Ventilator Waveforms: Basic Interpretation and Analysis

1. Ventilator Waveforms: Basic Interpretation and Analysis Vivek Iyer MD, MPH Steven Holets, RRT CCRA Rolf Hubmayr, MD Edited for ATS by: Cameron Dezfulian, MD

2. Outline 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 system • Development of the concept of ventilator waveforms • Illustrations and videos of waveforms to illustrate their practical applications and usefulness.

3. Types of Ventilator Waveforms: Scalars and Loops Scalars are waveform representations of pressure, flow or volume on the y axis vs time on the x axis flow vs time scalar Inspiratory arm expiratory arm pressure vs time scalar volume vs time scalar

4. Types of Ventilator Waveforms: Scalars and Loops Loopsare representations of pressure vs volume or flow vsvolume Expiratory arm Pressure Vs volume loop volume pressure Inspiratory arm Flow Vs volume loop Expiratory arm flow volume

5. Understanding the flow-time waveform • There are two components to the flow-time waveform • The inspiratory arm: • Active in nature • The character is determined by the ventilatory flow settings. • The expiratory arm: • Passive in nature • The character is determined mainly by elastic recoil of the patients lungs and airway resistance. • Also affected by patient respiratory effort (if any) • There are two commonly used types of flow patterns available on most ventilators • The ‘square wave’ or ‘constant flow’ pattern • The ‘ramp’ (decelerating) type pattern

6. The ‘square wave’ flow pattern The inspiratory flow rate remains constant over the entire inspiration. Inspiratory arm flow The expiratory flow is passive and is determined by airways resistance and the elastic recoil of the lungs time Expiratory arm Inspiratory time = Tidal volume Flow rate

7. flow time The ‘decelerating ramp’ flow pattern The inspiratory flow rate decelerates as a function of time to reach zero flow at end inspiration Inspiratory arm For a given tidal volume, the inspiratory time is higher in this type of flow pattern as compared to the square wave pattern Expiratory arm Inspiratory time = Tidal volume Flow rate

8. Now let us try to understand the following in the next few slides A basic ventilator circuit diagram Airway pressures The equation of motion for the respiratory system The pressure-time waveform

9. Understanding the basic ventilator circuit diagram ventilator Essentially the circuit diagram of a mechanically ventilated patient can be broken down into two parts….. The ventilator makes up the first part of the circuit. Its pump like action is depicted simplistically as a piston that moves in a reciprocating fashion during the respiratory cycle. The patient’s own respiratory system Makes up the 2nd part of the circuit. The diaphragm is also shown as a 2nd piston; causing air to be drawn into the lungs during contraction. ET Tube These two systems are connected by an endotracheal tube which we can consider as an extension of the patients airways. airways Diaphragm Chestwall

10. ET Tube Paw Airway pressure Paw = Flow Resistance + Volume Compliance airways PPL Pleural pressure Chest wall Flow resistance Volume compliance Diaphragm Palv Alveolar pressure Understanding airway pressures The respiratory system can be thought of as a mechanical system consisting of a resistive (airways) and elastic (lungs and chest wall) element in series Lungs + Chest wall (elastic element) THUS Airways (resistive element) Airways (resistive element) Lungs + Chest wall (elastic element) The contribution of the elastic element (lungs + chest wall) depends on the degree of lung inflation and the underlying compliance of the lungs and the chest wall The contribution of airway resistance pressure depends on the rate of airflow and the underlying resistance (caliber) of the airways

11. Understanding basic respiratory mechanics Thus the equation of motion for the respiratory system is ventilator P applied (t) = Pres (t) + Pel (t) Elungs RET tube ET Tube Raw Ers airways Rairways Echest wall Thus to move air into the lungs at a given time (t), the ventilator has to generate a pressure (P applied) that is sufficient to overcome thepressure generated by the elastic (Pel (t))and airway (Paw)resistances offered by the respiratory system at that time. The total ‘elastic’ resistance(Ers)offered by the respiratory system is equal to the sum of elastic resistances offered by the Lung E lungsand the chest wall E chest wall The 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) Let us now understand how the respiratory systems’ inherent elastance and resistance to airflow determines the pressures generated within a mechanically ventilated system. Diaphragm

