breathe on their own. Their principle of operation is based on mechanical exchange of gases, and their circuitry includes electro-valves for their control and flowmeters as transducers. MV can become very complex devices, training of clinicians using them is crucial in order to avoid risks to patients. Hazards to patients may occur when used for more than 48 hours, however new techniques like high frequency ventilation can reduce this. There is a wide range of devices present on the market and they all vary on their modes of operation. Portable devices are now popular and present important features. Mechanical ventilators are used every day in hospitals and represent a critical part on vital support.


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