Presentation on theme: "Ventilator Waveforms: Basic Interpretation and Analysis"— Presentation transcript:
1Ventilator Waveforms: Basic Interpretation and Analysis Vivek Iyer MD, MPHSteven Holets, RRT CCRAEdited for ATS by:Cameron Dezfulian, MD and Nitin Seam, MD
2Outline of this presentation Goal:To provide an introduction to the concept of ventilator waveform analysis in an interactive fashion.Content:Outline of types of ventilatory waveforms.Introduction to respiratory mechanics and the ‘Equation Of Motion’ for the respiratory systemDevelopment of the concept of ventilator waveformsIllustrations and videos of waveforms to illustrate their practical applications and usefulness.
3Types of Ventilator Waveforms: Scalars and Loops Scalars are waveform representations of pressure, flow or volume on the y axis vs time on the x axisFlow vs timescalarInspiratoryarmExpiratoryarmPressure vs timescalarVolume vs timescalar
4Types of Ventilator Waveforms: Scalars and Loops Loops are representations of pressure vs volume or flow vs volumeExpiratoryarmPressure Vs volumeloopvolumepressureInspiratoryarmFlow Vs volumeloopExpiratoryarmflowvolume
5Understanding the flow-time waveform InspirationThere are two commonly used types of flow patterns available on most ventilatorsThe ‘square wave’ or ‘constant flow’ patternThe ‘decelerating wave’ or ‘ramp type’ patternVolume preset mode: determined by ventilator settingPressure preset mode: reflects respiratory mechanics and patient effort.ExpirationIf muscles inactive: determined by elastic recoil of the respiratory system and resistance of intubated airways.If muscles active: waveform influenced by patient effort in addition to above.
6The ‘square wave’ flow pattern The inspiratory flow rateremains constant overthe entire inspiration.InspiratoryarmflowThe expiratory flow isdetermined by theelastic recoil of therespiratory system andresistance of intubatedairwaystimeExpiratoryarmInspiratory time = Tidal volumeFlow rate
7The ‘decelerating ramp’ flow pattern The inspiratory flow ratedecelerates as a functionof time to reach zero flowat end inspirationtimeflowInspiratoryarmFor a given tidal volume,the inspiratory time islonger in this type of flowpattern as compared tothe square wave patternExpiratoryarmInspiratory time = Tidal volumeFlow rate
8Now let us try to understand the following in the next few slides Airway pressuresA basic ventilator circuit diagramThe equation of motion for the respiratory systemThe pressure-time waveform
9Understanding airway pressures The respiratory system can be thought of as a mechanicalsystem consisting of resistive (airways +ET tube) and elastic(lungs and chest wall) elements in seriesTHUSPaw = Flow X Resistance + Volume x 1/ComplianceDiaphragmET TubeairwaysChest wallPPLPleural pressurePawAirway pressurePalvAlveolar pressureET tube + Airways(resistive element)Lungs + Chest wall(elastic element)Airways + ET tube(resistive element)Lungs + Chest wall(elastic element)Resistive pressure varies with airflowand the diameter of ETT and airways.The elastic pressure varies with volume andstiffness of lungs and chest wall.Pel = Volume x 1/ComplianceFlow resistance
10Understanding the basic ventilator circuit diagram Essentially the circuit diagram of amechanically ventilated patient can bebroken down into two parts…..The ventilator makes up the first partof the circuit. Its pump like action isdepicted simplistically as a pistonthat moves in a reciprocating fashionduring the respiratory cycle.The patient’s own respiratory systemmakes up the 2nd part of the circuit.The diaphragm is also shown as a2nd piston; causing air to be drawn intothe lungs during contraction.ET TubeThese two systems are connected byan endotracheal tube which we canconsider as an extension of thepatient’s airways.airwaysDiaphragmChest wall
