Ventilation Sam Petty Clinical Specialist Physiotherapist John Farman ICU Addenbrooke’s Hospital

Nomenclature Which ventilators ? All modes called different things for the same basic principal e.g. PS/CPAP on Servo i and ASB on Draeger All mean air in:air out

Why do we ventilate?

Why do we ventilate Airway protection Normalise CO2 Oxygenation Lower consumption/unload respiratory muscles Apnoea e.g. High SCI

Consider Inspiration involves pumping high flow gas into a patient ventilation Reliant on normal passive recoil Bypasses usual URT therefore need to consider humidification Requires an interface: NIV, ETT, tracheostomy All contain built in software that recognises ‘normal’ flows in mammals therefore try to be as natural as possible Dual limb in invasive ventilation which provides data (important if machine is capable of running NIV and interface choices)

How do we choose which mode? Broadly speaking 3 choices Full mandatory ventilation Synchronised intermittent mandatory ventilation Supported ventilation

Mandatory modes When you require full control Patient will require sedation +/- muscle relaxant Tolerance/synchronicity Set volume or pressure Set RR PEEP Triger Rise time (rate of pressurisation)

Mandatory ventilation How to calculate Vt

Mandatory ventilation How to calculate Vt Height of patient 6-8 mls/kg of ideal body weight E.g. 170cm male Ideal weight 60-72 kg 360 mls is the lowest based on 6 mls/kg 576 mls is the highest based on 8 mls/kg 462 mls is the mid point based on mid range weight x 7 mls/kg

Mandatory ventilation If using Volume pre-set mode set Vt at chosen level If using Pressure pre-set mode set pressure that achieves approximately the chosen volume

So can a patient trigger a breath?

Volume Control Machine driven

Volume Control

Benefits of VC Shorter inspiratory time Longer inspiratory time Less risk of gas trapping Less effect on CVS system Longer inspiratory time improve oxygenation Higher mean airway pressure Re-distribution of gas Lower peak airway pressure More time available to deliver set Vt

Disadvantages of VC Not synchronous Inappropriate triggering will cause excessive minute ventilation Decrease in compliance of lung tissue will increase the risk of barotrauma

Pressure control Set pressure dependent on Vt you wish to achieve As with VC the patient can trigger the breath and will receive a mandatory breath Pressure is not the same as the alveolar pressure as this is dissipated across the airways Constant pressure applied. High initial flow as more alveoli to fill. Deceleration of flow falling to zero by end inspiration Changes in resistance/compliance will cause a change in Vt

Advantages PC Avoids high inspiratory pressures Rests muscles of respiration

Disadvantages PC Not synchronised Inappropriate triggering results in excessive MV Changes in lung compliance or resistance will result in a change in Vt

Pressure Support/ASB Patient ready to wean Set: FiO2 PEEP Trigger type and sensitivity Inspiratory rise time Cycle off Patient will determine RR and Vt

PS/ASB Trigger: flow v’s pressure Risk of too difficult or too easy – autotriggering Inspiratory flow – decelerating flow due to lung unit filling End of inspiration – determined by fall in flow End of expiration – when next breath is triggered

What to look for VT inspiration: VT expiration They should remain similar (generally within 20-30 mls variance and they will swap around) Is the patient achieving Vt RR limits ‘double triggering’ Apnoea ‘gas trapping’

Gas trapping

Gas trapping Likely in bronchospasm Long inspiratory time (therefore short expiratory time) High RR (ie. Absolute expiratory time is short) Progressive increase in intra-thoracic pressure Decreased venous return Decreased cardiac output May lead to PEA arrest

Synchronised modes Combination of mandatory mode + PS +PEEP Minimum mandatory RR Any additional breaths PS

Does asynchrony matter? Poor sleep Increased agitation Increased use of sedatives/muscle relaxants Prolonged ventilation Respiratory muscle weakness Increased LoS (both ICU and hospital) Less likely to be mobile

Finally – where?

Any Questions?