Presentation on theme: "Ventilator Waveforms: Basic Interpretation and Analysis Vivek Iyer MD, MPH Steven Holets, RRT CCRA Edited for ATS by: Cameron Dezfulian, MD and Nitin Seam,"— Presentation transcript:
Ventilator Waveforms: Basic Interpretation and Analysis Vivek Iyer MD, MPH Steven Holets, RRT CCRA Edited for ATS by: Cameron Dezfulian, MD and Nitin Seam, MD
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.
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 Pressure vs time scalar Flow vs time scalar Volume vs time scalar Inspiratory arm Expiratory arm
Pressure Vs volume loop Flow Vs volume loop Inspiratory arm Expiratory arm Expiratory arm volume flow pressure volume Loops are representations of pressure vs volume or flow vs volume Types of Ventilator Waveforms: Scalars and Loops
Understanding the flow-time waveform Inspiration There are two commonly used types of flow patterns available on most ventilators The square wave or constant flow pattern The decelerating wave or ramp type pattern Volume preset mode: determined by ventilator setting Pressure preset mode: reflects respiratory mechanics and patient effort. Expiration If 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.
The square wave flow pattern The inspiratory flow rate remains constant over the entire inspiration. time flow Inspiratory arm Expiratory arm Inspiratory time = Tidal volume Flow rate The expiratory flow is determined by the elastic recoil of the respiratory system and resistance of intubated airways
The decelerating ramp flow pattern The inspiratory flow rate decelerates as a function of time to reach zero flow at end inspiration For a given tidal volume, the inspiratory time is longer in this type of flow pattern as compared to the square wave pattern time flow Inspiratory arm Expiratory arm Inspiratory time = Tidal volume Flow rate
Now let us try to understand the following in the next few slides Airway pressures A basic ventilator circuit diagram The equation of motion for the respiratory system The pressure-time waveform
Understanding airway pressures The 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 Diaphragm ET Tube airways Chest wall P PL Pleural pressure P aw Airway pressure P alv Alveolar pressure ET tube + Airways (resistive element) Resistive pressure varies with airflow and the diameter of ETT and airways. Flow resistance The elastic pressure varies with volume and stiffness of lungs and chest wall. Pel = Volume x 1/Compliance P aw = Flow X Resistance + Volume x 1/Compliance THUS Lungs + Chest wall (elastic element) Airways + ET tube (resistive element) Lungs + Chest wall (elastic element)
Understanding the basic ventilator circuit diagram ventilator Diaphragm 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 patients own respiratory system makes up the 2 nd part of the circuit. The diaphragm is also shown as a 2 nd piston; causing air to be drawn into the lungs during contraction. These two systems are connected by an endotracheal tube which we can consider as an extension of the patients airways. ET Tube airways Chest wall
Let us now understand how the respiratory systems inherent elastance and resistance to airflow determines the pressures generated within a mechanically ventilated system. Ventilator Diaphragm R ET tube R airways R aw Understanding basic respiratory mechanics The total airway resistance (R aw ) in the mechanically ventilated patient is equal to the sum of the resistances offered by the endotracheal tube (R ET tube ) and the patients airways ( R airways ) The total elastic resistance (E rs ) offered by the respiratory system is equal to the sum of elastic resistances offered by the Lung E lungs and the chest wall E chest wall E lungs E chest wall Thus to move air into the lungs at any given time (t), the ventilator has to generate sufficient pressure ( P aw ( t ) ) to overcome the combined elastic ( P el (t) ) and resistance ( P res(t) ) properties of the respiratory system E rs ET Tube airways Thus the equation of motion for the respiratory system is P aw (t)=P res (t)+P el (t)
ventilator Diaphragm P peak P res R ET tube R airways P res P plat Understanding the pressure-time waveform using a square wave flow pattern time pressure 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 P res to overcome the airway resistance. Note: No volume is delivered at this time. After this, the pressure rises in a linear fashion to finally reach P peak. Again at end inspiration, air flow is zero and the pressure drops by an amount equal to P res to reach the plateau pressure P plat. The pressure returns to baseline during passive expiration. P res
Now lets look at some different pressure-time waveforms using a square wave flow pattern This is a normal pressure-time waveform With normal peak pressures ( P peak ) ; plateau pressures (P plat )and airway resistance pressures (P res ) time pressure P res P plat P res Scenario # 1 P aw(peak) = Flow x Resistance + Volume x 1/ Compliance time flow Square wave flow pattern P aw(peak)