11Understanding basic respiratory mechanics Thus the equation of motion for the respiratory systemisVentilatorPaw (t)=Pres (t)+Pel (t)ElungsRET tubeET TubeRawErsairwaysRairwaysEchest wallLet us now understand how the respiratory systems’inherent elastance and resistance to airflowdetermines the pressures generated within amechanically ventilated system.The total ‘elastic’ resistance (Ers) offered by therespiratory system is equal to the sum ofelastic resistances offered by theLung E lungs and thechest wall E chest wallThus to move air into the lungs at any given time (t),the ventilator has to generate sufficientpressure (Paw(t)) to overcome the combinedelastic (Pel (t)) and resistance (Pres(t)) propertiesof the respiratory systemThe total ‘airway’ resistance (Raw)in the mechanically ventilated patientis equal to the sum of the resistances offeredby the endotracheal tube (R ET tube)and the patient’s airways ( R airways)Diaphragm
12Understanding the pressure-time waveform using a ‘square wave’ flow patternPpeakPrespressureventilatorPplatPresRET tubetimePresRairwaysAfter this, the pressure rises in a linear fashionto finally reach Ppeak. Again at end inspiration,air flow is zero and the pressure drops by anamount equal to Pres to reach the plateaupressure Pplat. The pressure returns tobaseline during passive expiration.At the beginning of the inspiratory cycle,the ventilator has to generate a pressure Presto overcome the airway resistance.Note: No volume is delivered at this time.The pressure-time waveform is a reflectionof the pressures generated within theairways during each phase of theventilatory cycle.Diaphragm
13Now let’s look at some different pressure-time waveforms using a ‘square wave’ flow pattern Paw(peak) = Flow x Resistance + Volume x 1/ ComplianceScenario # 1Paw(peak)pressurePresPplatPrestimeflow‘Square wave’flow patterntimeThis is a normal pressure-time waveformWith normal peak pressures ( Ppeak) ;plateau pressures (Pplat )andairway resistance pressures (Pres)
14Waveform showing high airways resistance Paw(peak) = Flow x Resistance + Volume x 1/ Compliance + PEEPScenario # 2NormalPpeakpressurePrese.g. ET tubeblockagePplatPrestimeflow‘Square wave’flow patterntimeThe increase in the peak airway pressure is drivenentirely by an increase in the airways resistancepressure. Note the normal plateau pressure.This is an abnormal pressure-time waveform14
16Waveform showing high inspiratory flow rates Paw(peak) = Flow x Resistance + Volume x 1/compliance + PEEPScenario # 3Paw(peak)NormalpressurePrese.g. high flowratesPplatPrestimeflow‘Square wave’flow patterntimeNormal (low)flow rateThe increase in the peak airway pressure is causedby high inspiratory flow rate and airways resistance.Note the shortened inspiratory time and high flowThis is an abnormal pressure-time waveform
17Waveform showing decreased lung compliance Paw(peak) = Flow x Resistance + Volume x 1/ Compliance + PEEPScenario # 4Paw(peak)Normalpressuree.g. ARDSPresPplatPrestimeflow‘Square wave’flow patterntimeThe increase in the peak airway pressure is drivenby the decrease in the lung compliance.Increased airways resistance is oftenalso a part of this scenario.This is an abnormal pressure-time waveform
19Now lets look at pressure-time tracings using a ‘decelerating ramp’ flow pattern High initial pressure spike reflecting airways resistance and flow settings.As flow declines pressure drops eventually reflecting plateau pressure40 cmH20timeLow compliance, pressure rises until end of flow. High end inspiratory peak and plateau pressures60 cmH20timeNormal resistance and compliance, initial pressure rise ( flow setting and ETT resistance) then slope flattens as flow and volume delivery ends.20 cmH20time
20Now let us try to understand the practical aspects of ventilator waveform analysis in an interactive fashion.
