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Flow rate Scalar

  • tells us how fast or slow the breath is being delivered to the patient

  • can be set on some ventilators directly (eg. Drager) or indirectly:

    • manipulating Inspiratory time or Rise time​

  • usually set at 60 L/min

  • patient will work harder if not set properly

Constant "square" Flow wave

  • decrease in mean airway pressure and improvements in venous return and cardiac output

Decelerating Flow Wave

  • indicates pressure mode breath

  • may be preferred over the constant "square" waveform because the same TV is delivered with a lower peak pressure

Inspiratory Flow rate

  • looking at the pressure / time scalar, you will have a "scooped out" appearance which means that the patients effort exceeds set flow - they are inhaling with such negative force, they are out striping the flow coming from the ventilator; you want it to look "shark fin"

  • the notch behind is artifact that is caused by the ventilator circuit

Flow / Time Scalar

- can be used to assess: 


  • Air Trapping

    • expiratory flow doesn't return to baseline

    • example: emphysema


  • Airway Obstruction

  • Bronchodilator response

    • the expiratory curve should return to base sooner

  • Active exhalation

  • Flow waveform shape

  • Inspiratory Flow

    • "zero-flow state" and I-time is too long for the patient

  • Asynchrony

  • Triggering effort

Ventilator Terminology (Kapadia):

  • triggering (breath initiation)

  • breath delivery

  • cycling (breath termination)


Triggering (breath initiation)

  • may be performed by the patient or the ventilator (based on time if unconscious)

    • flow-triggering: air flow threshold set by the clinician

    • pressure-triggering: the patient triggers the ventilator by having the pressure reach a threshold set by the clinician 

Limit / Target (breath delivery):

  • the ventilator is limited or targets a certain flow (flow-targeted breath) or pressure (pressure-targeted breath)

  • flow-limited breath: the clinician manually sets the flow, which builds up quickly and is maintained at a constant level (usually 60L/min). The pressure starts to build up in the lungs along with the tidal volume until the cycling parameter selected in achieved

    • looks like a "square"

    • selected when you want the inspiratory time to be minimized thus allowing more time for exhalation (eg. COPD)


  • pressure-limit breath: the flow of gas occurs upto a pressure limit. The flow shoots up and then comes down to baseline (there is no flow at this point, the lungs are not inflating and thus pressure is maintained). The pressure is held for the inspiratory time (eg. 1.5 sec)

    • looks like a "ramp", also known as "decelerating"

    • useful when ventilating heterogeneous lung (eg. ARDS)


  • rise time is the speed at which the targeted pressure is reached

Cycling (breath termination):

  • 4 signals for cycling:

    • volume

    • pressure

    • time

    • flow

  • volume-cycled: achived when a certain amount of tidal volume has been delivered

  • pressure-cycled: rarely used; often used as a backup when airway pressure reaches the set high pressure alarm level 

  • time-cycled: inspiratory valve will close and expiratory valve will open after a certain amount of time has passed (eg. respiratory rate, I:E ratio)

  • flow-cycled: when a certain percentage of the flow has decreased, cycling occurs



  • pressure control

    • pressure-targeted breath with a fixed inspiratory time

    • the pressure is applied for a period of time determined by the clinician; thus all breaths have the same inspiratory time

  • volume control, time-cycled

    • cycling into exhalation is dependant on the i-time (not when the patient reaches a set TV)

    • only when the i-time has been reached will the ventilator then cycle into exhalation

Types of Breath

  • Mandatory

    • T​

    • L

    • C

  • Assisted

    • T​

    • L

    • C

  • Spontaneous

    • T​

    • L

    • C

6 methods of positive pressure ventilation:

  • volume control

  • pressure control

  • assist-control

  • pressure support

  • intermittent mandatory ventilation

  • positive end-expiratory pressure

Continuous Mandatory Ventilation (CMV) / Assist Control (AC)

  • every breath will be the SAME volume regardless if it was triggered by the ventilator or the patient

  • trigger: time (if unconscious) or patient (red circles)

  • limit: flow

  • cycle: tidal volume

  • at the start of the breath cycle, the ventilator senses a patients attempt at inhalation by detecting negative airway pressure or inspiratory flow. This pressure or flow needs to reach a threshold before anything happens - called the trigger sensitivity. If no patient effort is sensed, the ventilator will deliver a breath

  • once triggered, the ventilator delivers the preselected tidal volume by inflating the lungs at a constant flow rate until the preselcted tidal volume is reached; the ventilator does this using whatever pressure needed to reach that tidal volume.