Waveform showing high airways resistance This is an abnormal pressure-time waveform time pressure P peak P res P plat P res Scenario # 2 The increase in the peak airway pressure is driven entirely by an increase in the airways resistance pressure. Note the normal plateau pressure. e.g. ET tube blockage P aw(peak) = Flow x Resistance + Volume x 1/ Compliance + PEEP time flow Square wave flow pattern Normal
Waveform showing increased airways resistance P peak P plat P res Square wave flow pattern
Waveform showing high inspiratory flow rates This is an abnormal pressure-time waveform time pressure P aw(peak) P res P plat P res Scenario # 3 The increase in the peak airway pressure is caused by high inspiratory flow rate and airways resistance. Note the shortened inspiratory time and high flow e.g. high flow rates P aw(peak) = Flow x Resistance + Volume x 1/compliance + PEEP time flow Square wave flow pattern Normal Normal (low) flow rate
Waveform showing decreased lung compliance This is an abnormal pressure-time waveform time pressure P res P plat P res Scenario # 4 The increase in the peak airway pressure is driven by the decrease in the lung compliance. Increased airways resistance is often also a part of this scenario. e.g. ARDS Normal time flow Square wave flow pattern P aw(peak) P aw(peak) = Flow x Resistance + Volume x 1/ Compliance + PEEP
Waveform showing decreased lung compliance P peak P plat P res Square wave flow pattern
Now 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 pressure Low compliance, pressure rises until end of flow. High end inspiratory peak and plateau pressures Normal resistance and compliance, initial pressure rise ( flow setting and ETT resistance) then slope flattens as flow and volume delivery ends. 40 cmH20 60 cmH20 20 cmH20 time
Now let us try to understand the practical aspects of ventilator waveform analysis in an interactive fashion.
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
Some ventilators with waveform displays Siemens Servo 300A Respironics Esprit Puritan Bennett 7200Puritan Bennett 840 Dräger Evita XL Bear 1000 series
Waveform selection on different ventilators Push to start waveforms Time scale Size adjustment Select different waveforms PB 840 Ventilator
Push to select waveforms Respironics Espirit ventilator Waveform selection on different ventilators
Press to adjust size Switch between loops and scalars Switch between waveforms Waveform selection on different ventilators Respironics Espirit ventilator
Variables that govern how a ventilator functions and interacts with the patient Control variable The Mode of Ventilation Pressure, flow, or volume controlled Triggering variable pressure, flow or volume sensing that initiates the vent cycle Cycle variable Pressure, volume, flow, or time that ends the inspiratory phase Limit Variable Volume, pressure or flow can be set to be constant or reach a maximum
So what waveforms should I be observing and analyzing? HINT Look at the waveforms that aredynamic for the current ventilator settings
Mode of ventilation -> useful waveforms Mode of ventilation Independent variables Dependent variables Waveforms that will be useful Waveforms that normally remain unchanged Volume Control/ Assist- Control Tidal volume, RR, Flow rate, PEEP, I/E ratio P aw Pressure-time: Changes in P ip, P plat Flow-time (expiratory): Changes in compliance Pressure-volume loop: Overdistension, optimal PEEP Volume-time Flow time (inspiratory) Flow-volume loop Pressure Control P aw, Inspiratory time (RR), PEEP and I/E ratio V t, flow Volume-time and flow- time: Changes in V t and compliance Pressure-volume loop: Overdistension, optimal PEEP Pressure-time Pressure support/ CPAP PS and PEEP V t, and RR, flow, I/E Ratio Volume- time Flow- time (for V t and V E ) V t =tidal volume; RR=respiratory rate; P aw =airway pressure; PEEP= positive end expiratory pressure; I/E ratio= inspiratory/expiratory time; V E = minute ventilation; Pip = Peak inspiratory pressure; Pplat = Plateau pressure
Waveforms 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
Waveforms to observe during pressure targeted (PS or PCV) ventilation Pressure-time waveform: –Is static and reflects the pressure targets you selected for ventilation Flow-time and volume-time waveform: –Are dynamic and reflect the patients intrinsic respiratory effort and changes in compliance and resistance of the respiratory system PS= pressure support ventilation; PCV= pressure control ventilation
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 patients respiratory muscle strength (independent variables)
Now let us begin riding the waves by looking at a few ventilator waveforms!
