MECHANICAL VENTILATION

Dr ANVESH NARIMETI

UNIT 1 INTERNAL MEDICINE

• Mechanical ventilation is a therapeutic method that is used to assist or replace spontaneous breathing

• Objectives- understand the concept of negative pressure and positive pressure ventilation

• Mechanical ventilation is any means in which physical devices or machines are used to either assist or replace spotaneous respiration

Indications

• Four major indications a)Need for high levels of inspired

oxygen(hypoxic respiratory failure) b)Need for assisted ventilation (hypercapnic

respiratory failure or surgical procedures) c)Protection of airways against aspiration d)Relief from upper airway obstruction TYPES 2 types of ventilation a)Negative pressure ventilationb)Positive pressure ventilation

• a)Negative pressure ventilation-Pressure lower than atmospheric pressures is applied to extra thoracic space during the inspiration

• b)Positive pressure ventilation-pressure higher than atmospheric pressures is applied to the intra alveolar space during inspiration

Negative pressure ventilation• These are called IRON LUNGS• Only historic imporatance • Helped a lot during polio outbreak in 1952

Positive pressure ventilation

NORMAL LUNG MECHANICS• VOLUMES –TV,IRV,ERV,RV• CAPACITIES-IC,FRC(ERV+RV)• VC=IRV+TV+ERV• RV+VC=TLC• DEAD SPACE VOLUME-volume of air in the airways

and lungs that do not participate in gas exchange a)Anatomic dead space-air in conducting

airways not lined by diffusing membranes b)Physiologic dead space-sum of anotomic

dead space and volume of air in alveoli which are ventilated and not perfused

Compliance-how volume of a space changes with pressure changes

MOST IMPORTANT THING IS TO UNDERSTAND

INSPIRATORY AIRWAY PRESSURE P peak and P plateau

AND TO UNDERSTAND HOW TO USE P peak and P plateau to monitor AIRWAY RESISTANCE AND LUNG COMPLIANCE

• Measuring airway pressure is an most direct way to continously monitor lung mechanics

• Ventilation pressure=pressure delivered to proximal airway

=resistive pressure+elastic pressure

• P peak pressure-maximum pressure in the proximal airway at the end of inspiration

• P plateue=equlibrium pressure reached if the expiratory tubing is occluded at the end of inspiration-it is a surrogate of intra alveolar pressure

• Increase in P peak pressure in absence of an increase in P plateus= indicates increase in AIRWAY RESISTANCE

1. Brochospasm2. Extrinsic airway compression3. Mucous plugs4. Excessive aspiration5. Foreign body aspiration

Monitoring compliance • Compliance = change in volume/change in

pressure• So increase in plateue pressure indicates

compliance is decreasing1. Pulmonary edema2. Pleural effusion3. Pneumo thorax4. Et tube in rt main stem bronchus5. Abdominal distension

Normal gas exchange

• Most important is to calculate (A-a)gradient• It measure how effectively oxygen moves from

the alveoli into pulmonary vasculature• Normal (A-a)gradient=[age/4]+4• When there is increase in(A-a)gradient the

possible causes

HOW TO MONITOR GAS EXCHANGE

• Pulse oximetry• Arterial blood gas analysis• Capnography

Pulse oximetry

• It provides a real time measurement of the percentage of the haemoglobin that is bound to oxygen in arterial blood

Capnography

• Continous measurement of carbondioxide tension in expired air which can serve as a real time surrogate marker of carbondioxide tension in arterial blood

• If Paco2-Pet co2= if more than 51. Low cardiac output2. Copd3. Pulmonary embolism4. Advanced age

NON INVASIVE POSITIVE PRESSURE VENTILATION

NPPV- means to support the failing respiratory function by delivering oxygen enriched gas under pressure without requiring endotracheal intubation

• It is best used as short term strategy to buy time • Noninvasive ventilation usually is provided by using a

tight-fitting face mask or nasal mask similar to the masks traditionally used for treatment of sleep apnea.

• Noninvasive ventilation has proved highly effective in patients with respiratory failure from acute exacerbations of chronic obstructive pulmonary disease and is most frequently implemented by using bilevel positive airway pressure ventilation or pressure support ventilation.

• The major limitation to its widespread application has been patient intolerance because the tight-fitting mask required for NIV can cause both physical and emotional discomfort

• Benefits of NPPV1. Avoids trauma secondary to intubation2. Avoids the need of sedation3. Ability to communicate4. Aloows intermittent drinking eating if

aspiration risk is felt low5. Avoids ventilator associated pneumonia

Contraindications to NPPV1. Cardio pulmonary arrest2. Haemodynamic instability3. Fascial trauma and deformity4. Severe upper gastrointestinal bleed5. Severe encephalopathy6. Inability to cooperate and protect airway7. Inability to clear secretions8. Upper airway obstructions9. High risk for aspiration

• NPPV has two options CPAP- Continous Positive Airway Pressure BPAP- Bilevel Positive Airway Pressure• In both of these modes, a preset positive

pressure is applied during inspiration and a lower pressure is applied during expiration at the mask