  • flow wave (the patient is limited to the flow rate and pattern selected)

    • constant square waveform (good when you want to minimize inspiratory time thus allowing more time for exhalation, example COPD)

    • ramp decelerating waveform (good for ventilating a heterogenous lung, example ARDS)

    • flow rate and pattern is usually selected for patient comfort and ventilator synchrony

    • inspiratory flow will last until the tidal volume is delivered at which time the breath is cycled-off - hence volume-cycled mechanical ventilation



  • delivers constant volume despite changes in lung properties (eg. if airway resistance increases or lung compliance decreases, the ventilator will generate higher pressures to deliver the preselected tidal volume); this maintains the desired minute ventilation 



  • rapid breathing could lead to hyperventilation and respiragtory alkalosis which can lead to "breath stacking"


Intermittent Mandatory Ventilation

  • this mode came about because of the high respiratory rates associated with AC in neonates 

  • IMV was designed to allow spontaneous breaths in between ventilator breaths (spontaneous breaths are patient intiated and terminated); there would be a flow (about 60L/min) running around the circuit available during patient initiated breath

  • SIMV and AC are the same exact mode in a heavily sedated patient

  • synchronized intermittent mandatory ventilation (SIMV) is when the ventilator breath is synchronized with the patients spontaneous breath - these are NOT assisted breaths

    • ​SIMV volume control + pressure support

    • SIMV pressure control and pressure support

    • SIMV PRVC and pressure support


mandatory (non-synchronized) breaths

  • trigger: time

  • limit: flow (volume)

  • cycle: volume


assisted (synchronized) breaths

  • trigger: pressure or flow

  • limit: flow (volume)

  • cycle: time 


SIMV + volume cycled

  • trigger: time or patient

  • limit: flow

  • cycle: volume


SIMV + pressure limited

  • trigger: time or patient

  • limit: pressure

  • cycle: time

Pressure Control - CMV

  • trigger: time or patient 

  • limit: pressure 

  • cycle: time

  • absolute vs. delta

  • inspiratory time: 1.5 sec (how long the breath will be held)

  • the ventilator delivers a flow to maintain the present pressure at a present respiratory rate over a present inspiratory time

SIMV (synchronized intermittent mandatory ventilation) with pressure support

  • mixture of mandatory breaths (some of which are synchronized with spontaneous breaths) and assisted breaths 


  • if no trigger, the set TV will be given

  • when a spontaneous breath is initiated, the patient determines the volume (eg. if they want a lower volume, they will receive a lower volume)

  • this gives you a good idea if the patient is ready to weaned off mechanical ventilation

  • if the patient can not take a big enough breath, pressure support is added to the spontaneous breath (you don't need to switch back to AC)

  • if you find yourself adding pressure support > 20 cmH2O, the patient is not ready to take spontaneous breath and you might consider changing back to AC mode


  • can turn off the piston but maintain MAP by adjusting your bias flow - great for tracheal/pneumothorax to prevent further barotrauma

Pressure Support / Continuous Spontaneous Ventilation​

  • trigger: patient effort (don't use if sedated or unconscious)

  • limit: pressure (the pressure only builds up for as long as the patient is breathing in)

  • cycle: flow (when the patient exhales the ventilator will cycle into exhalation)

  • once the patient triggers, the pressure will build up to what ever level you set the pressure at (eg. 10cm H2O). As the patient is breathing in, they are creating inspiratory flow. The tidal volume will also go up as long as they are breathing in. As soon as the patient starts to exhale and the peak flow drops to about 25% of the initial peak flow, the ventilator will then cycle into exhalation

  • the purpose of this mode is to augment the tidal volume to decrease the amount of work the patient has to do

  • inspiratory time is controlled by the patient so each breath will look different

  • the pressure support level will be the same for every breath

  • tidal volume will be different for each breath

  • only occurs on inhalation

  • is always spontaneous

  • have variable inspiratory times

  • the breath is terminated when flow reaches a present proportion of the peak flow (cycling sensitivity); for example, if a patient has a peak flow of 60 L/min and cycling sensitivity is set to 25%, then assisted inspiration is halted when flow reaches 15 L/min

  • these breaths have random start times 

  • similar to AC, but instead of delivering a certain volume, the patient will get a certain amount of pressure

  • patient initiates all the breaths

  • popular weaning mode



Ideal Tidal Volume

  • 8cc/kg (ideal body weight)




Air goes in many ways: (each one of these ways is a different mode)

  • inflate to certain volume

  • inflate to certain pressure

  • inflate at certain rate

  • adjust the flow rate (determines if the TV is given slow or fast)



