![]() So the basic approach to give a breath can be given by four basic modes: VC-CMV, VC-IMV, PC-CMV, or PC-IMV. Patient effort can result in variations in the spontaneous breaths. With the IMV approach the patient can breathe spontaneously in between the mandatory breaths then when time comes for a mandatory breath to occur the ventilator will provide the mandatory breath. In CMV, no purely spontaneous breaths occur. With the CMV approach anytime the patient triggers the ventilator to get a breath the ventilator delivers the breath by either giving the preset volume in VC or the preset pressure in PC. Inspiratory flow in PC is always decelerating- a square flow pattern cannot be selected.Įither of these controls can be set up using a continuous mandatory ventilation (CMV) or intermittent mandatory ventilation (IMV). Note: inspiratory flow in VC can be selected by the operator to be either square or decelerating. Ventilator Modesįor most patients receiving invasive mechanical ventilation, either a preset tidal volume is used (called volume-controlled ventilation, VC) or a preset pressure is used (called pressure-controlled ventilation, PC) to deliver a breath. When pressure is added it is called Continuous Positive Airway Pressure (CPAP) when providing noninvasive support or Positive End-Expiratory Pressure (PEEP) when providing invasive support (ie the patient has an endotracheal tube or tracheostomy tube). The baseline pressure may be zero (pressure is not elevated between breaths) or elevated above zero to a positive pressure that is held in the lungs by the action of the exhalation valve in the ventilator. The cycle phase is a function of the preset inspiratory time and preset tidal volume (or flow over time to deliver a targeted tidal volume). Newer generations of ventilators can also provide a combination of fixed and variable flows in the use of dual modes such as volume-assured pressure support and pressure augmentation. Inspiratory flow delivered by the ventilator is most often either a square flow pattern where flow is at a set value (LPM) and constant or a decelerating (or ramp) flow pattern where flow starts at a high level then tapers down with no preset value for peak flow. For example: with a rate or frequency set at 10 breaths per minute (BPM) in a patient who is not making any efforts to breath, a breath will be given every 6 seconds to achieve 10 BPM. If the patient does not trigger any breaths, the ventilator will deliver breaths based on time. A third trigger is time-based on the setting for the respiratory rate. ![]() The trigger can occur by the patient’s inspiratory (negative) pressure reaching a set point or by the patient’s inspiratory flow reaching a set point. Many aspects of these four phases can be altered by changing settings on the ventilator and by use of waveforms the optimum settings can be achieved the best way to ventilate a patient and reduce asynchrony (this occurs when the actions of the ventilator and the actions of the patient are not working in harmony). Mechanical Ventilation Basicsīreaths delivered by a mechanical ventilator are defined by four phases: the trigger phase (how the breath is initiated), the inspiratory phase (mainly dealing with the flow of gas into the lungs, or how the breath gets delivered), the cycle phase (how inspiration ends and expiration begins), and the expiratory phase (mainly dealing with the baseline pressure during the period between breaths). The focus will be on the invasive approach to ventilation in adult patients where an artificial airway is used to provide an interface between the patient and the ventilator. This article will provide a look at many of the basic ventilator settings and examine how various waveforms and loops can be used to evaluate the effectiveness of mechanical ventilation in supporting the patient. ![]() Ventilator waveforms and loops are part of the standard monitoring package for all ICU ventilators but understanding what is being displayed can sometimes be difficult. ![]()
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