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Maintaining proper ventilation is vital in many medical conditions, and we have the technology to keep people alive and their organs functioning when no other treatments can provide them with the care they need. A ventilator does exactly what its name implies, which is to help patients breathe, through the use of tubing that supplies oxygen and removes carbon dioxide from their bodies. The technology used in medical ventilators has been steadily advancing over the years, and there are now three main types of ventilators in use today. These types include mechanical ventilators, volume-cycled ventilators, and pressure support ventilators.
Mechanical Ventilation Basics
We usually categorize breath into four phases: the trigger phase (how the breath is initiated), the inspiratory phase (mostly relating to the gas flow entering the lungs, or how the breath gets delivered), the cycle phase (how inspiration ends and expiration begins), and the expiratory phase (relating to the base pressure when in between breaths). Alterations in these four phases can be achieved through modifying settings on the ventilator, which relies on waveforms, with the best configuration possible by optimizing the ventilator and reducing asynchrony.
The trigger can occur when an inspiratory (negative) pressure or inspiratory flow reaches a set point. Another trigger occurs when the respiratory rate is set to a certain level. In the absence of a breath trigger, the ventilator will deliver breaths according to time. For example: when a patient is not making any effort to breathe, a breath will be given every six seconds to achieve 10 breaths per minute (BPM).
There are two types of ventilation flow patterns offered by ventilators: one with a fixed flow rate (LPM) and constant flow or one with a decelerating (or ramp) flow pattern.
Using dual modes such as volume-assured pressure support and pressure augmentation, newer generations of ventilators can provide both fixed and variable flows. A cycle phase is determined by the inspiratory time and tidal volume settings (or the flow rate over time). Depending on the ventilator’s action, the baseline pressure may be zero (no pressure is exceeded between breaths) or a positive pressure that is maintained in the lungs by the exhalation valve. Continuous Positive Airway Pressure (CPAP) is used when providing noninvasive support, while Positive End-Expiratory Pressure (PEEP) is used when providing invasive support (e.g., patients with tracheostomies or endotracheal tubes).
Ventilator Modes
Either a preset tidal volume is used or a preset pressure is used to provide patients with a breath when receiving invasive mechanical ventilation. inspiratory flow can be set in the VC mode to be either square or decelerating. However, inspiratory flow in PC is always decelerating – no square flow can be selected.
Either of these control options can be set up with a continuous mandatory ventilation (CMV) or intermittent mandatory ventilation (IMV). With the CMV approach, any time the patient triggers the ventilator to take a breath, the ventilator either delivers the preset volume or preset pressure, or the patient may choose to take a deep breath and self-trigger the machine for their breath. In CMV, there are no normal breaths without any association to physical exertion.
With the IMV approach, the patient breathes autonomously until a mandatory breath is called for; when it does occur, the machine takes over for a brief moment, to give the patient a rest. However, the patient is free to control his or her breaths. One way to breathe is to simply combine these four modes together: VC-CMV, VC-IMV, PC-CMV, or PC-IMV. see Fig. 1 below.
The addition of positive end expiratory pressure (PEEP) makes these four methods the ideal choices. With PEEP, the patient is still not exhaling at the end of exhalation or back to a zero pressure baseline, but exhaling early at the end with a pressure remaining in the airways. With this, patients’ FRC will increase, their airways will stay open, and their work of breathing will be eased.
The other thing to mention about pressure support ventilation is that it also allows for variation in both types of IMV or between VC-IMV or PC-IMV (depending on whether or not it also includes adding PEEP). Pressure support ventilation produces an extra boost in all of the spontaneous breaths, such that each breath can reach a preset pressure. Using CPAP (Continuous Positive Airway Pressure) increases a patient’s breathing and tidal volume (amount of air taken in when breathing) during sleep.
Beyond basic modes of delivery such as volume-assured pressure support and pressure augmentation that combine a preset volume target with a pressure approach, with the lung compliance changing, the pressure to achieve the desired volume will change.
In order to maintain a specific volume, the ventilator adjusts pressure upward as the lungs stiffen or become less compliant (like with pneumonia or fibrosis). During the fallback toward the target volume, the volume will increase as the lungs have less stiffness or greater compliance. Changes in a reservoir’s own pressure will occur gradually, and safety measures will be taken to maintain a safe pressure.
Ventilator Settings
Mechanical ventilation settings are ordered by care providers. Additional settings are established by respiratory therapists to reduce asynchrony, use alarm limits (high- or low-level alarms), and use humidification. A provider will usually order the control approach, the mode, the desired tidal volume (for VC) or inspiratory pressure (for PC), the rate or frequency (f), the desired inspired oxygen level, and added PEEP, if in the IMV mode. It is possible to order volume controlled continuous mandatory ventilation with a tidal volume of 400 mL, a frequency of 12 BPM, 60% oxygen per breath, and a PEEP of 8 cmH2O. Here is an abbreviated version:
VC-CMV, VT 400 mL, f-12, FiO2 .60, + 8 cmH2O PEEP
Below is a proposed plan for pressure controlled intermittent mandatory ventilation with a peak inspiratory pressure of 20 cmH2O, a frequency of 14 BPM, 40% oxygen for each breath, an addition of 5 cmH2O PEEP, and 5 cmH2O pressure support.
PC-IMV, PIP 20, f-14, FiO2 .40, + 5 cmH2O PEEP, +5 PS
All other conditions required for appropriate ventilation, necessary for the therapy of respiratory problems, will be arranged by the therapist handling the patient.