21Clinical applications of ventilator waveform analysis Ventilator waveforms can be very useful in many different situations including:Diagnosing a ventilator that is ‘alarming’Detecting obstructive flow patterns on the ventilatorDetecting air trapping and dynamic hyperinflationDetecting lung overdistentionDetecting respiratory circuit secretion build-upDetecting patient-ventilator interactionsDyssynchronyDouble triggeringWasted effortsFlow starvation
23Waveform selection on different ventilators PB 840VentilatorSelect differentwaveformsSize adjustmentTime scalePush to start waveforms
24Waveform selection on different ventilators RespironicsEspiritventilatorPush to select waveforms
25Waveform selection on different ventilators Switch betweenwaveformsRespironicsEspiritventilatorPress to adjust sizeSwitch betweenloops and scalars
26Variables that govern how a ventilator functions and interacts with the patient Control variable‘The Mode of Ventilation’Pressure, flow, or volumecontrolledLimit VariableVolume, pressure or flowcan be set to be constantor reach a maximumTriggering variablepressure, flow or volumesensing that initiatesthe vent cycleCycle variablePressure, volume, flow,or time that ends theinspiratory phase
27So what waveforms should I be observing and analyzing? HINTLook at the waveforms that are ‘dynamic’ for the current ventilator settings
28Mode of ventilation -> useful waveforms Independent variablesDependent variablesWaveforms that will be usefulWaveforms that normally remain unchangedVolume Control/ Assist-ControlTidal volume, RR, Flow rate, PEEP, I/E ratioPawPressure-time:Changes in Pip, PplatFlow-time (expiratory): Changes in compliancePressure-volume loop: Overdistension, optimal PEEPVolume-timeFlow time (inspiratory)Flow-volume loopPressure ControlPaw, Inspiratory time (RR), PEEP and I/E ratioVt, flowVolume-time and flow-time: Changes in Vt and compliancePressure-timePressure support/CPAPPS and PEEPVt,and RR, flow, I/E RatioVolume- timeFlow- time(for Vt and VE)Vt=tidal volume; RR=respiratory rate; Paw=airway pressure; PEEP= positive end expiratory pressure;I/E ratio= inspiratory/expiratory time; VE= minute ventilation; Pip = Peak inspiratory pressure; Pplat = Plateau pressure
29Waveforms to observe during volume assist/control ventilation Pressure-time waveform:Is ‘dynamic’ and is affected by patient effort and changes in respiratory system compliance and resistance.Flow-time waveform:The inspiratory arm of the flow time waveform is ‘static’ and reflects what you set for flows.But the expiratory flow-time waveform is ‘dynamic’ and reflects the elastic recoil pressure of respiratory system and patient effort
30Waveforms to observe during pressure targeted (PS or PCV) ventilation Pressure-time waveform:Is ‘static’ and reflects the pressure targets you selected for ventilationFlow-time and volume-time waveform:Are ‘dynamic’ and reflect the patient’s intrinsic respiratory effort and changes in compliance and resistance of the respiratory systemPS= pressure support ventilation; PCV= pressure control ventilation
31Waveforms to observe during pressure targeted ventilation: PCV*** Pressure-time waveform usually will not changeFlow-time and volume-time waveform will be affected by changes in compliance, resistance and the patient’s respiratory muscle strength (independent variables)
32Now let us begin riding the ‘waves’ by looking at a few ventilator waveforms!
36Decelerating flow volume assist/control mode Any abnormalities? : NoPEARL: At similar flow rates, the inspiratory time is shorter (and peak pressures higher) for the square wave flow as compared to the decelerating flow pattern.
37Basic ventilator waveforms Mode of ventilation: CPAP + PSAirway pressures: limited by ventilator settingFlow rate: determined by patient effort and rise time setting.Inspiratory time: determined by patient effort and ventilator cycle criteria (E-sens/E-trigger) setting.Respiratory rate: patient controlledWaveforms shown: flow-time and volume-time
38CPAP with Pressure Support Any abnormalities?: NoPEARL: notice how each breath differs in flow rate and tidal volume.
