Mechanical ventilation supports patients in many ways, such as decreasing the work of breathing, supporting and improving gas exchange, and recruiting collapsed alveoli. These benefits are often lifesaving, but mechanical ventilation can also cause harm by opening the door for infection, contributing to muscle atrophy and ventilator dependence, contributing to an increased work of breathing, or damaging the fragile lung tissues, leading to development of complications such as pneumothorax or acute respiratory distress syndrome (ARDS). These issues point to the delicate balance of providing ventilatory support while avoiding harm. This article examines how ventilator waveforms can help achieve this balance in adults and give clues to how well the patient-ventilator system is functioning. It also discusses the effects of changes in the ventilator settings.
Basic ventilator settings: A quick review
Ventilator settings for adults include several parameters, such as the trigger mode. The trigger mode describes the way a ventilator-delivered breath is triggered or initiated.
- Pressure-triggered: A ventilator-delivered breath is initiated by the patient's negative inspiratory pressure (generated by the patient trying to initiate a breath) greater than the trigger sensitivity setting of the ventilator.1
- Flow-triggered: A decrease in a continuous flow of gas through the ventilator circuit as a result of the patient's inspiratory effort triggers a ventilator-delivered breath.
- Neurally adjusted ventilatory assist (NAVA) ventilation: An esophageal catheter incorporates electrodes to sense electrical activity of the diaphragm (patient effort) to trigger a ventilator-delivered breath.2
- Time-triggered: If the ventilator setting includes a breath rate or frequency, the ventilator-delivered breath may be triggered based on the timing of the breath relative to the frequency if the ventilator does not sense a patient's inspiratory effort as described above. For example, if the frequency is set at 12 breaths a minute, a ventilator-delivered breath will be given every 5 seconds (time-triggered). If frequency is set at 6 breaths a minute, a ventilator-delivered breath will be given every 10 seconds. If the patient is not providing any effort to breathe (apneic), the set frequency rate will determine the timing of all breaths. Patients who are making efforts to breathe can trigger more breaths above the set frequency rate. Generally, conventional ventilation in adult patients will use a flow-triggered approach as this tends to reduce the work of breathing compared with pressure triggering.3 Patient-ventilatory asynchrony exists if the phases of ventilator-delivered breaths do not match that of the patient.1 Understanding how the patient's effort triggers a ventilator-delivered breath is important. If the patient makes an inspiratory effort and the ventilator does not respond to this effort, the patient may become agitated (reflected in increased heart rate, decreased oxygen saturation, and dyspnea). Other examples of asynchrony related to initiation of a ventilator-delivered breath include double-triggering (when the ventilator delivers two breaths in rapid sequence, also known as breath stacking) and triggering by cardiogenic oscillation (during systole and diastole, the intrathoracic pressures change enough to cause a trigger for a ventilator-delivered breath).4
Pressure or volume-control
Mechanical ventilation is commonly set up to be either volume-control (VC) or pressure-control (PC) ventilation. VC ventilation delivers a set tidal volume (VT) for each mandatory ventilator-delivered breath (such as 400 mL), while PC ventilation delivers a set pressure for each mandatory ventilator-delivered breath (such as 22 cm H2O). For VC ventilation, the VT is usually set using a formula involving predicted body weight (PBW) (VT = 6 to 8 mL/kg PBW). PC ventilation is usually set with an appropriate pressure target to achieve an adequate VT (usually around 6 to 8 mL/kg PBW) and minute ventilation to have acceptable arterial blood gases—particularly the partial pressure of carbon dioxide (PaCO2), which is normally 35 to 45 mm Hg. When caring for patients with certain diseases (for example, ARDS), the desired (set) VT is reduced using a low VT ventilation strategy (also known as lung protective ventilation) 4 to 8 mL/kg with a target of 4 to 6 mL/kg PBW.5 The low VT strategy is used to reduce alveolar overdistension in patients with ARDS who require mechanical ventilation.6
Three ventilator modes
Ventilator modes include continuous mandatory ventilation (CMV), intermittent mandatory ventilation (IMV), or continuous spontaneous ventilation (CSV).
CMV, which is sometimes referred to as assist control or controlled mechanical ventilation, provides the most support: Any time a patient triggers a breath, the ventilator provides a fully supported breath. When the ventilator senses no effort from the patient, a mandatory breath is given based on the time trigger. In the CMV mode, the patient has the least amount of work because the ventilator is providing every breath either by time or by patient trigger.
In IMV, the patient can breathe spontaneously between the mandatory breaths and the mandatory ventilator breaths are delivered only when the time comes for the next mandatory breath. Note that as the time for the mandatory breath approaches in IMV, the ventilator will provide a breath if no patient inspiratory effort is sensed (time-triggered). But if the patient makes an inspiratory effort, the ventilator will synchronize with the patient's effort and provide a mandatory breath (this is why the IMV mode is sometimes called SIMV or synchronized intermittent mandatory ventilation). In the IMV mode, the patient is performing more work because of the combination of mandatory and spontaneous breaths when the patient is making efforts to breathe.
