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= + = F*R + V/C = R*dV/dt + V/CC = 5080 / 2R = 0.020.03 2*/ ()

Pplat = Vt/C = Vt/(Pplat PEEP)Ppeak Pplat = R*F R = (Ppeak Pplat)/FF = Vt/Ti R = (Ppeak Pplat) * Ti/Vt

ComplianceBurton SL & Hubmayr RD: Determinants of Patient-Ventilator Interactions: Bedside Waveform Analysis, in Tobin MJ (ed): Principles & Practice of Intensive Care Monitoring

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Ref: Burton SL and Hubmayr RD: Determinants of Patient-Ventilator Interactions: Bedside Waveform Analysis,s in Tobin MJ (ed): Principles and Practice of Intensive Care Monitoring. New York, McGraw-Hill, Inc, 1998, p. 656.Note that the FRC rests on a favorable part of the compliance curve. Small changes in pressure result in large changes in volume. If a patients compliance is on this part of the curve, then a given tidal volume will result in low peak pressures. If the compliance worsens (i.e., moves to the left or far right) then the pressure needed to deliver that same tidal volume will increase and the PIP will increase. The same is true if a set pressure is delivered - as compliance improves, the tidal volume will increase. If compliance worsens then a smaller tidal volume will result for the same PAP. Ideally, you want the alveolus to be at the bottom inflection point (FRC) at the beginning of each breath (end of each breath). In the ventilator circuit, the peak inspiratory pressure is a combination of the resistance related pressure and the compliance related pressure. An end inspiratory pause eliminates the flow related pressure and thus reflects on the compliance related pressure.These are some of the basic terms used with ventilators. CPAP is equivalent to PEEP except the term is usually used when referring to patients who are not intubated (i.e., on nasal CPAP).2222222222Spontaneous breathing while on a ventilator has often been associated with imposed work of breathing and patient ventilator synchrony problems. The work to initiate a breath is only the first part of the total work a patient must expend during spontaneous or assisted efforts. Effective breathing coordination between patient and ventilator output requires careful attention to four additional distinct elements of a pressure based breath, especially when patient efforts or fighting are present. The first is (A) the work to trigger, followed by (B) the rise to pressure, (C) pressure overshoot and sustaining the breath, and ( D) transition to expiration. The 840 is superior over all of our competition in our ability to provide patient ventilator synchrony.2323232323Well start off by discussing the work to trigger.2525252525Once the patient has triggered the ventilator into inspiration, the next concern regarding patient comfort and coordination between the ventilator and the patient is flow acceleration percent.2626262626Although presently debated, some clinicians desire a rapid rise to pressure in PC with ARDS patients because it results in higher mean airway pressures. Others desire a less aggressive rise so as not to cause the most compliant areas of the lung to inflate more quickly potentially causing sheer forces between less compliance lung units. If PC is being used to deliver mandatory breaths to improve synchrony for patients with spontaneous efforts, adjusting rise to pressure can improve patient comfort similar to PS breaths.Literature indicates that inappropriate flow rates, too high or too low, at any time during the inspiratory phase in Pressure Support Ventilation may result in increased inspiratory muscle effort (WOB) and will increase the likelihood of patient discomfort and patient / ventilator asynchrony. Studies have looked at various indications of patient comfort. In patients with a high inspiratory drive and explosive flow demands early in the inspiratory phase, a more aggressive (faster) rise to pressure may meet inspiratory flow demands better. In patients with a more modest demand, studies have shown increased patient comfort and larger tidal volumes with a less aggressive rise. FAP can make a big difference in the NICU and PICU, and is a clinical advantage compared to other ventilators.2727272727Some clinicians may want to deliver a similar shape of pressure curve for all patients. This can easily be done by setting the FAP that creates the desired rise to pressure. Some clinicians will want to titrate FAP for individual patients. If additional adjustment is desired, the optimal FAP setting can be easily determined by analysis of the pressure time curve and patient observation for signs of increased comfort and synchrony. The 840 will allow either strategy. With FAP initial flow needs can be individualized to ensure optimal airway flow and pressure characteristics to maximize meeting the patients inspiratory flow requirements and minimize problems associated with exceeding them.Observe the minimal pressure overshoot with the FAP at 100% and the pressure relief on the flow curve, a benefit of the active exhalation valve.2828282828The higher the value of FAP%, the more aggressive the ventilator will be in its rise to meet the set target pressure. The figure depicted here illustrates pressure and flow time curves depicting an FAP with an aggressive, moderate and low rise to pressure for both pressure control and pressure support ventilation.3030303030Next we will talk about preventing pressure overshoot and and sustaining the breath as it relates to the active exhalation valve.3131313131The 840s active exhalation valve is an electromagnetic voice-coil that is designed to be active continuously during both the inspiratory and expiratory phases of ventilation. This valve maintains the target pressure, while giving the patient control of spontaneous breathing during the set inspiratory time in pressure control or pressure support ventilation. It does this by pressing on the exhalation diaphragm with the same pressure set by the clinician. If pressure rises above that setting during inspiration for any reason the excess flow is vented off and the desired pressure is maintained.The active valve allows for minimal transient overshoot resulting from low compliance or high resistance and an aggressive rise to pressure. The overshoot is minimal due in part to the FAP but is assured by the venting of excess pressure through the active exhalation valve. If the patients coughs any time during the inspiratory phase, again the valve vents the resulting pressure and a high pressure alarm is avoided.3232323232Depicted here we see spontaneous efforts occurring while the target pressure is maintained during PCV. Because the ventilator allows spontaneous efforts during the inspiratory time, the benefit may be increased patient/ventilator synchrony, which could result in a decrease in the use of heavy sedation or paralysis. Overall the advantage is the ability to delivery PCV, control MAP and PEEP, and with our active exhalation valve facilitate patient synchrony. Current opinion suggests a trend toward less aggressive use of heavy sedation and paralysis. This literature has emphasized that the added time a patient remains in ICU is proportional to how long the patient was on paralysis. The active exhalation valve is an integral part of patient ventilator synchrony during PCV as well as modes like BiLevel and APRV.3333333333In the spontaneously breathing patient in PS the next issue is transition into exhalation.343434343418Most ventilators currently on the market today terminate the inspiratory phase in Pressure Support Ventilation (PS) according to set termination criteria. This criteria is usually a percent of the peak flow for that particular breath. Clinicians have been limited in the past due to the inability to adjust this termination criteria. Expiratory Sensitivity (ESENS) defines the percentage of the projected peak inspiratory flow (VMAX) at which the ventilator terminates flow and thus cycles from inspiration to expiration during spontaneous breathing. A fixed PS termination criterion may potentially result in several clinical issues. If inspiratory flow terminates too early, it can lead to a decreased tidal volume, or increased inspiratory muscle load if the patients inspiratory effort persists after the ventilator has cycled flow off. Conversely, if inspiratory flow persists beyond patient effort which may happen when leaks are present, it can result in unnecessary expiratory work and patient/ventilator dysynchrony.3535353535Here we see a ESENS is setting of 20%, but due to a leak, the inspiratory flow continues because the termination criterion cannot be met. In the second figure ESENS has been increased to 40 %, which enables the patient to easily transition from inspiration to expiration.ESENS complements Flow Acceleration Percent. FAP should be adjusted first to match the patients inspiratory drive, thus establishing the peak flow (VMAX). The ESENS setting can then be adjusted if needed to cease inspiratory flow and cycle into exhalation at a point that is most appropriate for the patient (most patients will be comfortable with the default setting of 10%).3636363636