39Basic ventilator waveforms Mode of ventilation: pressure control ventilation (PCV)Airway pressures: ventilator controlledRespiratory rate: ventilator controlledTidal volume: dependent variable (lung compliance)Flow waveform: ventilator controlled (decelerating in this instance)Flow rate: dependent variable (varies with changes in resistance, compliance and Pmus)Waveforms shown: flow-time and pressure-time
40Pressure Assist/Control – Decelerating Flow Any abnormalities? : NoPEARL: Tidal volumes and flow rates are determined by respiratory system mechanics. Increasing inspiratory time after flow ends will only decrease expiratory time, without any increase in tidal volume.
41Let us now shift gears and see how waveforms can help us recognize some common ventilator related problems!
43Let us briefly revisit the flow-time waveform As previously noted, the flow-time waveform has both an inspiratory and an expiratory arm.The shape of the expiratory arm is determined by:the elastic recoil of the lungsthe airways resistanceand any respiratory muscle effort made by the patient during expiration (due to patient-ventilator interaction/dys=synchrony)It should always be looked at as part of any waveform analysis and can be diagnostic of various conditions like COPD, auto-PEEP, wasted efforts, overdistention etc.
45Recognizing lung overdistension Suspect this when:There are high peak and plateauPressures…Accompanied by high expiratoryflow ratesPEARL: Think oflow lung compliance (e.g. ARDS),excessive tidal volumes,right mainstem intubation etc
46The Stress IndexIn AC volume ventilation using a constant flow waveform observe the pressure time scalar.Normal, linear change in airway pressure Stress index =1Upward concavity indicates decreased compliance and lung overdistensionStress index > 1Downward concavity indicates increased compliance and potential alveolar recruitmentStress index < 1flowPawNote: Patient effort must be absenttime
47The pressure-volume loop can tell us a lot about lung physiology! Compliance (C)is markedly reduced in theinjured lung on the right ascompared to the normal lungon the leftNormallungUpper inflection point (UIP)above this pressure,additional alveolar recruitmentrequires disproportionateincreases in appliedairway pressureARDSLower inflection point (LIP)Can be thought of as theminimum baselinepressure (PEEP)needed for optimalalveolar recruitment
48Observe a pressure-volume loop illustrating the concept of overdistension PeakinspiratorypressureUpperinflectionpointThe LIP purportedly shown here really represents attainment of the set inspiratory flow, not a true LIP. I’ve done enough of these VP curves on test lungs (and patients) to recognize that the flow rate has not been set sufficiently low to remove the flow artifact. I think you should emphasize that flow must be set very low in order for the pressure in this graph to meaningfully reflect a VP curve.Added comment. Seeing that LIP on inspiratory limb has fallen out of favor should we delete?Note: During normal ventilation the LIP cannot be assessed due to the effect of the inspiratory flow which shifts the curve to the right48
50Detecting Auto-PEEP Recognize Auto-PEEP when Expiratory flow continues and fails to return tothe baseline prior to the newinspiratory cycle
51The development of auto- PEEP over several breaths in a simulation Notice how the expiratory flow failsto return to the baseline indicatingair trapping (AutoPEEP)Also notice how air trapping causesan increase in airway pressuredue to increasing end expiratorypressure and end inspiratorylung volume.51
52Development of auto-PEEP Notice how the expiratoryflow fails to return tothe baseline causingprogressive air trappingClick here to watch videoAlso notice how theprogressive air trappingcauses a gradualincrease in airwaypressures because ofdecreasing compliance
53Understanding how flow rates affect I/E ratios and the development of auto PEEP Decreasing the flow rateIncrease the inspiratory timeand consequently decrease theexpiratory time(decreased I/E ratio)Thus allowing incomplete emptyingof the lung and the developmentof air trapping and auto-PEEPLluis Blanch MD, PhD et al: Respiratory Care Jan 2005 Vol 50 No 1