Both CMV and IMV can be set up as either VC or PC ventilation as described above. Abbreviations for these variations will be VC-CMV or PC-CMV; VC-IMV or PC-IMV.
CSV is a PC ventilator mode that has no set frequency; all breaths are spontaneous. However, a back-up frequency in the alarm settings provides ventilator-delivered breaths should a patient become apneic.
Options: PEEP and PSV
Positive end-expiratory pressure (PEEP) can be added to any mode (CMV, IMV, or CSV). Adding PEEP raises the baseline pressure from 0 cm H2O to the PEEP setting (such as 5 cm H2O) by holding air in the lungs at the end of exhalation. This increases functional residual capacity, helps keep alveoli open, and aids in oxygenation.
Pressure support ventilation (PSV) is an option that can be added only to the ventilator modes that allow for spontaneous ventilation (namely, IMV and CSV). PSV will give an extra “push” of air during a spontaneous inspiration to augment the VT. The level of the push is determined by how much pressure is selected for PSV (such as 5 cm H2O). This extra boost will, at the minimum, help overcome the resistance to breathing imposed by an endotracheal tube. As the PSV setting is increased to higher levels, more significant changes can be seen in increased minute ventilation and/or reduction in the patient's spontaneous frequency because this helps decrease the work of breathing. (See Basic ventilator settings.)
Other basic settings
Other settings include fraction of inspired oxygen (FiO2) (supplemental oxygen from 21% to 100%); frequency (breaths per minute); sensitivity (how much effort is needed to trigger a breath by either pressure or flow described earlier); inspiratory flow patterns (most often either a descending ramp or a square wave); and the alarm package to monitor issues such as high or low pressure, high or low frequency, and apnea. Cycling from inspiration to expiration (how a breath ends and exhalation begins) involves other adjustments that can be made by the clinician.
The prescription for mechanical ventilation usually includes the mode (including the control variable for PC or VC ventilation), frequency, FiO2, PEEP, and, if using a mode that allows spontaneous breathing, PSV. The other parameters described above are set by the respiratory therapist or nurse and are not usually included in the ventilator prescription. These include sensitivity, flow pattern, alarms, and cycling from inspiration to expiration.
Scalars and loops
Ventilator waveforms (also called graphics) provide a look at three aspects of mechanical ventilation: pressure (measured in cm H2O), flow (measured in L/min and showing inspiratory and expiratory flow pattern), and volume (measured in mL). The ventilator screen shows these three plotted over time (described as scalars) or may look at two plotted in relationship with each other (described as loops).
Inspiration is seen in the green tracings followed by exhalation in yellow (see Pressure, flow, and volume scalars). In A: For 4 breaths over a 12-second time frame, note that flow during inspiration is above the baseline and expiratory flow drops below the baseline. Breaths were delivered using the VC-CMV mode with a 400 mL VT, a decelerating or descending ramp inspiratory flow, frequency of 15 breaths/min, and PEEP set at 5 cm H2O. Because there are no spontaneous breaths in CMV, there is no option to provide pressure support to this mode.
PEEP can be seen on the pressure scalar (note the 0 pressure on the left side of the screen and the baseline pressure between breaths sitting at 5 cm H2O). This image was captured by using the “screen capture” or “freeze screen” function on the ventilator and scrolling backward to examine the events that occurred in the patient-ventilator system. The first breath is time-triggered. Breaths 2 and 3 were triggered by the patient, which can be seen in the small downward dip in the pressure scalar. After a short pause, the ventilator senses no patient effort, so the fourth breath is given (another time-triggered breath). Each breath is the same when examining the duration of inspiration (inspiratory time), the flow pattern (flow is set at 50 L/min), PIP, and the delivered tidal volume. PIP for each breath is about 12 cm H2O.
Note that in a VC approach (either VC-CMV or VC-IMV), changes in lung compliance will cause changes in peak inspiratory pressure (PIP) for the mandatory breaths. If compliance decreases, the lungs are getting “stiffer” and harder to ventilate. The delivered tidal volume stays the same so the PIP will be higher. Conversely, if compliance increases (the lungs are less stiff, easier to ventilate), the PIP will decrease.
In B: For 6 breaths over a 22-second time frame, breaths were delivered using the VC-IMV mode with a 400 mL VT, a descending ramp inspiratory flow, PEEP set at 5 cm H2O, and pressure support set at 5 cm H2O. The frequency is set at 10 breaths/min so a mandatory breath will occur every 6 seconds and can be either time-triggered or patient-triggered. All breaths are triggered by the patient's effort (seen in the pressure scalar with a downward dip preceding each breath) except the very last breath (6) that is time-triggered.