Simply put, the lower the ESENS, the longer the time before inspiration ends. The higher the ESENS setting, the shorter the inspiratory time. In general, the most appropriate ESENS setting is compatible with the patients condition, neither extending nor shortening the patients intrinsic inspiratory phase. ESENS can be adjusted by patient assessment. If inspiratory times appear to exceed the inspiratory efforts of the patient, a higher ESENS can be attempted to see if patient comfort improves. If the breath appears to be terminating prematurely, a lower ESENS can be set and patient comfort re-evaluated as well as its effect on tidal volume.555551211

AutoFlowWhat about new modes?There are so many modes of ventilation out there that it can get a bit confusing. The same mode can even have a different name depending on which manufacture your talking about.The initial settings for both volume and pressure are similar with one exception - you set a tidal volume or a peak pressure (or PAP). A starting tidal volume is usually 10-12 cc/kg. The starting PAP is what is needed to adequately move the patients chest and to generate breath sounds. This number can be between 15-20 mmH2O above PEEP. Note that in pressure ventilation as compliance decreases, the volume decreases.Note that in volume ventilation, as compliance decreases, pressure increases.

Note that with assist/control, every patient inspiratory effort triggers a mechanical breath.Ref: Ingento EP and Drazen J: Mechanical ventilators, in Hall JB, Scmidt GA, and Wood LDH(eds.): Principles of Critical Care. New York, McGraw-Hill, Inc., 1992, p.144.

In assist control modes (pressure control, volume control), the machine will deliver a full breath whether it is triggered by patient effort (note the negative deflection in the uppermost graph indicating patient effort) or triggered by the machine (the machine will act if a set amount of time (T) elapses without discernible patient effort).Ref: Ingento EP and Drazen J: Mechanical ventilators, in Hall JB, Scmidt GA, and Wood LDH(eds.): Principles of Critical Care. New York, McGraw-Hill, Inc., 1992, p.145.

Positive pressure, volume-cycled breaths are delivered at a preset rate similar to control mode ventilation, except that between breaths, the inspiratory valve to the patient is open, allowing for spontaneous breathing.Note the variance in volume delivery between the spontaneous breaths and the mechanical breaths Note that during SIMV, the use of a trigger allows for the synchronization of delivery of the mechanical breath with the spontaneous breath.l 4This slide summarizes the difference between SIMV and Assist/Control. Note that in SIMV, breaths between the mechanical breaths are spontaneous

In Assist/Control, every spontaneous inspiration triggers a mechanical breath. 4Note that the patient breaths spontaneously on an elevated baseline pressure. By convention, even when PEEP=0, we still call this mode CPAP1515151515This slide illustrates spontaneous breathing at PEEPH and PEEPL. It is possible to synchronize the transition from PH to PL with the patients respiratory cycle (i.e. the fall to PL synchronized with the patients expiratory phase. Also, notice at PEEPH that there is a spontaneous breath that is pressure supported

Also notice at PEEPL that the spontaneous breath that is pressure supported.1616161616The spontaneous breaths at both PEEPH and PEEPL can be Pressure Supported if the PS level is high enough to exceed PEEPH. As you wean, the delta P between PEEPH and PEEPL gets smaller which allows for more support of the spontaneous breaths at PEEPH.1818181818This is a graphical representation of BiLevel/APRV. Again you see PEEPH and PEEPL and TH and TL . As you can see the difference is in the short release time or TL. Weaning is accomplished the same, by changing the Delta P and/or frequency.All ventilators can be classified as to what begins the breath, what is controlled or limited during the breath and what terminates inspiration.