Ventilator graphics constitute a valuable tool and a thorough understanding of the associated patterns, problems, and corrections will help RCPs provide high-quality care .

Mechanical ventilation often involves patients who have complicated medical and/or surgical problems that create major challenges for the health care team that is trying to manage the patient-ventilator system. Critical care for these patients, from the premature infant to the trauma victim to the patient with burns and smoke-inhalation injuries, includes deciding which ventilator settings to use. As mechanical ventilators evolved, many began to include displays of graphic information, either as an option or as standard equipment. Graphics can aid clinicians in deciding which ventilator mode is most appropriate, or in fine-tuning the settings for a given mode in order to achieve the best combination for the patient. Beyond the RCP, few members of the health care team have the knowledge and understanding needed to apply the information given in a graphic in order to make the right ventilator adjustments. This situation makes the RCP the resident expert and makes learning about this application a high priority for the RCPs who work in the intensive care environment.

Ventilator Waveforms
Waveforms usually plot one of three parameters (pressure, flow, or volume) against time. Time is plotted on the horizontal (x) axis and the other parameter is plotted on the vertical (y) axis. The ventilator has default settings for the scales used to depict the graphics; sometimes these scales need to be changed in order for the user to see certain details of a breath or to see several breaths on the screen for comparison. Figure 1 shows a pressure-time graphic of three breaths. The first breath (A) shows a negative deflection, representing the patient’s triggering of the breath. The ventilator sensitivity is set so that an inspiratory effort by the patient will trigger the delivery of a breath. The second breath (B) is not triggered by the patient, but is, instead, initiated by the machine (time triggered). The third breath (C) shows an improperly set sensitivity; the patient is having to generate a large negative inspiratory pressure before the trigger point is reached and a breath is given. This increases the work of breathing and is very uncomfortable for the patient. Making the machine more sensitive will correct this problem. The patient’s inspiratory effort should result in an immediate flow of gas. The usual sensitivity setting for a pressure-triggered breath is –1 to –2 cm H2O. It should be noted that if the inspiratory flow is inadequate, the patient may generate a large negative inspiratory effort and continue pulling as the breath is being delivered. This will make the pressure waveform look irregular and nonlinear as the breath is given. Increasing the flow should correct this. If the pressure-time waveform shows a breath-triggering problem not related to sensitivity, the RCP should consider checking the gas source, the driving pressure, and the machine calibration. Time, pressure, and flow are the most common triggering mechanisms.¹

Figure 1. Pressure-time of three breaths. Figure 2. Flow patterns of volume-control ventilation.

Flow-delivery waveforms are the next parameters to consider. Flow delivery can be set as square (rectangular), ascending ramp, descending ramp, sine (sinusoidal), or decay (exponential). Most ventilators will give a choice of two or three of these, but this depends on the mode of ventilation, since some flow patterns are preset due to the characteristics of the breath delivery. For example, volume-control ventilation may have several choices of flow patterns including square, ascending ramp, descending ramp, and sine wave, while pressure-control ventilation uses a descending ramp or decaying flow pattern. See Figure 2 for examples of these flow patterns.

Modes of ventilation
Modes of ventilation are generally volume control or pressure control, and either of these modes will give clinicians the option of augmenting a spontaneous breath between the mandatory breaths by using pressure support. Mandatory breaths occur when either the patient or the machine triggers the breath to start and the breath itself is cycled into expiration by the machine. Spontaneous breaths occur when the patient initiates the breath and cycles the breath into expiration. Pressure-support ventilation augments the spontaneous breaths by adding flow (in a decelerating pattern) to reach a preset inspiratory pressure; this results in an increased tidal volume. Pressure support is available only in those modes that allow for spontaneous breaths. Positive end-expiratory pressure (PEEP) is one other common addition to volume-control and pressure-control modes.

Figure 3 shows a side-by-side comparison of the pressure-time, volume-time, and flow-time waveforms for volume-control versus pressure-control ventilation over four breaths. Both examples show that the ventilator settings include 10 cm H2O of PEEP, shown by the baseline tracing at +10 on the pressure-time waveforms. On both tracings, the first and last breaths are mandatory, the first breath is time triggered, and the last three breaths are patient triggered (as seen in the triggering deflection on the pressure-time waveform). Pressure support of 20 cm H2O is being delivered during the two spontaneous (second and third) breaths).

Figure 3. Volume-control vs pressure-control ventilation over four breaths.