54Understanding how inspiratory time affect I/E ratios and the development of auto-PEEP In a similar fashion, an increase in inspiratory time can also cause a decrease in the I: E ratio and favor the development of auto-PEEP by not allowing enough time for complete lung emptying between breaths.Watch in the next video how auto-PEEP develops in a patient on Pressure control ventilation at a RR of 20, just by increasing the inspiratory time from 0.85 sec to 1.0 sec (no auto-PEEP develops) and then to 1.5 sec (development of auto PEEP)
55Ventilator settings before and after the development of auto-PEEP Mode of ventilation: PCV ( pressure control ventilation)Waveforms depicted: flow-time and pressure-timeInspiratory pressure: 15cm H2O with PEEP of 5 cm H2ORespiratory rate: 20 bpmVentilatorsettingsInitialSubsequentlyFinal settingsInspiratory time0.85 sec1.0 sec1.5 secExpiratory time2.15 sec2.0 secI : E ratio1 : 2.51: 21: 1Auto PEEPNoYes
56Development of auto-PEEP with inadequate expiratory time Click here to watch video
58Recognizing prolonged expiration (air trapping) Recognizeairway obstructionwhenExpiratory flow quickly tapers offand then enters a prolongedlow-flow state without returning tobaseline (auto- PEEP)This is classic for the flowlimitation and decreased lungelastance characteristic of COPDor status asthmaticus
60Recognizing: Wasted efforts Double triggering Flow starvation Active expiration
61Recognizing ineffective/wasted patient effort Patient inspiratory effortfails to trigger ventresulting in a wasted effortResults in fatigue, tachycardia,increased metabolic needs,fever etcCauses: High AutoPEEP,respiratory muscle weakness,inappropriate sensitivity settings
62Recognizing double triggering High peak airwaypressures anddouble the inspiratoryvolumeContinued patient inspiratory effortthrough the end of a deliveredbreath causes the ventilator to trigger againand deliver a 2nd breath immediately afterthe first breath.This results in high lung volumes and pressures.Causes: patient flow or volume demand exceeds ventilator settingsConsider: Increasing tidal volume, switching modes i.e. pressure support, increasing sedation or neuromuscular paralysis as appropriate
64Recognizing flow starvation Look at the pressure-timewaveformIf you see this kind ofscooping or distortion insteadof a smooth rise in thepressure curve….Diagnose flow starvationin the setting of patientdiscomfort, fatigue,dyspnea, etc on the vent
65Recognizing active expiration (pressure support) Look at the flow-time& pressure-timewaveformNotice the high and variableexpiratory flow rates due tovarying expiratory muscle effortThe patient’s active expiratory effortsduring the inspiratoryphase causes a pressure spike.PEARL: This is a high drive state where increased sedation/paralysis and mode change may be appropriate for lung protection.
68Recognizing airway or tubing secretions Normal flow-volumeloopFlow volume loopshowing a ‘saw tooth’pattern typical ofretained secretions
69Characteristic scalars due to secretion build up in the tubing circuit
70Recognizing ventilator auto-cycling Think about auto-cycling when the respiratory rate increases suddenly without any patient input and if the exhaled tidal volume and minute ventilation suddenly decrease.Typically occurs because of a leak anywhere in the system starting from the ventilator right up to the patients lungse.g. leaks in the circuit, ET tube cuff leak, lungs (pneumothorax)May also result from condensate in the circuitThe exhaled tidal volume will be lower than the set parameters and this may set off a ventilator alarm for low exhaled tidal volume, low minute ventilation, circuit disconnect or rapid respiratory rate.
71Waveform video showing the ventilator ‘auto cycling’ Click here to watch video
72Take home pointsVentilator waveform analysis is an integral component in the management of a mechanically ventilated patient.Develop a habit of looking at the right waveform for the given mode of patient ventilation.Always look at the inspiratory and expiratory components of the flow-time waveform.Don’t hesitate to change the scale or speed of the waveform to aid in your interpretation.
73Additional links Follow these links for more waveform videos: Excessive airway secretions:Flow starvation