Using Ventilator Graphics to Identify Patient …

1 Using Ventilator Graphics to Identify Patient - Ventilator AsynchronyJon O Nilsestuen PhD RRT FAARC and Kenneth D Hargett RRTI ntroductionTrigger Asynchrony (Phase 1)Trigger Asynchrony Can Occur in Any Ventilation ModeFlow Asynchrony (Phase 2)Volume Ventilation With a Fixed Flow PatternPressure Ventilation With Variable FlowTermination Asynchrony (Phase 3)Delayed TerminationPremature TerminationExpiratory Asynchrony (Phase 4)SummaryPatient- Ventilator interaction can be described as the relationship between 2 respiratory pumps: (1) thepatient s pulmonary system, which is controlled by the neuromuscular system and influenced by themechanical characteristics of the lungs and thorax, and (2) the Ventilator , which is controlled by theventilator settings and the function of the flow valve. When the 2 pumps function in synchrony, everyphase of the breath is perfectly matched. Anything that upsets the harmony between the 2 pumps resultsin asynchrony and causes Patient discomfort and unnecessarily increases work of breathing.

2 This articlediscusses asynchrony relative to the 4 phases of a breath and illustrates how asynchrony can be iden-tified with the 3 standard Ventilator waveforms: pressure, flow, and volume. The 4 phases of a breathare: (1) The trigger mechanism (ie, initiation of the inspiration), which is influenced by the trigger-sensitivity setting, Patient effort, and valve responsiveness. (2) The inspiratory-flow phase. During bothvolume-controlled and pressure-controlled ventilation the Patient s flow demand should be carefullyevaluated, Using the pressure and flow waveforms. (3) Breath termination (ie, the end of the inspiration).Ideally, the Ventilator terminates inspiratory flow in synchrony with the Patient s neural timing, butfrequently the Ventilator terminates inspiration either early or late, relative to the Patient s neuraltiming. During volume-controlled ventilation we can adjust variables that affect inspiratory time (eg,peak flow, tidal volume).

3 During pressure-controlled or pressure-support ventilation we can adjustvariables that affect when the inspiration terminates (eg, inspiratory time, expiratory sensitivity). (4)Expiratory phase. patients with obstructive lung disease are particularly prone to developing intrinsicpositive end-expiratory pressure (auto-PEEP) and therefore have difficulty triggering the evaluation for the presence of auto-PEEP should be routinely performed and corrective ad-justments made when words: Ventilator Graphics , waveforms, asynchrony, Patient -venti-lator interface.[Respir Care 2005;50(2):202 232. 2005 Daedalus Enterprises]IntroductionVentilator Graphics are available on almost all currentmechanical ventilators and have been available for evalu-ating the Patient - Ventilator interface for more than a de-cade. Both direct and anecdotal evidence (see below), how-ever, suggests that bedside use of ventilators graphicscapabilities is widely underutilized, and standard ap-proaches or guidelines for Graphics interpretation are FEBRUARY2005 VOL50 NO2In a recent Joint Commission on Accreditation of HealthCare Organizations report on Ventilator safety,1humanerror was identified as an important factor.

4 Further inves-tigation of the reported errors identified orientation andtraining as the most frequent root cause of problems ( ). In addition, only about one third of the members of therespiratory care profession who are responsible for thebedside monitoring of patients receiving mechanical ven-tilation subscribe to the RESPIRATORYCAREJ ournal, whichindirectly suggests that some information regarding graph-ics monitoring is probably not widely distributed. One ofthe reasons for the current Journal Conference was to gathergraphical information, because it is not widely publishedor synthesized in the recent literature. Also, during ourpresentations on Ventilator waveforms at the InternationalRespiratory Congresses in recent years, we used an audi-ence-participation feedback system to test the audiencewith waveform-analysis questions (unpublished data). Thecorrect response rate ranged from 25% to 75% for fairlysimple Graphics analysis questions, which we believe sug-gestswidedifferencesinthegeneralunde rstandingofwave-form describing the interaction between the Patient and theventilator, Kondili et al2offered an insightful definition ofpatient- Ventilator synchrony/asynchrony: During mechan-ical ventilation the respiratory system is affected by 2pumps: the Ventilator controlled by the physician [or cli-nician] and the Patient s own respiratory muscle interaction is an expression of these 2controllers, which should be in harmony if the result is tobe appropriate for the Patient .

5 Though the number of additional factors affecting eachof the 2 controllers is fairly substantial (Table 1), the basicdescription of the 2 pumps being in harmony provides aclear goal for what Patient - Ventilator synchrony should be;therefore, anything that falls short of that results in somedegree of asynchrony, which deserves analysis and cor-rection. Failure to make appropriate adjustments to im-prove synchrony can cause various complications and del-eterious consequences for the Patient (Table 2).The present article reviews Graphics methods for recog-nizing Patient - Ventilator asynchrony and describes how graph-ics can be used to improve Patient - Ventilator O Nilsestuen PhD RRT FAARC is affiliated with the Department ofRespiratory Care, University of Texas Medical Branch, School of AlliedHealth Sciences, Galveston, Texas. Kenneth D Hargett RRT is affiliatedwith the Department of Respiratory Care, The Methodist Hospital, TexasMedical Center, Houston, O Nilsestuen PhD RRT FAARC presented a version of this article atthe 34th RESPIRATORYCAREJ ournal Conference, Applied RespiratoryPhysiology: Use of Ventilator Waveforms and Mechanics in the Man-agement of Critically Ill patients , held April 16 19, 2004, in Cancu n, : Jon O Nilsestuen PhD RRT FAARC, Department ofRespiratory Care, University of Texas Medical Branch, School of AlliedHealth Sciences, 301 University Boulevard, Galveston TX : 1.