When examining pressure-time waveforms for either volume-control or pressure-control ventilation with the addition of PEEP, clinicians should notice the baseline pressure between breaths; it should be fairly flat. If the baseline pressure drifts downward, there may be a leak in the system (at the exhalation valve, at a connection in the ventilator circuit, or around the endotracheal tube). Loss-of-PEEP (or low-PEEP) alarms may alert the RCP to the problem, but this may not occur if the leak is small or the alarm setting is too lenient. The baseline may show slight movement up and down due to the heartbeat (cardiac oscillation). A difference between delivered tidal volume and measured exhaled tidal volume or a variation in the volume-time waveforms comparing similar types of breaths (two mandatory, time-triggered breaths) may also point to a leak in the system.

Figure 4. Volume- and flow-time waveforms during the expiratory phase of the breath.

Finding AutoPEEP
AutoPEEP (also called intrinsic PEEP) and air trapping are other problems that can be uncovered by examining the waveforms. AutoPEEP often happens in patients with high respiratory rates or high minute volumes, or when PEEP settings are at 10 cm H2O or higher.² The basic problem occurs when inspiration starts before the end of the previous breath (before a completion of exhalation).³ When autoPEEP is present, there will be an increase in the work of breathing when the patient initiates a breath. This occurs because the patient must create a larger negative pressure (or negative flow) to reach the set trigger point. Work of breathing is also increased as autoPEEP causes the diaphragm to be flattened, reducing the effectiveness of the muscle’s contraction. The volume-time and flow-time waveforms show this problem during the expiratory phase of the breath, as shown in Figure 4 (page 50). At the end of exhalation, the volume-time waveform approaches the baseline then starts upward immediately with the next breath. Conversely, at the end of exhalation on the flow-time curve, there is an abrupt movement up to the baseline and an immediate starting of inspiratory flow for the next breath.

When the problem of autoPEEP is seen on the ventilator’s waveforms, the RCP needs to consider several possible causes and remedies. The patient may need suction in order to clear obstructing secretions out of the airways, or it may be time for a bronchodilator treatment, which can increase airway diameter. More air is exhaled as a result of these actions, reducing the trapped air. Increasing the flow rate, decreasing the inspiratory time, or decreasing the tidal volume can prolong expiratory time and allow for more exhalation. Other possibilities include decreasing the breath rate while increasing the tidal volume, moving to a larger endotracheal tube, or changing to a different mode of ventilation.

Airway collapse may also be the cause of autoPEEP. In this situation, adding PEEP can help prop or splint the airways open and stop the air trapping. Patients with chronic obstructive pulmonary disease are more prone to have this problem as the normal supporting structures in the lung are weakened or destroyed by the effects of the disease. The amount of PEEP to add should be determined by having an expiratory pause or hold at the end of exhalation and observing the airway-pressure measurement; as it stabilizes, it will show the amount of autoPEEP or intrinsic PEEP. The RCP should set the PEEP level at no more than 85% of the measured autoPEEP level and should be sure to adjust the low-PEEP alarm to the appropriate level.⁴ It should be kept in mind that adding PEEP can present other problems related to barotrauma, decreased venous return, decreased cardiac output, and increased hyperinflation.

Figure 5. Patient-triggered breath. Figure 6. Decelerating-ramp flow pattern on a flow-volume loop.

Using Loops
Loops allow the practitioner to analyze the inspiratory and expiratory phases of each breath using either flow-volume or pressure-volume tracings. On the flow-volume loop, volume is plotted on the x axis and flow on the y axis. Positive flow from a positive-pressure breath often appears above the horizontal axis, with expiratory flow below the axis, but this pattern may be reversed, depending on the ventilator being used. In the examples given here, positive flow from the ventilator (during inspiration) will be above the horizontal axis and negative flow (during exhalation) will be below the axis. On most pressure-volume loops, the pressure is plotted on the x axis; volume, on the y axis. Patient-triggered breaths will look different from time-triggered or machine-triggered breaths on the pressure-volume loops as the patient generates a negative pressure at the beginning of inspiration. Figure 5 (page 50) shows a patient-triggered breath and the resulting pressure-volume loop that traces the inspiration and exhalation. Figure 6 shows a decelerating-ramp flow pattern on a flow-volume loop. It shows the rapid increase in flow of early inspiration reaching peak flow, then decreasing to the end of inspiration and reaching zero flow. There is no time factor in these tracings, and exhalation follows immediately after the inspiratory phase on each of these loops.