Other indications of NPPV

• Obstructive sleep apnoea• Neuro muscular disease• Fascilitating weaning from ventilator• In immunocompramised states

VENTILATOR MODES

• We should know three variables that determine and define a ventilator mode• We should know most commonly

used ventilator modes

3 Variables

1. Trigger variable2. Control variable3. Cycling variable

Trigger variable

• It defines how the ventilator determines when to intiate a machine driven breath

• Common options1. Time triggered(no spontaneous breathing)2. Pressure triggered(spontaneous breathing)3. Flow triggered• un common options1. Volume,chest wall electrical impedence and

motion

Control variable

• It defines what aspect of inspiration is the primary variable controlled by the ventilator during inspiration

• Options1. Presssure controlled2. Flow controlled

Cycling variable

• It defines what signals the ventilator to terminate inspiration

• Options1. Volume cycled2. Flow cycle(when 25%of peak flow decreases)3. Time cycle

• Compliance=change in volume/change in pressure

• Volume targetted ventilation high compliance=low airway pressures low compliance=high airway pressures• Pressure targetted ventilation high compliance=increase volume low compliance = low volume

BASIC VENTILATOR MODES

• ASSIST CONTROL(AC)• SYNCHRONISED INTERMITTENT MANDATORY

VENTILATION(SIMV)• PRESSURE CONTROL VENTILATION(PCV)• PRESSURE SUPPORT VENTILATION(PSV)

ASSIST CONTROL(AC)

SYNCHRONISED INTERMITTENT MANDATORY VENTILATION(SIMV)

PRESSURE CONTROL VENTILATION

PRESSURE SUPPORT VENTILATION(PSV

DUAL CONTROLLED MODES

• Uses instantaneous feed back to control the aspects of lung volume and airway pressure simultaneously

• PRVC-Pressure Regulated Volume Control• Volume support• Volume Assured Pressure Support

VENTILATOR OPTIONS• We should understand basic options used

during mechanical ventilation• OPTIONS1. MODE2. FiO23. TIDAL VOLUME4. RESPIRATORY RATE5. PEEP6. PRESSURE SUPPORT7. FLOW SHAPE/CONTOUR8. I:E RATIO

FiO2• Applicable to all modes• Should be titrated to lowest possible level at the

same time maintaining adequate oxygenatiom• First set at 100% and then titrate down over

several hours indicated by pulse oximeter or serial ABG

• FiO2 >60% is toxic to lungs• If adequate oxygenation requires FiO2 >60%

then additional strategies like1. Increasing PEEP2. Recruitment manuever3. Trial of another mode

TIDAL VOLUME• Mostly applicable to volume cycled

modes(SIMV,AC)• WEIGHT BASED1. Healthy lungs-10ml/kg2. COPD -8ml/kg3. ARDS -6ml/kg• High tidal volume-lower Paco2,high ph,high

Pplateue• Low tidal volume-high Paco2,low ph,low P

plateue

RESPIRATORY RATE

• For time triggered mode and SIMV• TYPICAL-10-20 breaths/min• High respiratory rate- low paco2,high ph,high

risk of auto peep• Low respiratory rate- high paco2,low ph,low

risk of auto peep

POSITIVE END EXPIRATORY PRESSURE(PEEP)

• Continous positive pressure present through out all phase of ventilation

• Used in almost all pateints• Physiological effects-1. Increase alveolar recruitment,and alveolar surface area–

so improves oxygenation2. Decrease preload and LV after load-increase cardiac out

put in congestive cardiac failure3. Increase right ventricular load- increase right to left shunt• SET PEEP TO LOWEST POSSIBLE VALUE TO ALLOW FiO2

<60% WITH MINIMUM OF 5 CM OF H2O

Pressure support• Amount of additional positive pressure

beyond PEEP that is provided during the inspiration

• An integral parameter used in pressure support ventilation and BPAP, SIMV

FLOW SHAPE/CONTOUR• It describes the pattern of air flow during

inspiration• Set by clinician in volume targetted mode and

always decelerating shape in pressure targetted as a consequence of lung mechanics

I:E RATIO• Ratio between amount of time spent in

inspiration and amount of time spent in expiration

VENTILATOR ASSOCIATED LUNG INJURY

• It can occur in any lungs but most commonly in ARDS/ALI

• FOUR TYPES1. BARO TRAUMA2. VOLUM TRAUMA3. BIO TRAUMA4. CYCLIC ATELECTASIS

PROTECTIVE LUNG VENTILATION

• Target tidal volume close to 6 mL/kg of ideal body weight.

• Prevent plateau pressure over 30 cmH2O.• Lowest possible fraction of inspired oxygen

(FIO2) to keep SaO2 90%.• Adjust the PEEP to maintain alveolar patency

while preventing overdistention and closure/reopening

General support during mechanical ventilation

• Sedation and analgesia• DVT propylaxis• Bed sores prevention• Ulcer prophylaxis• Nutrition • Delayed gastric emptying is common responds

to promotility agents such as metoclopramide

Tracheostomy • It is generally agreed that if a

patient is in need of MV for more than 10–14 days, a tracheostomy is indicated and should be planned under optimal conditions

THANK YOU