Mechanical Ventilation Systems PDF

BMT 353 Therapeutic Medical Equipment Mechanical Ventilation Systems References: - THE BIOMEDICAL ENGINEERING HANDBOOK , Joseph D. Bronzino, Donald R. Peterson, CRC Press, 2015 - ECRI  Mechanical Ventilation Systems Physiology R...

BMT 353 Therapeutic Medical Equipment Mechanical Ventilation Systems References: - THE BIOMEDICAL ENGINEERING HANDBOOK , Joseph D. Bronzino, Donald R. Peterson, CRC Press, 2015 - ECRI  Mechanical Ventilation Systems Physiology Reminder: Respiratory system is a biological system consisting of specific organs and structures used for the process of respiration in an organism. The function of the respiratory system: bring in oxygen from the atmosphere and get rid of carbon dioxide from the blood. Respiratory failure is a life-threatening condition in which the respiratory apparatus is unable to provide adequate oxygenation (delivery of oxygen to the blood) and/or ventilation (removal of carbon dioxide from the blood). Alveoi  Mechanical Ventilation Systems Introduction - Mechanical ventilators (respirators) are used to artificially ventilate the lungs of patients who are unable to naturally breathe from the atmosphere. A large variety of ventilators are now available for short-term treatment of acute respiratory problems as well as for long-term therapy for chronic respiratory conditions. - Ventilators are chiefly used in intensive care unit, home care, and emergency medicine (as standalone units) and in anesthesia (as a component of an anesthesia machine). - From the beginning, the designers of the mechanical ventilators realized that the main task of a respirator is to ventilate the lungs in a manner as close to natural respiration as possible. Since natural inspiration is a result of negative pressure in the pleural cavity generated by distention of the diaphragm, designers initially developed ventilators that created the same effect. These ventilators are called negative pressure ventilators. 3  Mechanical Ventilation Systems Negative Pressure Ventilator The flow of air to the lungs is created by generating a negative pressure around the patient’s thoracic cage. The negative pressure moves the thoracic walls outward, expanding the intrathoracic volume and dropping the pressure inside the lungs. The pressure gradient between the atmosphere and the lungs causes the flow of atmospheric air into the lungs. The inspiratory and expiratory phases of the respiration are controlled by cycling the pressure inside the body chamber between a sub-atmospheric level (inspiration) and the atmospheric level (exhalation). Negative pressure ventilator has the following disadvantages: 1-obtaining a seal around the chest wall to create negative pressure is difficult 2-made the patient less accessible for patient care and monitoring 3-synchronization of the machine cycle with the patient’s effort was difficult A Simplified illustration of a 4-the machines are noisy and bulky negative pressure Ventilator 4  Mechanical Ventilation Systems Positive-pressure ventilators generate the inspiratory flow by applying a positive pressure (greater than the atmospheric pressure) to the airways. The figure below shows a simplified block diagram of a positive-pressure ventilator. From: 2003 ECRI 5  Mechanical Ventilation Systems Block diagram of positive pressure ventilator O2 Control Spirometer Main Valve Valve Timer Patient Valve Bag Filter Compressor Bacteria Nubulizer Humidifier Filter 6 Mechanical Ventilation Systems Controls of positive pressure Ventilators 7  Mechanical Ventilation Systems Principles of Operation Major Components of Positive Pressure ventilator: flexible breathing circuit Heating and Humidification control system Nebulizer devices. An Air Compressor Filters gas supply Power Supply monitors and alarms. Most ventilators are microprocessor controlled, and they regulate the followings based on control settings: pressure Volume Flow fraction of inspired oxygen (FiO2) 8  Mechanical Ventilation Systems Principles of Operation In General, Ventilators use a double-limb breathing circuit to transport the gas from the ventilator to the patient (inspiratory limb) and return the exhaled gas to the ventilator through the expiratory limb. During inspiratory phase, exhalation valve is closed to maintain pressure in the breathing circuit and lungs. After the inspiratory phase, the gas is released to ambient air through this valve The breathing circuit has sites for: heating the delivered gas Humidifying the delivered gas monitoring airway pressure Conditioning the delivered gas with nebulized medications Collecting condensation. Many ventilators have sensors that can measure airway pressure or flow and provide feedback to the ventilator to automatically adjust its output. 