Ventilator Waveforms
Flow, pressure, and volume scalars are indicators of changes in these variables over time, and can be displayed by themselves or in combination (either two or all three). The following 3 variables occur simultaneously during a breath. The screen can display several breaths and allow observation of ventilation trends over time with a slow “sweep” speed. When the sweep speed is fast, there will be fewer breaths (perhaps even just one breath), and the delivery of each breath can be examined in greater detail. When looking at the flow, pressure, and/or time scalars, many ventilators give the operator the option of freezing the display. Otherwise, the screen will update over time with the current view.
For a more detailed view and explanation of ventilator waveforms, see Examples 1-8 below.
Conclusion
With scalars and loops, ventilation graphics can be used to visually assess the patient-ventilator system and reveal potential problems. The skills needed to recognize problems can be sharpened by studying examples such as those in this article. Changing variables such as sensitivity, inspiratory flow and volume, pressure support, PEEP, breath rate, and other settings can reduce work of breathing, reduce potential damage from mechanical ventilation, and improve patient comfort.
Various ventilation and ventilator waveforms have been discussed in this article; books, web-based materials, and publications from ventilator manufacturers go deeper into explaining how graphics can help ventilate a patient effectively and safely.
Examples
Example 1. Scalars for Volume-Controlled Continuous Mandatory Ventilation (VC-CMV) with PEEP.
It can be seen from the three graphs how inspiration, shown in green, relates to exhale, shown in yellow, as well as an inhalation represented by a rectangular sine wave on the central graph. The labels of inspiration and expiration are below a line. On the side, there is three markers on the line for breathing. A drop from baseline after a patient pause shows time trigger. Breath B is time-triggered. (FiO2 is not shown in this illustration) settings are VC-CMV, 450 mL, f – 6 BPM, +5 cmH2O PEEP. The scale at the bottom represents tidal volume. In VC-CMV all patient efforts that are sensed by the ventilator will result in delivery of the same set tidal volume for breathing.
Example 2. Scalars for VC-CMV with PEEP, decelerating or ramp flow.
There is a decelerating (ramp) wave inspiratory flow shown in the middle scalar. Inspiration is above the baseline and expiration is below. The first two breaths are triggered by the patient’s inspiratory effort, which can be seen in the pressure scalar (on top), indicated by letter A. This illustration shows the settings for the third breath: VC-CMV, 450 mL, FC-6, +5 cmH2O PEEP, f-6 BPM, and the last scalar shows the tidal volume. When VC-CMV is used, all patient efforts will trigger delivery of a set tidal volume to the patient and any time-triggered breaths will also trigger delivery of a set tidal volume.
Example 3. Prolonged exhalation.
Observe the duration of the exhalation in this example. Chronic obstructive pulmonary disease or severe bronchospasm may result in a prolonged expiratory time. There is a time trigger for the first breath, and a patient trigger for the second breath. The prolonged exhalation might be exacerbated by secretions in the airways that interfere with the process. Air-trapping or auto-PEEP might occur if this patient has a higher respiratory rate. To correct this, you could shorten the inspiratory time setting, give bronchodilation medication, suction secretions, decrease the set breath rate, or replace a smaller ET tube with a larger one.
Example 4. Spontaneous Breathing in VC-IMV with PEEP and no PSV.
In the figure, there are 6 breaths, all of which are caused by the patient’s own efforts, as evidenced by drops in the baseline pressure before each inhalation. PEEP has been set to 5 cmH2O, and each spontaneous inhalation has a tidal volume of about 100-150 mL. But each mandatory, intermittent breath consists of 2 and 6, which are delivered with a slowing airflow and a 450 mL tidal volume.
Example 5. VC-SIMV with PEEP and Pressure Support Ventilation of 5 cmH2O.
There are three patient-triggered breaths in this illustration. PEEP is set at 5 cmH2O. Breath 1 and 3 are spontaneous breaths boosted to 400 mL with 5 cmH2O pressure support. Breath 2 is an intermittent mandatory breath that peaks at 16-17 cmH2O PEEP.
Example 6. Patient effort not being sensed by the ventilator.
One breath is time-triggered (the first) and five breath attempts are attempted by the patient (shown in the pressure scalar). Only one breath is delivered from the five efforts. There is an asynchrony in the mandatory breaths in which the patient’s breathing is squared and the tidal volume is 450 mL. There is a PIP of 16-17 cmH2O and a PEEP of 5 cmH2O. This asynchrony makes breathing harder for the patient. Patient efforts will result in a breath being delivered if sensitivity is set lower than normal.
Example 7. Normal Pressure-Volume loop and Flow-Volume loops.
A pressure-volume loop is shown in the top graphic and a flow-volume loop is shown in the bottom graphic. Inspiration is shown in green, exhalation is shown in yellow. A test lung was ventilated using VT 450 mL, square flow waveforms, and 5 cmH2O PEEP.
Example 8. Pressure-Volume loop and Flow-Volume loops with overdistension
A patient with an overinflated lung is shown here, due to a large tidal volume and an overshoot in pressure. The tidal volume is set at 680 mL, and the overdistension is seen near the end of inspiration in the P-V loop. A rapid increase in pressure is accompanied by a much smaller increase in volume, resulting in a “bird’s beak” appearance. It can cause barotrauma (due to the high peak inspiratory pressure) and volutrauma (due to the large tidal volume) to the lungs. The tidal volume delivered to the lungs can be reduced to correct this.