Breaths 1, 3, 5, and 6 are mandatory breaths. Breaths 2 and 4 are pressure-supported breaths. The mandatory breaths look identical when examining the duration of inspiration (inspiratory time), the flow pattern (flow is set at 50 L/min.), PIP, and the delivered tidal volume. The pressure-supported breath scalars show the extra “push” of air during a spontaneous inspiration. Pressure support always has a descending ramp flow and the amount of support is given above the baseline. Accordingly, these 2 breaths start at 5 cm H2O (the PEEP level establishes the baseline) and inspiratory pressure moves up to 10 cm H2O reflecting the PS setting of 5 cm H2O above PEEP. The 2 pressure-supported breaths are around 300 mL in volume.
Note that in pressure support the patient can take bigger breaths (as in taking a sigh), so spontaneous tidal volume may vary. Also, if the lung compliance changes over time, the tidal volume will change in the PS breaths. Since these PS breaths are limited by pressure, if compliance decreases the lungs are getting “stiffer” and harder to ventilate, and the volume of the spontaneous breaths will decrease. Conversely, if compliance increases, the lungs are less stiff and easier to ventilate, and the spontaneous volume will increase. This same concept applies in pressure-control modes (PC-CMV and PC-IMV). In these modes, the mandatory breaths target an inspiratory pressure setting so changing compliance will affect the delivered tidal volume.
In C: Three breaths provided by the ventilator, three breaths are provided by the ventilator but the patient has made three efforts to trigger a breath that were missed by the ventilator. The first breath is time-triggered (no effort is seen in the pressure scalar). Then the patient made three attempts to breathe (seen in the pressure scalar) with no response from the ventilator. The next breath given by the ventilator is also time-triggered as there is no relationship between the patient's attempt and the mechanical breath (note the delay between the effort and the mandatory breath). The last breath shows the patient making a big effort to trigger a breath and the ventilator responded. This example shows that the sensitivity needs to be changed so that the patient efforts result in a delivered breath. With a baseline PEEP of 5 cm H2O, the patient had to drop the circuit pressure down to 0 cm H2O to trigger a breath. This imposes an increase work of breathing that can cause agitation, increased heart rate, decreased oxygen saturation, and respiratory muscle fatigue. Compare this example to previous figures where the ventilator is sensing patient efforts and is responding to a 1 cm H2O (or less) drop from the baseline. Both flow and pressure triggering will show patient effort in the pressure scalar and each effort should be related to a response from the ventilator.
When a patient coughs, the ventilator settings are VC-IMV, VT 300mL, frequency 9 breaths/min, PEEP 5 cm H2O, PSV cm H2O, and a descending flow pattern. (See A patient who is coughing.) The first breath is a PSV breath. The second breath is a patient-triggered mandatory breath. The next tracings show multiple coughs that occurred between the time frame for a mandatory breath, and the patient has a few coughs superimposed on a mandatory breath at the end of the recorded screenshot. Note the pressure scalar: no major increases occurred with these coughs. Had the pressure during a cough been excessive and reached the PIP pressure alarm setting, the pressure alarm would sound and the ventilator would stop providing flow and cycle into exhalation to prevent possible harm to the lung (assuming the alarm setting is appropriate).
Intrinsic PEEP (PEEPi), also called occult PEEP, auto-PEEP, or air trapping, is measured in the pressure scalar (see Measurement of PEEPi in the pressure scalar). Patients with prolonged exhalation may not reach full exhalation before the next breath is delivered by the ventilator. When this happens, there is a buildup of air in the lungs that is not exhaled. To uncover this, the operator can add a brief expiratory pause that delays the next breath. During this delay, PEEPi will appear as a shift in the baseline pressure upward. In this example, PEEPi of about 6 cm H2O is detected during the expiratory pause. Patients with chronic obstructive pulmonary disease or severe asthma have a prolonged exhalation and are prone to developing PEEPi. There are many remedies for this depending on the cause. Decreasing the VT will decrease inspiratory time and increase expiratory time as well as decrease the volume of gas that must exit the lungs. Increasing inspiratory flow will get the breath in quicker and allow for a longer time to exhale. Decreasing the frequency also allows more time to exhale. In the presence of airway secretions causing prolonged exhalation, suctioning can alleviate this problem and relieve PEEPi. In cases involving bronchospasm and prolonged exhalation, administering a bronchodilator to open the airways allows for quicker exhalation. The cause of the PEEPi may be related to an artificial airway that is too small. Replacing a small endotracheal tube with a larger airway can reduce or remove PEEPi. Some patients have premature collapse of the small airways which causes PEEPi. To remedy this problem, the ventilator PEEP setting can be increased to match the amount of PEEPi. For example, when the PEEP setting is 5 cm H2O and the PEEPi is 6 cm H2O, changing the PEEP setting to 11 cm H2O keeps the collapsing airways open to allow full exhalation and the PEEPi will disappear.
Pressure-volume or flow-volume relationships are given graphically in loops. (See Pressure-volume loops.)
A delicate balance
Ventilator waveforms can help achieve the delicate balance of providing ventilatory support without causing patient harm. By providing clues about how well the patient-ventilator system is functioning, they can ensure patients get the care they need when on mechanical ventilation.
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