9  Mechanical Ventilation Systems Ventilation Modes There are two main categories of ventilation modes:  Mandatory  spontaneous. Mandatory mode refers to the strategy for treating patients who need the respirator to completely take over the task of ventilating their lungs. However, some patients are able to exert the respiratory effort needed to breathe on their own, but may need to remain on the ventilator to receive oxygen-enriched air flow or slightly elevated airway pressure. When a ventilator delivers a breath to the patient according to the level of effort exerted by the patient, it is said that the ventilator operates in spontaneous mode. 10  Mechanical Ventilation Systems Ventilation Modes In many cases, it is necessary to first treat the patient with mandatory ventilation and as the patient’s condition improves, spontaneous ventilation is introduced to wean the patient from mandatory breathing and restore natural breathing. For this purpose, several schemes for combining mandatory and spontaneous breathing have been established. For instance, when a patient is able to generate the needed effort for few breaths, but not sufficient for completely proper ventilation, mandatory breaths are added intermittently to supplement the patient’s spontaneous breathing. Such a scheme is simply referred to as synchronized intermittent mandatory ventilation (SIMV). 11  Mechanical Ventilation Systems Mandatory Ventilation There are two distinct approaches for delivering mandatory breaths:  Volume-controlled ventilation  Pressure-controlled ventilation Volume-controlled ventilation refers to delivering a specified tidal volume to the patient during the inspiratory phase. Pressure-controlled ventilation refers to raising the airway pressure to a desired level (set by the therapist) during the inspiratory phase of each breath. Regardless of the type, a ventilator operating in mandatory mode must control all aspects of breathing, such as tidal volume, respiration rate, inspiratory flow pattern, and oxygen concentration of the breath. This is often labeled as controlled mandatory ventilation (CMV), a term that encompasses both volume- as well as pressure- controlled ventilation. 12  Mechanical Ventilation Systems Mandatory Ventilation Figure 2 shows the flow and pressure waveforms for volume-controlled ventilation. In this illustration, the inspiratory flow waveform is chosen to be a half sine wave. In Figure 2a, ti is the inspiration duration, te the exhalation period, and Qi the amplitude of inspiratory flow. The ventilator delivers a tidal volume equal to the area under the flow waveform in Figure 2a at regular intervals (ti + te) set by the therapist. The resulting pressure waveform is shown in Figure 2b. It is noted that during volume-controlled ventilation, the ventilator attempts to deliver the desired volume of breath, irrespective of the patient’s respiratory mechanics. The resulting pressure waveform, such as the one shown in Figure 2b, will be different depending on the patient’s respiratory mechanics. Of course, for safety purposes, the ventilator must limit the maximum applied airway pressure according to the therapist’s setting. 13  Mechanical Ventilation Systems Mandatory Ventilation: volume-controlled ventilation Figure 2: (a) Inspiratory flow for a mandatory volume-controlled ventilation breath and (b) airway pressure resulting from the breath delivery with a nonzero PEEP. 14  Mechanical Ventilation Systems Mandatory Ventilation As can be seen in Figure 2b, the airway pressure at the end of exhalation may not end at atmospheric pressure. The positive end expiratory pressure or PEEP is sometimes used to prevent the alveoli from collapsing during expiration. In other cases, the expiration pressure is allowed to return to the atmospheric level. Figure 3 shows a plot of the pressure and flow during a mandatory pressure-controlled ventilation. In this case, the respirator raises the airway pressure and maintains it at the desired level, Pi, which is set by the therapist, independent of the patient’s respiratory mechanics. Although the ventilator maintains the same pressure trajectory for patients with different respiratory mechanics, the resulting flow trajectory, shown in Figure 3b, will depend on the respiratory mechanics of each patient. As in the case of mandatory volume-controlled ventilation, the total volume of delivered breaths is monitored to ensure that patients receive adequate ventilation. 