Basic Concepts in Adult Mechanical Ventilation

Pa

Points of Discussion

1.Basic respiratory dynamics

2.Basic ventilator parameters

3.Control variables: pressure, volume

4.Phase variables: trigger, limit and cycle

5.Conditional variables

6.Breath types

Inspiration Inspiration

Spontaneous Breathing

Exhalation Exhalation

Precondition of Inspiration

Pa < PbSpontaneous

breathPb > Pa

Mechanical ventilation

PbPb

Pa

Gas Flow

PaPa << PbPb

PleuralPressure

Three - Dimensional Spring

Inspiration

Mechanical Breath

Spontaneous Breath

Pressure

Time

Alveolar Pressure Changes

Spontaneous Inspiration

Volume Change

Gas Flow

Pressure Difference

Mechanical Ventilation

Pressure Difference

Volume Change

Gas Flow

“The Feature of the Tube”

R = P

F

Airway Resistance

Pressure Difference = Flow Rate x Resistance of the Tube

Volume Change = Pressure Difference x Compliance of the Balloon

Volume Change = Pressure Difference x Compliance of the Balloon

Volume

Pressure

V

P

C = V P

Compliance

Compliance

Measures compliance of the lung and thorax Tidal volume / Plateau-PEEP Units = ml/cmH20

C= V P

R = PF

Compliance and Resistance

Resistance Measures airway resistance

Length Viscosity Flow Radius4

Peak-plateau / Flowrate Units = cmH20/Lps

Peak and Plateau Pressures

Peak airway pressure reflects Baseline (PEEP) Pressure due to compliance (L+T) Pressure due to resistance

Plateau pressure (breath hold) reflects Baseline (PEEP) Pressure due to compliance (L+T) (alveolar distending pressure)

Peak Alveolar and Transpulmonary PressuresP(t) = VT/CR+ Flow x RR + PEEP tot

meanPaw

External PEEPIntrinsic PEEP

Plateau pressure

Peak Airway Pressure+

_

_ _

_ _++

++ Palveolar

Ppleural

Ptranspulmonary = Palveolar - Ppleural Pplat = Maximum Palveolar

Alveolar Pressure

Transpulmonary pressure is a key determinant of alveolar distension.

Basic Ventilator ParametersFiO2

Fractional concentration of inspired oxygen delivered expressed as a % (21-100)

Breath Rate (f)The number of times

over a one minute period inspiration is initiated (bpm)

Tidal volume (VT)The amount of gas that

is delivered during inspiration expressed in mls or Liters. Inspired or exhaled.

FlowThe velocity of gas

flow or volume of gas per minute

Terminology

Terminology: PIP & MAP

A=A1+A2+…+AnPressure

Inspiration + Exhalation

A

Mean Airway Pressure

Peak Inspiratory PressurePeak Inspiratory Pressure

Time

Terminology: PEEP, I:E Ratio

Pressure

Time

Positive EndExpiratory Pr.

T insp. T exp. I : E = 1 : 2 I : E = 1 : 2

I : E = 4 : 1 I : E = 4 : 1

PEEP

PIP

PIP

PEEPT insp.

Terminology: Flow and Volume

Minute Ventilation = Tidal Volume x Breath RateMinute Ventilation = Tidal Volume x Breath Rate

Pressure

Flow

Time

Tidal Volume Tidal Volume

Volume

Flow Rate

Time

Volume = Flow X Time

PEEPDefinition

Positive end expiratory pressureApplication of a constant, positive pressure such

that at end exhalation, airway pressure does not return to a 0 baseline

Used with other mechanical ventilation modes such as A/C, SIMV, or PCV

Referred to as CPAP when applied to spontaneous breaths

PEEPIncreases functional residual capacity (FRC) and

improves oxygenationRecruits collapsed alveoliSplints and distends patent alveoliRedistributes lung fluid from alveoli to perivascular

space

5 cm H5 cm H22O O

PEEP PEEP

ClassificationControl variable

Flow (volume)Pressure

Phase variableTrigger, limit, cycle, baseline

Conditional variablePatient effort and time

“Control Variable”

Which parameter remains constant despiteWhich parameter remains constant despite changes in pulmonary mechanics?changes in pulmonary mechanics?

Pressure Generated BreathPressure Generated Breath

Pressure

Flow

Time

Control variablesVolume Ventilation Pressure Ventilation

Pressure

Flowtime

Pressure

Flowtime

If compliance decreases the pressure increases to maintain the same Vt

11 22 33 44 55 66

SECSEC

11 22 33 44 55 66

PPawaw

cmHcmH 2200

6060

-20-20

120120

120120

SECSEC

INSPINSP

EXHEXH

FlowFlowL/minL/min

Volume Control Breath Types

As compliance changes - flow and volumes change

1 2 3 4 5 6

SEC

1 2 3 4 5 6

PawcmH20

60

-20

120

120

SEC

INSP

EXH

FlowL/min

Volume Targeted (Pressure Controlled)

Pressure then raises to assure that the set tidal volume is delivered

1 2 3 4 5 6

SEC

1 2 3 4 5 6

PawcmH20

60

-20

120

120

SEC

INSP

EXH

FlowL/min

New Volume Targeted Breath Pressure Variability is Controlled

Phase Variables

A. Trigger mechanismWhat causes the breath to

begin? Patient (assisted)Machine (controlled)

B. Limit variableWhich parameter is sustained

at a preset level during the breath?

FlowPressure

C. Cycle mechanismWhat causes the breath to end?VolumeTime

A

B C

Trigger Variable- Start of a BreathTime - control ventilationPressure - patient assistedFlow - patient assistedVolume - patient assistedManual - operator control

Inspiratory Trigger MechanismTime

Controlled Mechanical VentilationPressureFlowFlowChest impedanceChest impedanceAbdominal movementAbdominal movement

Inspiratory - delivery limitsMaximum value that can be reached

but will not end the breath-VolumeFlowPressure

Breath TypesMandatory

Ventilator does the workVentilator controls start and stop

SpontaneousPatient takes on workPatient controls start and stop

AssistedPatients triggers the breathThe ventilator delivers the breath as per control variable

SupportedPatients triggers the breathVentilator delivers pressure support Breath cycles at set flow

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