Mechanical Ventilator 2 Lecture (6)

Modes of Mechanical Ventilation A- Volume Modes B- Pressure Modes

A- Volume Modes 1- Assist-control (A/C) 2- Synchronized intermittent mandatory ventilation (SIMV)

1- Assist Control Mode A/C The ventilator provides the patient with a pre-set tidal volume at a pre-set rate . The patient may initiate a breath on his own, but the ventilator assists by delivering a specified tidal volume to the patient. Patient can initiate breaths that are delivered at the preset tidal volume. Patient can breathe at a higher rate than the preset number of breaths/minute

The total respiratory rate is determined by the number of spontaneous inspiration initiated by the patient plus the number of breaths set on the ventilator. If the patient wishes to breathe faster, he or she can trigger the ventilator and receive a full-volume breath.

2- Synchronized Intermittent Mandatory Ventilation (SIMV) The ventilator provides the patient with a pre-set number of breaths/minute at a specified tidal volume. In between the ventilator-delivered breaths, the patient is able to breathe spontaneously at his own tidal volume and rate with no assistance from the ventilator.

B- Pressure Modes 1- Pressure-controlled ventilation (PCV) 2- Pressure-support ventilation (PSV) 3- Continuous positive airway pressure (CPAP) 4- Positive end expiratory pressure (PEEP) 5- Noninvasive bi-level positive airway pressure ventilation (BiPAP)

1- Pressure-Controlled Ventilation Mode ( PCV) The PCV mode is used: If compliance is decreased. It is used when the patient has persistent oxygenation problems The inspiratory pressure level, respiratory rate, and inspiratory–expiratory (I:E) ratio must be selected.

In pressure controlled ventilation the breathing gas flows under constant pressure into the lungs during the selected inspiratory time. The flow is highest at the beginning of inspiration( i.e when the volume is lowest in the lungs). As the pressure is constant the flow is initially high and then decreases with increasing filling of the lungs. Like volume controlled ventilation PCV is time controlled.

Inverse ratio ventilation (IRV) mode reverses this ratio so that inspiratory time is equal to, or longer than, expiratory time (1:1 to 4:1). Inverse I:E ratios are used in conjunction with pressure control to improve oxygenation by expanding stiff alveoli by using longer distending times, thereby providing more opportunity for gas exchange and preventing alveolar collapse.

3- Pressure Support Ventilation ( PSV) The patient breathes spontaneously while the ventilator applies a pre-determined amount of positive pressure to the airways upon inspiration. Helps to overcome airway resistance and reducing the work of breathing. In PSV mode, the inspired tidal volume and respiratory rate must be monitored closely to detect changes in lung compliance.

4- Continuous Positive Airway Pressure (CPAP) Constant positive airway pressure during spontaneous breathing CPAP allows the nurse to observe the ability of the patient to breathe spontaneously while still on the ventilator.

5- Positive end expiratory pressure (PEEP) Positive pressure applied at the end of expiration during mandatory \ ventilator breath positive end-expiratory pressure with positive-pressure (machine) breaths.

6- Noninvasive Bilateral Positive Airway Pressure Ventilation (BiPAP) BiPAP is a noninvasive form of mechanical ventilation provided by means of a nasal mask or nasal prongs, or a full-face mask. The system allows the clinician to select two levels of positive-pressure support: An inspiratory pressure support level (referred to as IPAP) An expiratory pressure called EPAP (PEEP/CPAP level).

The ventilator pneumatic system This device is primarily used in intensive therapy to help improve the patients breathing by regulating the flow of gas in the lungs. The most common indicators of the ventilation are the absolute volume and changes of volume of the gas space in the lungs achieved during a few breathing maneuvers.

This system requires a set of sensors for pressure, volume, and flow This system requires a set of sensors for pressure, volume, and flow. The information from the sensors modulates the operations in the MCU. This MCU receives information from the airways, lungs, and chest wall through the sensors, and decides how the ventilator pumps.

The pneumatic system has two air supplies that can be oxygen and air, and can come from a pressurized tank or compressor.

Both sources are regulated by two input valves to control mixture composition, which comes from an air tank where the mixture is kept at certain pressure limits.

If the mixture composition is correct and is in the right pressure range, the system sends this air to the patient to control breathing.

For this system has an input and output valves connected to lungs simulator to control pressure in lungs and the respiratory frequency to maintain patient safety. The signal that shows lung volume is a differential signal, but this is not the signal measured directly from the lungs. To get this signal, it is necessary to transduce the pressure to voltage. This is done by using a pneumo-tachometer that contains a pressure sensor.

Lung volume measurements Tidal volume (TV)—Amount of gas inspired or expired with each breath. Inspiratory reserve volume (IRV)—Maximum amount of additional air that can be inspired at the end of a normal inspiration. Expiratory reserve volume (ERV)—Maximum volume of additional air that can be expired at the end of a normal expiration Residual volume (RV)—The volume of air remaining in the lungs after a maximum expiration

Total lung capacity (TLC) =RV+IRV+TV+ERV These measurements can be used in the following equations to express lung capacities: Total lung capacity (TLC) =RV+IRV+TV+ERV