Ventilator Waveforms: Interpretation - apsresp.org

1 Ventilator Waveforms: InterpretationAlbert L. Rafanan, MD, FPCCPP ulmonary, Critical Care and Sleep MedicineChong HuaHospital, Cebu CityTypes of Waveforms Scalarsare waveform representations of pressure, flow or volume on the y axis vstime on the x axis Loopsare representations of pressure vsvolume or flow vsvolume Scalar WaveformsLoopCommon problems that can be diagnosedby analyzing Ventilator waveformsAbnormal ventilatoryParameters/lung Overdistension,Auto PEEPCOPDP atient- Ventilator flow starvation,Double triggering,Wasted effortsActive expirationVentilatorycircuit related auto cycling andSecretion build up in theVentilatorycircuitLung Mechanics Use Scalar Pressure Time waveform with a square wave flow patternUnderstanding the basic Ventilator circuit diagramventilatorDiaphragmEssentially the circuit diagram of a mechanically ventilated patient can be broken down into two Ventilator makes up the first part of the circuit.

2 Its pump like action is depicted simplistically as a piston that moves in a reciprocating fashion during the respiratory patient s own respiratory system makes up the 2ndpart of the circuit. The diaphragm is also shown as a 2ndpiston; causing air to be drawn intothe lungs during two systems are connected byan endotrachealtube which we can consider as an extension of the patient s TubeairwaysChest wallUnderstanding airway pressuresThe respiratory system can be thought of as a mechanical system consisting of resistive (airways +ET tube) and elastic(lungs and chest wall) elements in series DiaphragmET TubeairwaysChest wallPPLP leural pressurePawAirway pressurePalvAlveolar pressureET tube + Airways(resistive element)

3 Resistive pressure varies with airflow and the diameter of ETT and resistanceThe elastic pressure varies with volume and stiffness of lungs and chest = Volume x 1/CompliancePaw= Flow X Resistance +Volume x 1/ComplianceTHUSL ungs + Chest wall(elastic element)Airways + ET tube(resistive element)Lungs + Chest wall(elastic element)Let us now understand how the respiratory systems inherent elastanceand resistance to airflow determines the pressures generated within a mechanically ventilated tubeRairwaysRawUnderstanding basic respiratory mechanicsThe total airway resistance (Raw)in the mechanically ventilated patient is equal to the sum of the resistances offered by the endotracheal tube (R ET tube)and the patient s airways ( R airways)The total elastic resistance(Ers)

4 Offered by the respiratory system is equal to the sum of elastic resistances offered by the Lung E lungsand thechest wall E chest wallElungsEchest wallThus to move air into the lungs at any given time (t),the Ventilator has to generate sufficient pressure (Paw(t)) to overcome thecombined elastic (Pel(t))and resistance (Pres(t))propertiesof the respiratory system ErsET TubeairwaysThus the equation of motion for the respiratory systemisPaw (t)=Pres(t)+Pel(t)ventilatorDiaphragmPpe akPresRET tubeRairwaysPresPplatUnderstanding the pressure-time waveformusing a square wave flow patterntimepressureThe pressure-time waveform is a reflectionof the pressures generated within the airways during each phase of the the beginning of the inspiratory cycle,the Ventilator has to generate a pressure Presto overcome the airway : No volume is delivered at this this, the pressure rises in a linear fashionto finally reach Ppeak.

5 Again at end inspiration, air flow is zero and the pressure drops by anamount equal to Presto reach the plateau pressure Pplat. The pressure returns to baseline during passive waveforms using a square wave flow patternThis is a normal pressure-time waveformWith normal peak pressures (Ppeak);plateau pressures (Pplat )and airway resistance pressures (Pres)timepressurePresPplatPresScenario # 1 Paw(peak)= Flow x Resistance +Volume x 1/ Compliancetimeflow Square wave flow patternPaw(peak) waveform showing high airways resistanceThis is an abnormalpressure-time waveformtimepressurePpeakPresPplatPresSc enario # 2 The increase in the peak airway pressure is drivenentirely by an increasein the airways resistancepressure.

6 Note the normal plateau ET tubeblockagePaw(peak)= Flow x Resistance + Volume x 1/ Compliance + PEEP timeflow Square wave flow patternNormalWaveform showing increased airways resistancePpeakPplatPres Square wave flowpatternWaveform showing high inspiratoryflow ratesThis is an abnormalpressure-time waveformtimepressurePaw(peak)PresPplatPr esScenario # 3 The increase in the peak airway pressure is caused by high inspiratory flow rate and airways resistance. Note the shortenedinspiratory time and high high flowratesPaw(peak)= Flowx Resistance + Volume x 1/compliance + PEEP timeflow Square wave flow patternNormalNormal (low) flow rateWaveform showing decreased lung compliance This is an abnormalpressure-time waveformtimepressurePresPplatPresScenari o # 4 The increase in the peak airway pressure is drivenby the decreasein the lung airways resistance is often also a part of this ARDSN ormaltimeflow Square wave flow patternPaw(peak)Paw(peak)

7 = Flow x Resistance + Volume x 1/ Compliance + PEEPC ommon problems that can be diagnosedby analyzing Ventilator waveformsAbnormal ventilatoryParameters/lung Overdistension,Auto PEEPCOPDP atient- Ventilator flow starvation,Double triggering,Wasted effortsActive expirationVentilatorycircuit related auto cycling andSecretion build up in theVentilatorycircuitRecognizing Lung OverdistensionFlow-time waveform 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 lungs the airways resistance and 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, overdistentionetc.

8 Recognizing lung overdistensionThere are high peak and plateau this when:PEARL: Think oflow lung compliance ( ARDS),excessive tidal volumes, right mainstemintubation etcAccompanied by high expiratory flow ratesThe Stress Index In AC volume ventilation using a constant flow waveform observe the pressure time scalar. Normal, linear change in airway pressure Stress index =1 Upward concavity indicates decreased compliance and lung overdistensionStress index > 1 Downward concavity indicates increased compliance and potential alveolar recruitment Stress index < 1flowtimePawNote.

9 Patient effort must be absentThe Stress IndexPressure-volume loop Lower inflection point (LIP)Can be thought of as theminimum baseline pressure (PEEP)needed for optimalalveolar recruitmentUpper inflection point (UIP)above this pressure, additional alveolar recruitmentrequires disproportionateincreases in applied airway pressureCompliance (C)is markedly reduced in the injured lung on the right as compared to the normal lungon the leftNormallungARDSO verdistensionUpperinflectionpointPeakins piratorypressureNote:During normal ventilation the LIP cannotbe assessed due to the effect of the inspiratory flow which shifts the curve to the rightRecognizing Auto-PEEPD ynamic Hyperinflation (Gas Trapping) Dynamic hyperexpansion, defined as premature termination of exhalation, often occurs when respiratory rate, inspiratory time, or both have been increased.

10 By not permitting exhalation to finish, an increase in mean airway pressure results. Gas trapping may occur leading to an elevation in PCO2. Careful attention must be paid to dynamic hyperexpansionin patients with obstructive lung disease whose long time constants and slow emptying can result in progressive air trapping, hypercarbia, and eventually decreased cardiac flow continues and fails to return to the baseline prior to the new inspiratory cycleDetecting Auto-PEEPR ecognize Auto-PEEP whenExpiratory flow continues and fails to return to the baseline prior to the newinspiratory cycleThe development of auto-PEEP over several breaths in a simulation Notice how the expiratory flow failsto return to the baseline indicatingair trapping(AutoPEEP)