Vol. 11 •Issue 2 • Page 39
Catch the Wave
Clinical Application of Ventilator Graphics
Graphic displays have been available on mechanical ventilators since the mid-1980s, but clinicians continue to struggle with the proper application of the information provided to them.
Ventilator graphics provide valuable insight to a patient’s lung condition and patient-ventilator interaction. When used properly, graphic displays can guide the clinician to appropriate ventilator settings. In addition to illustrating the ventilator’s performance (i.e., flow volume and pressure delivery), ventilator graphics are useful in determining the presence of auto-Positive End-Expiratory Pressure, assisting in the proper use of newly available features and determining critical opening pressures for the correct application of PEEP.
AUTO-PEEP
The most useful application of ventilator graphics is to determine the presence of auto-PEEP. Auto-PEEP has been associated with dynamic hyperinflation, hemodynamic compromise, carbon dioxide rebreathing and triggering difficulty. For the bedside clinician, the recognition of auto-PEEP is an essential skill in ventilator management.
Simply put, auto-PEEP is an incomplete exhalation. Gas is trapped in the lung at the beginning of the next inspiration. Auto-PEEP is present when the lung pressure exceeds the ventilator circuit pressure at the time another breath is initiated. Because there is a gradient in pressure, flow continues and has not reached zero. For that reason, the determination of auto-PEEP is easily seen on the flow-time graphic.
Figure 1 (page 42) illustrates a normal flow-time waveform. The depiction shows a square flow delivery of 60 liters/minute during a volume breath. The portion of the graph above zero represents inspiratory flow. The portion of the graph below zero represents expiratory flow, and completeness of the expiration is evidenced by the flow returning to zero prior to the beginning of the next breath.
Figure 2 (page 42) illustrates the development of auto-PEEP. The expiratory portion of the waveform does not return to zero prior to the beginning of the next inspiration due to insufficient expiratory time. It’s important to note that auto-PEEP may develop in patients due to their lung pathology and that auto-PEEP may appear and disappear. Some studies indicate that auto-PEEP may be present in 40 percent of mechanically ventilated patients. Auto-PEEP also may be due to inappropriate ventilator settings, and its recognition by noting the expiratory flow graphic not returning to zero is a simple observation. Correction or minimizing of auto-PEEP by observation of the flow graphic while making adjustments is an essential component of ventilator management.
The most common causes of auto-PEEP are the following: a ventilator rate that results in an inadequate expiratory time for the patient’s airway resistance, endotracheal tube size or compliance. The easiest way to reduce auto-PEEP is to allow sufficient expiratory time by decreasing the set ventilator rate or by decreasing inspiratory time for a given rate, thus allowing longer exhalation. Table 1 lists the ways to minimize auto-PEEP with the adjustment of ventilator settings.
Auto-PEEP is often difficult to measure, especially in the spontaneous-breathing patient. Many newer generation ventilators have a mechanism to measure auto-PEEP by occluding the expiratory port and allowing the lung pressure to equilibrate with the ventilator circuit. This is effective except if the patient is breathing spontaneously. Under those conditions, the maneuver is aborted and no reading is obtained.
Ventilator graphics can be used to semi-quantitate the amount of auto-PEEP present. Because auto-PEEP is an incomplete or interrupted exhalation, the more of the exhalation that is present the less the auto-PEEP. By observing where in the expiratory flow waveform the interruption occurs, the clinician can observe reductions in the trapped gas. Figure 3 (page 42) illustrates the presence of auto-PEEP as a failure of the expiratory flow to return to zero. A change in flow pattern from decelerating to square reduces inspiratory time, allowing longer expiratory time. The lessening of auto-PEEP is seen by the expiratory flow getting closer to zero before the next inspiration activates.
PRESSURE CURVES
Ventilator graphics are necessary to utilize some of the new features on current generation ventilators. For pressure breaths, pressure control and pressure support, the new ventilators can control the rate of rise to the target pressure. This is called rise time, slope or flow acceleration percent, depending on the manufacturer. The control either dampens or accelerates the flow control valve, making pressure rise rapidly or delaying the reaching of the pressure until very late in the inspiration.
Figure 4 illustrates two extremes of the setting and how it is observed on the pressure-time graphic. In the pressure waveform on the left, the pressure is not reached until the inspiration is almost complete. This would indicate that the maximum flow to generate the breath is diminished and that the rise in pressure is gradual. In the pressure waveform on the right, the pressure is reached almost instantaneously as a result of a rapid increase in flow to the maximum amount available.
Clinicians can use the pressure graphic to tailor these pressure breaths to the patient and the clinical condition. A rapid rise to pressure is used to increase mean airway pressure and improve oxygenation in pressure ventilation, and a slower rise to pressure might be used to reduce flows to ventilate past obstructions or reduce turbulence in the face of high airway resistance. The pressure graphic also can be used to adjust the rise to pressure and prevent overshoot or rebound when flow is too aggressive for a given clinical situation.
PRESSURE VOLUME LOOPS
Pressure volume (PV) loops offer a variety of information crucial to ventilator management but are often misinterpreted or improperly applied. The PV loop can indicate changes in compliance and resistance and under the correct application can indicate the critical opening pressure of the lung. Volume change plotted against pressure change without respect to time generates the PV loop. Because these are the same components that are included in the compliance formula (Compliance = Volume Change/Pressure Change) the PV loop graphically depicts dynamic compliance.
Figure 5 (page 43) superimposes two PV loops with different compliance. The loop on the left illustrates the better compliance, and the loop on the right illustrates that the compliance has decreased, resulting in a higher pressure being required to deliver the same volume. Generally, dynamic compliance changes throughout a patient’s ventilator course. This is especially true in patients who develop auto-PEEP. As the trapped gas in the lung builds up, the PV loop moves farther to the right and leans over. Eventually, the trapped volume gets expelled, and the PV loop stands more upright.
Recently, PV loops have been discussed in relation to determination of the critical opening pressure of the alveoli. In the past, this type of PV loop has been constructed meticulously at the bedside by using a super syringe method. This requires the injection of a small known volume (50-100 cc), measuring the pressure and plotting it on a volume pressure graph. This method is a static determination of the PV relationship. The slope of the pressure curve changes when the lung begins to open, indicating the lower inflection point. This data is then used to set the PEEP that will keep the lung open.
It also has been shown in the literature that the lower inflection point can be obtained on the ventilator so long as the PV loop is generated with very low inspiratory flows. These flows need to be less than 10 liters/minute to remove the resistive component of the PV loop and only reflect the opening pressure. It should be noted that the slow flow determination of the opening pressure slightly exaggerates the true value.
Many clinicians have confused the slow flow maneuver with the dynamic PV loops seen during routine ventilation. The dynamic loops cannot be used to determine opening pressure because of the high flows and the resistance that shows up in the loop at the lower right corner, which is where the inflection point is also observed.
Figure 6 illustrates the difference of a slow flow inflection maneuver and the dynamic loop on the same patient. The slow flow loop performed from a 0 PEEP setting with an inspiratory flow of 10 liters shows a distinct change in the slope of the loop as the lung begins to open, indicating the correct level of PEEP that needs to be set. The dynamic loop, which in this case is delivered at 60 liters/minute, shows the bulge indicative of resistance.
These examples are just a few of the applications of ventilator graphics that can be used in the clinical management of patients. Clinicians should familiarize themselves with these and other aspects of their particular devices.
Hargett is director of respiratory care services at the University of Texas M.D. Anderson Cancer Center, Houston. Kraus is manager of respiratory care at St. Luke’s Episcopal Hospital, Houston.
