# Ventilator Waveforms: Basic Interpretation and Analysis

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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 Flow vs time scalar Inspiratory arm Expiratory arm Pressure vs time scalar Volume vs time scalar

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

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. Inspiratory arm flow The expiratory flow is determined by the elastic recoil of the respiratory system and resistance of intubated airways time Expiratory arm Inspiratory time = Tidal volume Flow rate

The ‘decelerating ramp’ flow pattern
The inspiratory flow rate decelerates as a function of time to reach zero flow at end inspiration time flow Inspiratory arm For a given tidal volume, the inspiratory time is longer in this type of flow pattern as compared to the square wave pattern 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 THUS Paw = Flow X Resistance + Volume x 1/Compliance Diaphragm ET Tube airways Chest wall PPL Pleural pressure Paw Airway pressure Palv Alveolar pressure ET tube + Airways (resistive element) Lungs + Chest wall (elastic element) Airways + ET tube (resistive element) Lungs + Chest wall (elastic element) Resistive pressure varies with airflow and the diameter of ETT and airways. The elastic pressure varies with volume and stiffness of lungs and chest wall. Pel = Volume x 1/Compliance Flow resistance

Understanding the basic ventilator circuit diagram
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 patient’s airways. airways Diaphragm Chest wall

Understanding basic respiratory mechanics
Thus the equation of motion for the respiratory system is Ventilator Paw (t) = Pres (t) + Pel (t) Elungs RET tube ET Tube Raw Ers airways Rairways Echest wall Let us now understand how the respiratory systems’ inherent elastance and resistance to airflow determines the pressures generated within a mechanically ventilated system. The total ‘elastic’ resistance (Ers) 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 Thus to move air into the lungs at any given time (t), the ventilator has to generate sufficient pressure (Paw(t)) to overcome the combined elastic (Pel (t)) and resistance (Pres(t)) properties of the respiratory system 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) Diaphragm

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 Pres to reach the plateau pressure Pplat. The pressure returns to baseline during passive expiration. 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. The pressure-time waveform is a reflection of the pressures generated within the airways during each phase of the ventilatory cycle. Diaphragm

Now let’s look at some different pressure-time waveforms using a ‘square wave’ flow pattern
Paw(peak) = Flow x Resistance + Volume x 1/ Compliance Scenario # 1 Paw(peak) pressure Pres Pplat Pres time flow ‘Square wave’ flow pattern time This is a normal pressure-time waveform With normal peak pressures ( Ppeak) ; plateau pressures (Pplat )and airway resistance pressures (Pres)

Waveform showing high airways resistance
Paw(peak) = Flow x Resistance + Volume x 1/ Compliance + PEEP Scenario # 2 Normal Ppeak pressure Pres e.g. ET tube blockage Pplat Pres time flow ‘Square wave’ flow pattern 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 14

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

Waveform showing high inspiratory flow rates
Paw(peak) = Flow x Resistance + Volume x 1/compliance + PEEP Scenario # 3 Paw(peak) Normal pressure Pres e.g. high flow rates Pplat Pres time flow ‘Square wave’ flow pattern time Normal (low) flow rate 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 This is an abnormal pressure-time waveform

Waveform showing decreased lung compliance
Paw(peak) = Flow x Resistance + Volume x 1/ Compliance + PEEP Scenario # 4 Paw(peak) Normal pressure e.g. ARDS Pres Pplat Pres time flow ‘Square wave’ flow pattern time 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. This is an abnormal pressure-time waveform

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

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 40 cmH20 time Low compliance, pressure rises until end of flow. High end inspiratory peak and plateau pressures 60 cmH20 time Normal resistance and compliance, initial pressure rise ( flow setting and ETT resistance) then slope flattens as flow and volume delivery ends. 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
Puritan Bennett 840 Puritan Bennett 7200 Dräger Evita XL Siemens Servo 300A Bear 1000 series Respironics Esprit

Waveform selection on different ventilators
PB 840 Ventilator Select different waveforms Size adjustment Time scale Push to start waveforms

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

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

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

So what waveforms should I be observing and analyzing?
HINT Look at the waveforms that are ‘dynamic’ for the current ventilator settings

Mode of ventilation -> useful waveforms
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 Paw Pressure-time: Changes in Pip, Pplat Flow-time (expiratory): Changes in compliance Pressure-volume loop: Overdistension, optimal PEEP Volume-time Flow time (inspiratory) Flow-volume loop Pressure Control Paw, Inspiratory time (RR), PEEP and I/E ratio Vt, flow Volume-time and flow-time: Changes in Vt and compliance Pressure-time Pressure support/ CPAP PS and PEEP Vt,and RR, flow, I/E Ratio Volume- time Flow- 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

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 patient’s 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 patient’s respiratory muscle strength (independent variables)

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

Basic ventilator waveforms
Mode of ventilation: Assist/control volume – 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 (flow setting) Waveforms shown: flow-time and pressure-time

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!

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

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!

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
Suspect this when: There are high peak and plateau Pressures… Accompanied by high expiratory flow rates PEARL: Think of low lung compliance (e.g. ARDS), excessive tidal volumes, right mainstem intubation etc

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 Paw Note: Patient effort must be absent time

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

Observe a pressure-volume loop illustrating the concept of overdistension
Peak inspiratory pressure Upper inflection point The 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 right 48

Recognizing Auto-PEEP

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. 51

Development of auto-PEEP
Notice how the expiratory flow fails to return to the baseline causing progressive air trapping Click here to watch video Also notice how the progressive air trapping causes a gradual increase in airway pressures because of decreasing compliance

Understanding how flow rates affect I/E ratios and the development of auto PEEP
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 Lluis Blanch MD, PhD et al: Respiratory Care Jan 2005 Vol 50 No 1

Understanding 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)

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 H2O with PEEP of 5 cm H2O Respiratory rate: 20 bpm Ventilator settings Initial Subsequently Final settings Inspiratory time 0.85 sec 1.0 sec 1.5 sec Expiratory time 2.15 sec 2.0 sec I : E ratio 1 : 2.5 1: 2 1: 1 Auto PEEP No Yes

Development of auto-PEEP with inadequate expiratory time

Recognizing Expiratory Flow Limitation (e.g. COPD, asthma)

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

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
High peak airway pressures and double the inspiratory volume Continued patient inspiratory effort through the end of a delivered breath causes the ventilator to trigger again and deliver a 2nd breath immediately after the first breath. This results in high lung volumes and pressures. 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 Notice the high and variable expiratory flow rates due to varying expiratory muscle effort The patient’s active expiratory efforts during the inspiratory phase causes a pressure spike. PEARL: This is a high drive state where increased sedation/paralysis and mode change may be appropriate for lung protection.

Recognizing Airway Secretions & Ventilator Auto-Cycling

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’