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.
Basic ventilator waveforms Mode of ventilation: CPAP + PS –Airway pressures: limited by ventilator setting –Flow 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 controlled –Waveforms shown: flow-time and volume-time
CPAP with Pressure Support Any abnormalities?: No PEARL: notice how each breath differs in flow rate and tidal volume.
Basic ventilator waveforms Mode of ventilation: pressure control ventilation (PCV) –Airway pressures: ventilator controlled –Respiratory rate: ventilator controlled –Tidal 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
Pressure Assist/Control – Decelerating Flow Any abnormalities? : No PEARL: 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.
Let us now shift gears and see how waveforms can help us recognize some common ventilator related problems! Common problems that can be diagnosed by analyzing Ventilator waveforms Abnormal ventilatory Parameters/lung mechanics E.g.. Overdistension, Auto PEEP COPD Patient-ventilator Interactions E.g. flow starvation, Double triggering, Wasted efforts Active expiration Ventilatory circuit related problems E.g. auto cycling and Secretion build up in the Ventilatory circuit
Now let us learn to recognize Lung overdistension and the development of Auto PEEP Abnormal ventilatory Parameters/lung mechanics E.g.. Overdistension, Auto PEEP COPD Patient-ventilator Interactions E.g. flow starvation, Double triggering, Wasted efforts Active expiration Ventilatory circuit related problems E.g. auto cycling and Secretion build up in the Ventilatory circuit
Let 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 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, overdistention etc.
Recognizing Lung Overdistension
Recognizing lung overdistension There are high peak and plateau Pressures… Suspect this when: PEARL: Think of low lung compliance (e.g. ARDS), excessive tidal volumes, right mainstem intubation etc Accompanied by high expiratory flow rates
The 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 overdistension Stress index > 1 Downward concavity indicates increased compliance and potential alveolar recruitment Stress index < 1 flow time Paw Note: Patient effort must be absent
The pressure-volume loop can tell us a lot about lung physiology! Lower inflection point (LIP) Can be thought of as the minimum baseline pressure (PEEP) needed for optimal alveolar recruitment Upper inflection point (UIP) above this pressure, additional alveolar recruitment requires disproportionate increases in applied airway pressure Compliance (C) is markedly reduced in the injured lung on the right as compared to the normal lung on the left Normal lung ARDS
Observe a pressure-volume loop illustrating the concept of overdistension Upper inflection point Peak inspiratory pressure Note: During normal ventilation the LIP cannot be assessed due to the effect of the inspiratory flow which shifts the curve to the right
Detecting Auto-PEEP Recognize Auto-PEEP when Expiratory flow continues and fails to return to the baseline prior to the new inspiratory cycle
The development of auto- PEEP over several breaths in a simulation Notice how the expiratory flow fails to return to the baseline indicating air trapping (AutoPEEP) Also notice how air trapping causes an increase in airway pressure due to increasing end expiratory pressure and end inspiratory lung volume.