12. Understanding the pressure-time waveform using a ‘square wave’ flow pattern Ppeak Pres pressure ventilator Pplat Pres RET tube time Pres Rairways After this, the pressure rises in a linear fashion to finally reach Ppeak. Again at end inspiration, air flow is zero and the pressure drops by an amount equal to Presto reach the plateau pressure Pplat. The pressure returns to baseline during passive expiration The pressure-time waveform is a reflection of the pressures generated within the airways during each phase of the ventilatory cycle. At the beginning of the inspiratory cycle, the ventilator has to generate a pressure Pres to overcome the airway resistance. Note: No volume is delivered at this time. Diaphragm

13. Paw = Flow Resistance + Volume Compliance flow time ‘Square wave’ flow pattern Now let’s look at some different pressure-time waveforms using a ‘square wave’ flow pattern Scenario # 1 pressure Ppeak Normal values: Ppeak < 40 cm H2O Pplat < 30 cm H2O Pres< 10 cm H2O Pres Pplat Pres time This is a normal pressure-time waveform With normal peak pressures (Ppeak); plateau pressures (Pplat )and airway resistance pressures (Pres)

14. Paw = Flow Resistance + Volume + PEEP Compliance Normal flow time ‘Square wave’ flow pattern Waveform showing increased airways resistance Scenario # 2 Ppeak e.g. ET tube blockage pressure Pres Pplat Pres time The increase in the peak airway pressure is driven entirely by an increase in the airways resistance pressure. Note the normal plateau pressure. This is an abnormal pressure-time waveform

15. Waveform showing increased airways resistance ‘Square wave’ flow pattern Ppeak Pplat Pres

16. Paw = Flow Resistance + Volume + PEEP Compliance Normal flow time ‘Square wave’ flow pattern Waveform showing high airway resistance due to high flow rates Scenario # 3 Ppeak e.g. high flow rates pressure Pres Pplat Pres time Normal (low) flow rate The increase in the peak airway pressure is driven entirely by an increase in the airways resistance pressure caused by excessive flow rates. Note the shortened inspiratory time and high flow This is an abnormal pressure-time waveform

17. Paw = Flow Resistance + Volume + PEEP Compliance Normal flow time ‘Square wave’ flow pattern Waveform showing decreased lung compliance Scenario # 4 e.g. ARDS Ppeak pressure Pres Pplat Pres time The increase in the peak airway pressure is driven entirely by the decrease in the lung compliance. Increased airways resistance is often also a part of this scenario. This is an abnormal pressure-time waveform

18. Waveform showing decreased lung compliance ‘Square wave’ flow pattern Ppeak Pplat Pres

19. Now lets look at the same pressure-time tracings using a ‘decelerating ramp’ flow pattern Normal High Raw: (e.g. asthma) High PIP Normal PIP Normal Pplat Normal Pplat pressure High flow: (Note short Inspiratory time) High PIP Low CL: e.g. ARDS High PIP High Pplat Normal Pplat time

20. Now let us try to understand the practical aspects of ventilator waveform analysis in an interactive fashion.

21. Clinical 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 ventilator • Detecting air trapping and dynamic hyperinflation • Detecting lung overdistention • Detecting respiratory circuit secretion build-up • Detecting patient-ventilator interactions • Dyssynchrony • Double triggering • Wasted efforts • Flow starvation

22. Some ventilators with waveform displays Puritan Bennett 840 Puritan Bennett 7200 Dräger Evita XL Siemens Servo 300A Bear 1000 series Respironics Esprit

23. Waveform selection on different ventilators PB 840 Ventilator Select different waveforms Size adjustment Timescale Push to start waveforms

24. Waveform selection on different ventilators Respironics Espirit ventilator Push to select waveforms

25. Waveform selection on different ventilators Switch between waveforms Respironics Espirit ventilator Press to adjust size Switch between Loops and scalars

26. Variables that govern how a ventilator functions and interacts with the patient Control variable ‘The Mode of Ventilation’ Pressure, flow, or volume controlled Limit Variable Volume, pressure or flow can be set to be constant or reach a maximum Triggering variable pressure, flow or volume sensing that initiates the vent cycle Cycle variable Pressure, volume, flow, or time that ends the inspiratory phase

27. So what waveforms should I be observing and analyzing? Look at the waveforms that are varying based on the settings you have ordered