15  Mechanical Ventilation Systems Mandatory Ventilation: Pressure-controlled ventilation  Mechanical Ventilation Systems Mandatory Ventilation Owing to the need for monitoring both pressure and volume, in more recent years, new modes that combine several aspects of the volume- and pressure-controlled ventilation are devised. These modes are generally referred to as dual-control modes. One of these dual-control modes is the: Adaptive Pressure Control This new mode is a form of pressure-controlled ventilation that simultaneously keeps track of the delivered tidal volume. Figure 4 shows characteristic changes of pressure, volume, and flow of inspiratory flow in this mode. As shown in the top panel of Figure 4, for each breath, (a) through (e), the ventilator controls the inspiratory pressure to a level that may vary from breath to breath. Specifically, the ventilator controls the pressure, but also monitors the delivered tidal volume and compares it with the desired tidal volume. If the actual delivered tidal volume matches the desired level, such as in (a), then the level of controlled pressure for the next breath will be the same. However, if the next breath produced a larger than desired tidal volume, such as in (b), then the controlled pressure will be reduced in the next breath (c). Similarly, if the tidal volume falls short, such as in (d), then the controlled pressure in the next breath, (e), will be raised to a higher level to achieve the desired volume. Hence, the ventilator seeks delivering the desired tidal volume at fixed pressures that are adjusted from breath to breath. 17 Mechanical Ventilation Systems  Mechanical Ventilation Systems Spontaneous Ventilation  Mechanical Ventilation Systems Spontaneous Ventilation  Mechanical Ventilation Systems Spontaneous Ventilation  Mechanical Ventilation Systems Spontaneous Ventilation  Mechanical Ventilation Systems Spontaneous Ventilation  Mechanical Ventilation Systems Breath Delivery Control Figure 7 shows a simplified block diagram for delivering mandatory and spontaneous ventilation. Compressed air and oxygen are normally stored in high pressure tanks (≅1400 kPa) that are attached to the inlets of the ventilator. In some ventilators, an air compressor is used in place of a compressed air tank. Manufacturers of mechanical respirators have designed a variety of blending and metering devices. The primary purpose of the device is to enrich the inspiratory air flow with proper levels of oxygen and deliver a tidal volume according to the therapist’s specifications. The microprocessors are used to control the metering and blending of the breath delivery. In Figure 7, the air and oxygen valves are placed in closed feedback loops with the air and oxygen flow sensors. The microprocessor controls the valves to deliver the desired inspiratory air and oxygen flows for mandatory and spontaneous ventilation. During inhalation, the exhalation valve is closed to direct all of the delivered flows to the lungs. When exhalation begins, the microprocessor actuates the exhalation valve to achieve the desired PEEP level. The airway pressure sensor, shown on the right side of Figure 7, generates the feedback signal necessary for maintaining the desired PEEP (in both mandatory and spontaneous modes) and airway pressure during inspiratory breath delivery. 24 Mechanical Ventilation Systems Air Valve Oxygen Valve  Mechanical Ventilation Systems Defining Terms 26 Mechanical Ventilation Systems 27  Mechanical Ventilation Systems Technical Specifications From: 2003 ECRI CONTROLS Tidal volume, mL Manual sigh breath Insp flow, L/min Auto sigh breath Insp press, cm H2O Sigh volume, mL Ventilation rate, breaths/min Sigh rate Insp time, sec PEEP/CPAP, cm H2O Exp time, sec Nebulizer I:E ratio Sensitivity range, cm H2O Insp hold/plateau Trigger mechanism FiO2, % Manual norm breath OPERATING MODES EQUIPMENT ALARMS Pressure controlled Gas supply failure Volume controlled Power failure Spontaneous Vent inoperative Apnea-backup vent Low battery Self-diagnostic 28  Mechanical Ventilation Systems Technical Specifications From: 2003 ECRI MONITORED PARAMETERS INTERFACING Pressure: Displayed, Alarm Output ports Flow: Displayed, Alarm Remote alram/display Analog output Volume: Displayed, Alarm Report generation Temperature: Displayed, Alarm FiO2: Displayed, Alarm DISPLAY TYPES DATA DISPLAYED PNEUMATIC POWER ELECTRICAL POWER Voltage, AC Current, amps Watts Batteries 29  Mechanical Ventilation Systems Intensive care Ventilator From: 2003 ECRI 30  Mechanical Ventilation Systems Intensive care Ventilator - Pediatrics From: 2003 ECRI 31  Mechanical Ventilation Systems Ventilator - Portable From: 2003 ECRI 32  Mechanical Ventilation Systems Ventilator - Transport From: 2003 ECRI 33