Development of auto-PEEP 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 because of decreasing compliance Click here to watch video
Understanding how flow rates affect I/E ratios and the development of auto PEEP Lluis Blanch MD, PhD et al: Respiratory Care Jan 2005 Vol 50 No 1 Decreasing the flow rate Increase the inspiratory time and consequently decrease the expiratory time (decreased I/E ratio) Thus allowing incomplete emptying of the lung and the development of air trapping and 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) Understanding how inspiratory time affect I/E ratios and the development of auto-PEEP
Ventilator settings before and after the development of auto-PEEP Mode of ventilation: PCV ( pressure control ventilation) –Waveforms depicted: flow-time and pressure-time –Inspiratory pressure: 15cm H 2 O with PEEP of 5 cm H 2 O –Respiratory rate: 20 bpm Ventilator settings Initial settings SubsequentlyFinal settings Inspiratory time 0.85 sec1.0 sec1.5 sec Expiratory time 2.15 sec2.0 sec1.5 sec I : E ratio1 : 2.51: 21: 1 Auto PEEPNo Yes
Development of auto-PEEP with inadequate expiratory time Click here to watch video
Recognizing prolonged expiration (air trapping) Recognize airway obstruction when Expiratory flow quickly tapers off and then enters a prolonged low-flow state without returning to baseline (auto- PEEP) This is classic for the flow limitation and decreased lung elastance characteristic of COPD or status asthmaticus
Let us now move forward and Learn about diagnosing patient-ventilator Interactions by analyzing ventilator waveforms Abnormal ventilatory Parameters/lung mechanics E.g.. Overdistension, Auto PEEP COPD Patient-ventilator Interactions E.g. flow starvation, Double triggering, Wasted efforts Active expiration Ventilatory circuit related problems E.g. auto cycling and secretion build up in the Ventilatory circuit
Recognizing: Wasted efforts Double triggering Flow starvation Active expiration
Recognizing ineffective/wasted patient effort Patient inspiratory effort fails to trigger vent resulting in a wasted effort Results in fatigue, tachycardia, increased metabolic needs, fever etc Causes: High AutoPEEP, respiratory muscle weakness, inappropriate sensitivity settings
Recognizing double triggering Continued patient inspiratory effort through the end of a delivered breath causes the ventilator to trigger again and deliver a 2 nd breath immediately after the first breath. This results in high lung volumes and pressures. High peak airway pressures and double the inspiratory volume Causes: patient flow or volume demand exceeds ventilator settings Consider: Increasing tidal volume, switching modes i.e. pressure support, increasing sedation or neuromuscular paralysis as appropriate
Another example of double triggering
Recognizing flow starvation Look at the pressure-time waveform If you see this kind of scooping or distortion instead of a smooth rise in the pressure curve…. Diagnose flow starvation in the setting of patient discomfort, fatigue, dyspnea, etc on the vent
Recognizing active expiration (pressure support) Look at the flow-time & pressure-time waveform The patients active expiratory efforts during the inspiratory phase causes a pressure spike. Notice the high and variable expiratory flow rates due to varying expiratory muscle effort PEARL: This is a high drive state where increased sedation/paralysis and mode change may be appropriate for lung protection.
Lastly let us learn to recognize Ventilatory circuit related problems by analyzing ventilatory waveforms Abnormal ventilatory Parameters/lung mechanics E.g.. Overdistension, Auto PEEP COPD Patient-ventilator Interactions E.g. flow starvation, Double triggering, Wasted efforts Ventilatory circuit related problems E.g. auto cycling and secretion build up in the Ventilatory circuit
Recognizing airway or tubing secretions Normal flow-volume loop Flow volume loop showing a saw tooth pattern typical of retained secretions
Characteristic scalars due to secretion build up in the tubing circuit
Recognizing 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 lungs –e.g. leaks in the circuit, ET tube cuff leak, lungs (pneumothorax) May also result from condensate in the circuit The 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.
Waveform video showing the ventilator auto cycling Click here to watch video
Take home points Ventilator 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. Dont hesitate to change the scale or speed of the waveform to aid in your interpretation.
Additional links Follow these links for more waveform videos: –Excessive airway secretions: F93mPDMtl8I F93mPDMtl8I –Flow starvation ZoyFcvNqVq0 ZoyFcvNqVq0