6 The primary causes of Ventilator -related deaths, 1995 2003.(Adapted from Reference 1, with permission.)Table 1. Factors That Affect Patient - Ventilator SynchronyVentilator FactorsTrigger variables: esophageal pressure, flow, or shape signalSensitivity settingRise-time capabilityDesign, mode, and settings of the flow delivery systemFlow pattern selectedDesign of the exhalation valveHow positive end-expiratory pressure is generated by the softwareExtraneous flow (eg, from a nebulizer or added oxygen) Patient FactorsSedation level: pain, splintingInspiratory effort/respiratory drive; neural timingPathology of the respiratory system or abdomen; secretionsIntrinsic positive end-expiratory pressureSize and type of airwayPresence of leaksTable 2. Deleterious Effects of Patient - Ventilator AsynchronyPatient fights the ventilatorMore sedation requiredHigher work of breathingMuscle damageVentilation-perfusion problemsDynamic hyperinflationDelayed or prolonged weaningLonger stayHigher costsUSINGVENTILATORGRAPHICS TOIDENTIFYPATIENT-VENTILATORASYNCHRONYRE SPIRATORYCARE FEBRUARY2005 VOL50 NO2203 The use of guidelines, standards, and protocols for as-sessing and treating disease states improves Patient , a standard approach to analysis of ven-tilator waveforms should improve Patient comfort, reducework of breathing (WOB), and perhaps improve of Patient - Ventilator synchrony can be brokendown into 4 phases (Fig.)

7 2): evaluation of triggering; eval-uation of adequate flow delivery; evaluation of breath ter-mination; and evaluation of intrinsic positive end-expira-tory pressure (auto-PEEP), which is the primary clinicalcomplication associated with the expiratory phase. For or-ganization purposes and as a methodical approach for cli-nicians to use at the bedside, the present article is orga-nized according to these 4 Asynchrony (Phase 1)Definition: Trigger. The trigger variable is defined asthe variable that is manipulated to deliver inspiratory flow. 4 Although triggering composes only a small part of the entireinspiratory cycle, inappropriate setting or design may increasethe Patient s effort and inspiratory muscle work. 4,5 In a demand-flow system (pressure-trigger), the trig-ger variable is a set pressure that must be attained at theonset of inspiration for the Ventilator to deliver freshgas into the inspiratory circuit.

8 4 Most microprocessor-based ventilators use pressure-triggering to initiate boththe mandatory breaths (assist-control and synchronizedintermittent mandatory ventilation) and spontaneousbreaths (continuous positive airway pressure, synchro-nized intermittent mandatory ventilation, pressure sup-port ventilation).4 Definition: Trigger term has beendefined as muscular effort without Ventilator trigger. 6 Though this definition describes the problem when patienteffort fails to trigger the Ventilator , we will also discussseveral additional triggering problems: double-triggering,Fig. 2. Airway pressure (top), flow (middle), and volume (bottom) waveforms from a normal subject during synchronized intermittentmandatory ventilation with pressure support. The 4 phases of the breath are numbered. Phase 1 is the initiation of Patient effort, whichindicates achievement of the trigger threshold (2 cm H2O) that opens the inspiratory valve.

9 Phase 2 represents the relationship between flowdelivery, as determined by the Ventilator s flow algorithm, and the Patient effort (the first and third breaths are pressure support breaths,in which flow is partially dependent on Patient effort, and the second breath is the mandatory breath, which has a constant-flow pattern).Notice the scooped-out appearance of the pressure waveform during the mandatory breath, which indicates that the inspiratory flow wasinadequate. Phase 3 is the breath-termination point, which varies based on the type of breath; for the middle breath the inspiratory timeis set on the Ventilator , but the inspiratory time for the pressure support breath is based on the termination criterion, which in this case is5% of the peak flow. Phase 4 is the expiration portion of the breath. During this phase the breath should be inspected for evidence ofintrinsic positive end-expiratory pressure (auto-PEEP).

10 This expiratory flow waveform returns to zero prior to the next breath, whichindicates the absence of TOIDENTIFYPATIENT-VENTILATORASYNCHRONY20 4 RESPIRATORYCARE FEBRUARY2005 VOL50 NO2auto-triggering, and insensitive trigger (triggering that re-quires excessive Patient effort).Trigger asynchrony is only one type of problem asso-ciated with a Patient fighting the Ventilator . Though bed-side clinicians are inclined to think of trigger problems asbeing associated primarily with the sensitivity setting onthe Ventilator , the definition has been expanded to addressother variables that are influenced by the Patient s inspira-tory effort or respiratory drive, and the rate at which theventilator supplies gas to the circuit (Fig. 3).7 These vari-ables include (1) the traditional trigger pressure or valvesensitivity, which can be adjusted by the clinician; (2) thepressure maximum, which is the most negative pressure orlargest downward deflection in the airway pressure wave-form this value may be more negative than the triggerpressure if the Patient has a strong respiratory drive; (3)the inspiratory trigger time, which is the time elapsed be-tween the initial Patient effort and the point at which theairway pressure reaches the maximum baseline pressure for patients with low respiratory drive, it takes longer forthe airway pressure to reach the trigger pressure; (4) timeto return trigger pressure to zero or baseline this time isaffected by how rapidly the Ventilator is able to supplyflow to pressurize the circuit, and is influenced by theslope setting.