28. Mode of ventilation -> useful waveforms 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

29. Waveforms to observe during volume assist control ventilation • Pressure-time waveform: • Affected by patient effort and changes in resistance and compliance • Flow-time waveform: • Expiratory flow is not fixed, waveform is dependent on elastic recoil pressure of respiratory system/patient effort • Therefore this scalar is nearly always of interest

30. Waveforms to observe during pressure targeted ventilation: PCV • Pressure-time waveform usually will not change • Flow-time and volume-time waveform will be affected by changes in compliance, resistance and the patient’s respiratory muscle strength (independent variables)

31. Now let us begin riding the ‘waves’ by looking at a few ventilator waveforms!

32. Basic ventilator waveforms Mode of ventilation: Assist/control – square wave flow • Airway pressures: dependent on lung compliance, tidal volume and flow (dependent variable) • Tidal volumes, respiratory rate: ventilator controlled • Flow pattern: ventilator controlled (square wave pattern) • Inspiratory time: ventilator controlled • Waveforms shown: flow-time and pressure-time

33. Square wave volume assist/control mode • Any abnormalities? : No • PEARL: always look at both inspiratory and expiratory arms of the flow-time waveform. Make it a habit!

34. Basic ventilator waveforms Mode of ventilation: Assist/control – decelerating flow pattern • Airway pressures: dependent on lung compliance, tidal volume and flow (dependent variable) • Tidal volumes, respiratory rate: ventilator controlled • Flow pattern: ventilator controlled (decelerating wave pattern) • Inspiratory time: ventilator controlled • Waveforms shown: flow-time and pressure-time

35. Decelerating flow volume assist/control mode • Any abnormalities? : No • PEARL: 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.

36. Basic ventilator waveforms Mode of ventilation: CPAP + PS • Airway pressures: patient controlled (indirectly through control of volume and flow) • Flow pattern: patient controlled • Inspiratory time, respiratory rate: patient controlled • Waveforms shown: flow-time and volume-time

37. CPAP with Pressure Support • Any abnormalities?: No • PEARL: notice how each breath differs in flow pattern and tidal volume.

38. Basic ventilator waveforms Mode of ventilation: pressure control ventilation (PCV) • Airway pressures: ventilator controlled • Respiratory rate: ventilator controlled • Tidal Volumes: dependent variable (lung compliance) • Flow rates: ventilator controlled (decelerating in this instance) • Waveforms shown: flow-time and volume-time

39. Pressure Assist/Control – Decelerating Flow • Any abnormalities? : No • PEARL: tidal volumes and flow rates are determined by lung compliance. Increasing inspiratory time beyond a certain point will only decrease expiratory time, without any increases in tidal volumes achieved.

40. Let us now shift gears and see how waveforms can help us recognize some common ventilator related problems!

42. Let us briefly revisit the flow-time waveform • As previously noted, the flow-time waveform has both an inspiratory and an expiratory arm. • The expiratory arm is passive in nature and its character 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) • The expiratory arm can be thought of in some ways as passive bedside spirometry. • 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.

43. Recognizing Lung Overdistension

44. Recognizing lung overdistension Suspect this when: There are high peak and plateau Pressures… Accompanied by high expiratory Flow rates The pressure-time waveform Shows an abrupt increase in Pressure. PEARL: Think of right mainstem intubation, low lung compliance (e.g. ARDS), excessive tidal volumes etc

45. The pressure-volume loop can tell us a lot about lung physiology! Compliance (C) is markedly reduced in the injured lung on the right as compared to the normal lung on the left Normal lung Upper inflection point (UIP) above this pressure, additional alveolar recruitment requires disproportionate increases in applied airway pressure ARDS Lower inflection point (LIP) Can be thought of as the minimum baseline pressure (PEEP) needed for optimal alveolar recruitment

46. Observe a pressure-volume loop illustrating the concept of overdistension Peak Inspiratory pressure Upper Inflection point Lower Inflection point

47. Lung overdistension based on pressure-volume loops

48. RecognizingAuto-PEEP

49. Detecting Auto-PEEP Recognize Auto-PEEP when Expiratory flow continues and fails to return to the baseline prior to the new inspiratory cycle

50. The development of auto- PEEP over several breaths in a simulation Notice how the expiratory flow fails to return to the baseline causing progressive air trapping Also notice how the progressive air trapping causes a gradual increase in airway pressures due to decreasing compliance