INVASIVE MECHANICAL VENTILATION

D. Sara Salarian,

Nov 2006Kishore P.

Critical Care Conference

Why ventilate? Improve oxygenation Increase/maintain minute ventilation and

help CO2 clearance Decrease work of breathing Protect airway

Mask based device

Negative pressure ventilators“The Iron Lung”

Origins of mechanical ventilation

• Negative-pressure ventilators (“iron lungs”)

• Non-invasive ventilation first used in Boston Children’s Hospital in 1928

• Used extensively during polio outbreaks in 1940s – 1950s

• Positive-pressure ventilators

• Invasive ventilation first used at Massachusetts General Hospital in 1955

• Now the modern standard of mechanical ventilation

The era of intensive care medicine began with positive-pressure ventilation

The iron lung created negative pressure in abdomen as well as the chest, decreasing cardiac output.

Iron lung polio ward at Rancho Los Amigos Hospital in 1953.

Modes of Mechanical Ventilation

Spontaneous/Controlled/Dual Controlled Mechanical Ventilation (CMV) Assist Control (AC)/Volume Control (VC) Intermittent Mandatory Ventilation (SIMV) Pressure Control (PCV) Pressure Support Ventilation (PSV)

Nov 2006Kishore P.

Critical Care Conference

Patient system interaction

Assist control Assisted-pressure supportControlled MV

Spontaneous

SIMVIMV

+PEEP

CPAP

Breath Types

1.Spontaneous Breath • Inspiration is both initiated and

terminated by the patient. 2.Mandatory Breath

• Inspiration is either initiated or terminated by the ventilator.

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Breath Patterns

1.Continuous Mandatory Ventilation • CMV • All breaths mandatory

2.Intermittent Mandatory Ventilation • IMV or SIMV • Mandatory and spontaneous

breaths 3.Continuous Spontaneous

Ventilation • All breaths spontaneous

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Phase Variables

TRIGGER starts inspiration Example: pressure drop when patient

sucks in LIMIT preset inspiratory value

Example: preset maximum inspiratory flow

CYCLE stops inspiration Example: preset inspiratory time

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VOLUME-CYCLED VENTILATION

the controlled variables of tidal volume and inspiratory flow determine airway pressure and inspiratory time

Variations in airway resistance or lung compliance alter airway pressures but do not affect minute ventilation

There are three methods of initiating the inspiratory phase in volume-cycled mechanical ventilators: controlled, assist-control, and intermittent mandatory ventilation (IMV)

Nov 2006Kishore P.

Critical Care Conference

Modes of ventilationPressure controlled

Ventilator applies a predefined target pressure to the airway during inspiration

Adv.- decreased risk of barotrauma

Disadv.- with decreasing compliance or increasing resistance, tidal volume and minute ventilation fall

Comparison chartVolume Pressur

e Flow I-time

Spontaneous

variable

variable

variable

variable

VCV FIXED variable FIXED FIXED

PCV variable FIXED

variable FIXED

PSVvariable FIXED

Variable

variable

MODES OF PPV

C M VA /C V o lu m e

A /C P ressu re

S IM VS IM V V o lu m e

S IM V P ressu re

S P O N TA N E O U SC P A P w /P S V

C P A P w /o P S VO th er

3 C ateg ories o f P P V

Controlled mechanical ventilation (CMV)

minute ventilation is completely dependent upon the rate and tidal volume set on the ventilator. Any respiratory efforts made by the patient do not contribute to minute ventilation

Controlled ventilation is the required ventilatory mode in patients who are making no respiratory effort (eg, spinal cord injury or drug overdose and those who have been subjected to pharmacologic paralysis).

Controlled Mechanical Ventilation Advantages: rests muscles of respiration Disadvantages: requires sedation/neuro-

muscular blockade, potential adverse hemodynamic effects

Controlled Mode (Pressure-Targeted Ventilation)

Pressure

Flow

Volume

(L/min)

(cm H2O)

(ml)Time (sec)

Time-Cycled

Set PC level

Time Triggered, Pressure Limited, Time Cycled Ventilation

Pressure Control Ventilation C = VT / PC

Flow

Pressure

VolumeCl

Cl

Set PC level

Time (sec)

(L/min)

(cm H2O)

(ml)

Controlled Mode Volume Targeted

Flow

Pressure

Volume

Time (sec)

(L/min)

(cm H2O)

(ml)

ASSIST-CONTROL MECHANICAL VENTILATION

In the assist-control (A/C) mode, the ventilator senses an inspiratory effort by the patient and responds by delivering a preset tidal volume. Every inspiratory effort that satisfies the ventilator's demand valve trigger threshold initiates delivery of the preset tidal volume

Patient work is therefore required to trigger the ventilator and continues during inspiration

A control mode back-up rate is set on the ventilator to prevent hypoventilation

Assist-Control Ventilation Volume or time-cycled breaths + minimal

ventilator rate Additional breaths delivered with inspiratory effort

Order: AC Vt 500, RR12, 100% FiO2, 5 PEEP

Assist-Control Ventilation Advantages: reduced work of breathing; allows

patient to modify minute ventilation Disadvantages: potential adverse hemodynamic

effects or inappropriate hyperventilation

INTERMITTENT MANDATORY VENTILATION

With intermittent mandatory ventilation (IMV), the degree of ventilatory support is determined by the selected IMV rate. At regular intervals, the ventilator delivers a breath based upon a preset tidal volume and rate. In addition, the patient is allowed to breathe spontaneously through the ventilator circuit at a tidal volume and rate determined according to need and capacity.