Studies are under way using the pressure-volume loop to evaluate PEEP and peak inspiratory pressure (or mandatory tidal volume) settings. A point can sometimes be determined, early in the inspiratory phase, at which there is a change in the slope of the line that shows a more rapid increase in volume per unit of pressure. This is the lower inflection point. In the pattern of a typical pressure-volume loop on inspiration (with no PEEP added), the lower inflection point is thought to show the point at which alveoli begin to fill rapidly and alveolar recruitment begins. Some have recommended setting the PEEP level just above the lower inflection point, but this point can change (depending on inspiratory flow, with higher flows being related to a lower inflection point that is also higher).⁵ At the other end of the inspiratory tracing on the pressure-volume loop, overdistension from too great an inspiratory volume will show up as a bird-like beak as the lung’s maximum volume is reached in the face of continued inspiratory flow. The point at which this line begins to flatten and form the beak is the upper inflection

Figure 7. Lower and upper inflection points for a delivered volume. Figure 8. The beak represents overdistension as too much volume is delivered.

point. Figure 7 shows the lower inflection point (with tracings showing how this changes with increasing flow) and the upper inflection point for a delivered volume that is at the maximum setting (overdistension would begin to show up if delivered volume were increased). Figure 8 shows the beak representing overdistension as too much volume is delivered. In this situation, the volume needs to be reduced to avoid the problems related to overdistension (barotrauma, volutrauma, decreased venous return, and decreased cardiac output).

Comparisons of flow-volume loops can help assess the effectiveness of a bronchodilator. In patients with obstructive disease, the prebronchodilator line shows a “scooped-out” pattern on the expiratory side representing decreased expiratory flows and airway obstruction. Following the bronchodilator, the scooped-out appearance will often change to a more linear shape from peak expiratory flows down to the end of exhalation, which reflects the positive effect of the bronchodilator in relieving the obstruction.⁶

If the ventilator is delivering a decelerating flow, but the flow-volume loop shows a flattened inspiratory flow (similar to that of a flow-limited breath), there may be something that is artificially limiting flow. In this situation, the RCP should check for a bent or kinked endotracheal tube, tube occlusion (possibly because the patient is biting the tube), a saturated heat-moisture exchanger, or an occluded expiratory filter.

As the RCP becomes more familiar with ventilator graphics, the more unusual and difficult waveforms and loops will be easier to understand and correct. Patient comfort and the effectiveness of ventilation are two important aspects of care that can be improved using the information provided by the graphics monitor. If ventilator graphics, waveforms, and loops are unfamiliar,the RCP can try using a test lung with a ventilator that has graphics and observing what happens as changes are made in PEEP, flow, inspiratory time, the ratio of inspiration to expiration, pressure support, tidal volume, and mode. The RCP should become familiar with the screens, how to change the sweep time, what is involved in setup, and how to access and change the appearance of the trend information. Then the RCP should go to the bedside and observe the patient-ventilator system in conjunction with the breathing pattern. Ventilator graphics constitute a valuable tool, and a thorough understanding of the associated patterns, problems, and corrections will help the RCP provide high-quality, effective care.

William C. Pruitt, RRT, is instructor, Department of Cardiorespiratory Care, University of South Alabama, Mobile.

References
1. Branson R, Hess D, Chatburn R. Respiratory Care Equipment. 2nd ed. Philadelphia: Lippincott Williams & Wilkins; 1999.
2. Scanlon C, Wilkins R, Stoller J. Egan’s Fundamentals of Respiratory Care. 7th ed. St Louis: Mosby; 1999.
3. Ouellet P. Waveform and Loop Analysis in Mechanical Ventilation. Solna, Sweden: Siemens-Elema; 1997.
4. Branson R, Hess D, Chatburn R. Respiratory Care Equipment. 2nd ed. St Louis: Mosby; 1999.
5. Haas C. Volume-pressure curves during mechanical ventilation. AARC Times. 2000;24:64-68.
6. Pilbeam S. Mechanical Ventilation: Physiological and Clinical Applications. 2nd ed. St Louis: Mosby; 1998.

For further reading
Rittner F, Döring M. Curves and Loops in Mechanical Ventilation. Telford, Pa: Draeger Medical; 1996.
Waugh J, Deshpande V, Harwood R. Rapid Interpretation of Ventilator Waveforms. Upper Saddle River, NJ: Prentice-Hall; 1999.