  • the airways, as they taper, are responsible for producing pressure (Peak Pressure); this occurs when there is air flow

    • example: a problem with the airways will cause peak pressure to go up (bronchospasms, secretions, mucous plug, biting ETT)

  • once the air is in the lungs (alveoli) and there is no more air movement, the pressure is now determined by the compliance of the lung and not the airway resistance (Plateau Pressure); this occurs when the air stops

    • example: if there is a problem with lung compliance, plateau pressure goes up (pulmonary edema, pneumothorax, ARDS, pneumonia)

    • done with inspiratory hold

  • Peak - Plateau > 5 = elevated peak pressure

  • Ventilation can be increased by increasing TV or RR (TV more efficient because there is 150cc of dead space with every breath); in COPD patients, if the RR or TV is too high you could have "breath stacking" 

  • FiO2 > 50% can cause bronchitis

  • PEEP is the pressure left in the alveoli at the end of exhalation; PEEP opens more alveoli; too much pressure at the end of exhalation causes an increase in intrathoracic pressure which could compromise hemodynamics (increasing the pressure in the right atrium decreases the pressure difference in venous return)




Pressure / Volume Loop

  • "tail" cross over signifies that a patient is triggering a breath; if very large, could indicate an increased work of breathing - a lot of effort to trigger the brerth is required and may need more support

  • clock-wise = pure spontaneous negative pressure breath

  • "beaking" represents overdistention (barotrauma); decrease the volume down to the inflection point

  • shift right = decreased compliance

  • more PEEP needed = inhalation started but no volume has started going in (flat line); start PEEP at the inflection point


Flow / Volume Loop

  • all lines should be smooth

  • saw-tooth bottom line = sputum present

  • air trapping = when the line does not return to zero at the end of the breath


Modes of Ventilation

  • Assist Control (AC) / Continuous Mandatory Ventilation (CMV)

    • patient triggers the ventilator to deliver TV by taking a breath and creating negative pressure (appears as a negative deflection on the pressure curve)

    • the ventilator will assist the patient effort by giving them the set TV

    • PROS: patient is weak but needs a regular amount of volume, the patient only needs to achieve the trigger and the full volume set will be given

    • CONS: if the patient wants a smaller volume, this can't be done and leads to hyperventilation (uncomfortable). Switching them to SIMV might help. 

    • example: setting the rate at 12 will never allow the patient to breath less than 12 (considered a backup rate if for example overly sedated)

    • PV=nRT; compliance = V/P

    • by setting the TV, the ventilator will tell you what the pressure is

    • alarms can be set if the pressure goes up (change in compliance)

    • in pressure control, you set the pressure and a certain volume will be given based on the compliance of the lung

    • flow: will be constant (square) that can be set - how fast the TV is delivered - important in COPD; will go down once the target TV is reached

    • if the compliance of the patient changes, TV set will still be given BUT the pressure will be higher (you can set a pressure alarm on the ventilator to alert you)

    • SUMMARY: dial in the volume, pressure is variable depending on the compliance


  • Pressure Regulated Volume Control (PRVC)

    • similar to assist control (volume-control modes) - delivers a set TV if not trigger

    • pressure alarm will sound when PIP set to 5 less the set alarm in order to alert you that the pressure is increasing - once it reaches this pressure, the TV will stop and exhalation will begin

    • flow pattern is decelerating (not square which is set by the RT usually at 60L/min) which starts fast and slows down as they inhale. This produces a lower PIP. The flow is automatically set and adjusted by the ventilator and not RT. The ventilator knows which flow to set by looking at the last 3 breaths. Normally, square pattern creates a higher PIP.

    • PRVC is all about regulating the pressure to protect the lung


  • Pressure Control

    • a pressure is set at which you want to ventilate

    • if the compliance is low, you get low TV (vice versa)

    • patient or time can trigger a set change in pressure

    • alarms can be set if the volumes fall by a certain amount (change in compliance)


  • Continuous Positive Airway Pressure (CPAP) / PEEP

    • dialing in a presure that is continuous (not matter if breathing in or out)

    • pressure is present during inspiration and expiration

    • PEEP is present at the end of expiration (instead of having zero pressure at the end of expiration) - this recruits alveoli

    • the continuous pressure keeps the tongue out of the way in obstructive sleep apnea

Ventilator Asynchrony

Flow Mismatch

  • flow is set inappropriatly for patients need

  • "scooping" of the pressure waveform

  • flow needs to be increased to meet patients inspiratory demand

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