Most present day ventilators synchronize the intermittent ventilator breaths with inspiratory effort by the patient, a modality termed synchronized IMV or SIMV. However, this modification requires a trigger modality

Synchronized Intermittent Mandatory Ventilation (SIMV)

Potential advantages More comfortable for some

patients Less hemodynamic effects

Potential disadvantages Increased work of breathing

SIMV(Volume-Targeted Ventilation)

Spontaneous Breaths

Flow(L/m)

Pressure(cm H2O)

Volume(mL)

Pressure

Flow

Volume

(L/min)

(cm H2O)

(ml)

SIMV + PS (Pressure-Targeted

Ventilation)

PS Breath

Set PS levelSet PC level

Time (sec)

Time-Cycled Flow-Cycled

PRESSURE SUPPORT VENTILATION Pressure support ventilation (PSV) is flow-

cycled in that, once triggered by a demand valve, the preset pressure is sustained until the inspiratory flow tapers, usually to 25 percent of its maximal value [22]. PSV tends to be a comfortable ventilatory modality because the patient has greater control over ventilator cycling and flow rates. Close monitoring is required whenever PSV is used alone because neither tidal volume nor minute ventilation is guaranteed. PSV can be added during full or partial support with SIMV to overcome endotracheal tube and ventilator circuitry resistance encountered during spontaneous breaths

Pressure-Support Ventilation Pressure assist during spontaneous inspiration

with flow-cycled breath Pressure assist continues until inspiratory

effort decreases Delivered tidal volume dependent on

inspiratory effort and resistance/compliance of lung/thorax

Order: PS 10, PEEP 0, 50% FiO2

Pressure-Support Ventilation Potential advantages

Patient comfort Decreased work of breathing May enhance patient-ventilator synchrony Used with SIMV to support spontaneous

breaths

Pressure-Support Ventilation Potential disadvantages

Variable tidal volume if pulmonary resistance/compliance changes rapidly

If sole mode of ventilation, apnea alarm mode may be only backup

Gas leak from circuit may interfere with cycling

CPAP + PSV

Set PS level

CPAP level

Time (sec)

Flow(L/m)

Pressure(cm H2O)

Volume(mL)

Flow Cycling

VENTILATOR SETTINGS Ventilatory support requires consideration of

trigger mode and sensitivity, respiratory rate, tidal volume, flow rate, flow pattern, and the fraction of inspired oxygen (FiO2).

Ventilator Settings

Mode Rate Volume (VT) Pressure FIO2 PEEP I:E

Nov 2006Kishore P.

Critical Care Conference

Key concepts Determinants of CO2 clearance

- Ventilator factors* Rate* Tidal volume* Anatomical dead space

- Patient factors* Physiological dead space* CO2 production

Alveolar minute ventilation

Nov 2006Kishore P.

Critical Care Conference

Key concepts Determinants of Oxygenation

- Ventilator factors:* FiO2 ( fraction of oxygen in inspired air)* Mean airway pressure* PEEP ( positive end expiratory pressure)

- Patient factors* V/Q (ventilation/ perfusion) mismatch* Shunt* Diffusion defect* Reduced mixed venous oxygen

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Critical Care Conference

Adjust FiO2 and PEEP according to PaO2 and SpO2

Adjust TV and rate according to PCO2 and pH

POSITIVE END-EXPIRATORY PRESSURE PEEP: an elevation in

alveolar pressure above atmospheric pressure at the end of exhalation

Extrinsic PEEP (ePEEP): applied through a mechanical ventilator

ACV without PEEP

ACV with PEEP

Auto-PEEP Detection viathe Flow Waveform

Normal

--------------------------------------------------------

Flow does not return

to zero- Auto-PEEP

Air TrappingInspiration

Expiration

NormalPatient

Time (sec)

Flow

(L/m

in)

Air TrappingAuto-PEEP

}

Flow-Volume Loop

Volume (ml)

PEFR

FRC

Inspiration

Expiration

Flo w

(L/

mi n

)

PIFR

VT

Air TrappingInspiration

Expiration

Volume (ml)

Flow (L/min)

Does not returnto baseline

NormalAbnormal

Air Leak

Inspiration

Expiration

Volume (ml)

Flow (L/min)

Air Leak in mL

NormalAbnormal

Airway Secretions/Water in the Circuit

Inspiration

Expiration

Volume (ml)

Flow (L/min)

NormalAbnormal

Origins