Ventilation

Initiation of mechanical ventilation 

Objective of this station is to familiarize with clinical assessment of the patient before initiation of mechanical ventilation.

 Clinical parameters include:

• Level of consciousness and patients ability to cooperate
• Use of accessory muscles of respiration
• Restlessness, anxiety, sweating and posture preferred by the patient
• Presence of cyanosis
• Altered chest wall movements
• Pulse rate and rhythm disturbances
• Oxygen saturation
• Assess for effectiveness of cough reflex and phonation
• Examination of lungs

Note: The decision to initiate mechanical ventilation should be made considering multiple parameters and not a single parameter. Especially normal saturation does not mean ventilation is normal. (Misnomer of most of clinicians).
Skill of monitoring tidal volume and minute ventilation

Correct use of the Wright respirometer

• Wash hands and wear gloves
• Make the patient sit in as upright a posture as is comfortable
• Connect the respirometer to the patient through a mask or to endotrachal tube if already intubated.
• Instruct the to breathe normally

Skill of measurement of peak expiratory flow rate (PEFR)

Notes: Normal PEFR 450-700ml/min for men and 300-300l/min for women.

Lower values indicate high resistance to gas flow (asthma, COPD and bronchospasm)

• Wash hands and wear gloves
• Make the patient sit in as upright a posture as is comfortable
• Connect the peak flow meter holding vertical
• Instruct the patient to inhale as deeply as possible and exhale hard and as fast as possible into the peak flow meter.
• Note the PEFR (calculate average of three readings)

Setting the ventilator

Equipment:     Ventilator with tubing

                   Test lung

                   Humidifier  

• Heater wire should be properly incorporated to avoid air way burns. (patient end temperature should be at 37)
• Water traps and tubes should be placed below the patient’s level.
• Humidifier- warm water humidifier is preferred. Temperature at the chamber should be not more than 60⁰ C and alarm should be set at this level.
• In addition a filter may be used at the patient end and need of changing these filters everyday or when ever contaminated with secretions.

Ventilator self check: available in modern ventilators.

Guidelines for initiating mechanical ventilation:

FiO₂- common practice is to start with high FiO₂ e.g. 1.0 and then to titrate to achieve the desired SpO₂and PaO₂

Note: FiO₂ should be set at the minimum level which maintains SpO₂ (e.g. at or above 90%)

• Tidal volume (TV):  average TV 8-10ml/Kg.
• In pressure control mode set the pressure to get the desired TV
• Respiratory rate (RR):  adjusted to get desired minute volume once TV is set. Minute volume requirement vary with the metabolic demands and proportion of wasted ventilation.
• I: E ratio:  conventionally employ an I: E ratio of 1:2
• Note: Increasing I: E ratio will increase the mean airway pressure, which may cause air trapping and generate an auto PEEP.
• Inspiratory pause: inspiratory pause or plateau refers to the option of maintaining lung volume at end of inspiration for the time set by the operator. Longer inspiratory pause time results in alveolar recruitment. But it may damage relatively normal lung units.
• Inspiratory flow rate: this determines the rate at which the tidal volume is delivered. For e.g. during volume cycled ventilation, peak inspiratory flow rate should be at least four times the minute volume.
• MV     15 l/min
• Flow setting 60 l/min
• Positive End-expiratory Pressure (PEEP): aim is to improve oxygenation in hypoxaemic failure and reduce work of breathing (COPD).

Goal: maintain PaO₂ of more than 60-70 mmHg at FiO₂ < 0.5 it is important to select a level of PEEP which produces greatest improvement of oxygenation. Use of PEEP of 5-10cmH₂O is extremely effective with minimal haemodynamical disturbances.

 Different modes of ventilation

Ø  Volume control ventilation

         Delivers a volume disregarding the pressure that it may generate

Ø  Pressure control ventilation

         Delivers up to a pre set pressure disregarding the volume that it may deliver.

         The volume delivered changes with the lung compliance which may change time

to time. As a result a high or a low volume may be delivered resulting in hypoventilation or barotrauma.

Ø  Factors that terminate inspiration – time, pressure and volume

         This is called time, pressure and volume cycling

Ø  Factors that can initiate inspiration

         Generally this is time. However it can be triggered by pressure or flow.

         For triggering the patient should exert some breathing effort.

Ø  PEEP – positive end expiratory pressure pneumatically splint the alveoli at end expiration. Can be used in spontaneous or mechanical breaths. 

Ø  Pressure support (PS) - pressurizes the inspiratory limb to aid inspiration in a

         spontaneously breathing patient.

Ø  CPAP – continuous positive air way pressure

    Continuous pressure throughout the respiratory cycle by a tight fitting face mask.

    This provides both PS and PEEP. A higher CPAP may however interfere with

    expiration.

Ø  BIPAP – CPAP at two different levels. One level for inspiration and another for expiration. A lower expiratory pressure minimize the impedance to exhalation

Ø  SIMV – Synchronized intermittent mandatory ventilation

A set rate and a volume is delivered either in the pressure or the volume controlled  mode. If the patient is breathing the machine breath will always fall on a spontaneous breath. In between mechanical breaths the patient is free to breath. A pressure support can be added to facilitate the spontaneous breaths.