Mechanical Ventilation Flashcards
Mechanical Ventilation:
- replaces the function of the inspiratory muscles by delivering gas under positive pressure to the lungs. This substitutes for the respiratory pump
- Either the ventilator or the patient “triggers” inspiration. If Patient triggered - the ventilator delivers the breath as soon as it senses the beginning of the patients inspiration
Respiratory pump is:
The abdominal and thoracic structures that contribute to the expansion and contraction of the lungs
Compliance reflects:
the ability to change the shape of a structure when mechanical load applied. So Lung compliance is the ability of the alveoli and lung tissue to expand on inspiration
Intubation process occurs by:
- Endotracheal tubes (ETT): most common route to ventilation
- Tracheostomy: often follows if ventilation is predicted to be prolonged. Sometimes 1st choice of intubation if upper airway obstruction or severe facial trauma present. Helps with weaning process
- Nasal endotracheal tubes: less common in adult, often 1st choice in paediatrics. Normally used after failed attempts with ETT. May be used as 1st choice if mouth is inaccessible, i.e. mouth Ca, vocal cord Ca.
Mechanical Ventilation Indications include:
- Hospitals will vary: Most common indications: inadequate oxygenation (type 1 respiratory failure), inadequate ventilation (type 2 respiratory failure), inability to protect airway (disturbed consciousness)
- Post-operative, Head injuries, Polytrauma: to maintain coma after major trauma/surgery
- Spinal injury
- Airway obstruction
Complications of intubation include:
- CVS instability: intubation requires sedation and paralysis reducing BP. This is difficult to manage and often requires other strong meds to increase BP.
- Barotrauma: if ventilation settings mismatch
- V/Q mismatch
- Discomfort: ETT once awake may lead to coughing and gagging
- Excess secretions/infection: process is not sterile
- Complications of high O2,
- Gut & bowel dysfunction: sedatives and paralysing drugs
- Weakened respiratory muscles: if ventilation is prolonged or resp. muscles weak prior to intubation
Compliance varies depending on:
the elasticity and surface tension of the lungs. The stiffer the lung the less compliant, i.e. PF, CF, COPD, recurrent infection and inflammation causing fibrotic changes
Relationship between lung compliance and mechanical ventilation:
Poorly compliant lungs are harder to mechanically ventilate
Mechanical ventilation process includes:
- During complete mechanical ventilation an air & oxygen mixture is pushed into the lungs for inspiration.
- The gas flow is then stopped and air is allowed to be passively exhaled.
- Mechanical ventilation uses positive pressures
Effect of Ventilation on VQ:
- Accentuates the perfusion gradient
- Reverses the ventilation gradient: 1) diaphragm is passive 2) positive pressure takes the path of least resistance 3) lower regions compressed by increased perfusion 4) absorption atelectasis at higher oxygen concentrations
Ventilation settings include:
- Inspiratory pressure or tidal volume driven: depending on mode
- RR
- PEEP: required for gas exchange
- FiO2: oxygen requirement for gas exchange
- I:E ratio: manipulate length in inspiration or expiration to either help with oxygenation (2:1) or CO2 removal (1:3).
Invasive mechanical ventilation can be controlled by:
- Pressure controlled
- Volume controlled
- Dual mode
Pressure controlled ventilation means:
- Respiratory rate, inspiratory time are pre-set. Flow is delivered to a pre-set target pressure limit during inspiration until target pre-set pressure is reached & then expiration is allowed to happen passively.
- Pressure is constant and set so the volume can change from breath to breath depending on lung compliance. Better lung compliance leads to larger lung volume, poorer lung compliance leads to smaller volumes and does not guarantee adequate amount of volume, i.e. airway obstructed the pressure will be reached very quickly therefore only a small volume of gas is delivered. Hence, poor compliance may suggest areas of collapse and sputum retention
Volume controlled ventilation means:
- Inspiratory time, pause time and respiratory rate are all pre-set. Ventilator delivers a pre-set tidal volume.
- Airway pressure rises slowly as the ventilator reaches the desired volume (normal TV for age, gender) during inspiration and once reached expiration is allowed to occur passively.
- Peak airway pressure will vary from breath to breath as this mode is volume controlled and the ventilator will deliver its set volume irrespective of variations in lung compliance and resistance to flow. If low lung compliance patient at risk of barotrauma hence pneumothorax, clinically ventilators are advanced enough with pressure detection to prevent barotrauma.
Dual mode controlled ventilation means:
- Combines volume control and pressure control in an attempt to avoid barotrauma but maintain good lung volumes.
- Delivers a pre-set volume with the lowest possible pressure. If the volume falls below the pre set value the pressure level rises but only to a point (upper pressure limit). Expiration remains passive
- If the upper pressure limit is reached before the ventilator can deliver the appropriate amount of volume that has been set, the ventilator will alarm indicating setting need updating.
Main advantage of pressure controlled ventilation:
pressure can be controlled reducing the risk of barotrauma and volutrauma (overdistention of normal alveoli) in patients with stiff lungs
Main advantage of volume controlled ventilation:
reduces risk of collapse and sputum retention
Mechanical ventilation modes:
- Spontaneous modes: Pressure Support (PS)/Assisted Spontaneous Breath (ASB), Biphasic positive airway pressure (BiPAP) and CPAP need the patient to be able to initiate a breath or they won’t provide any support for ventilation
- Synchronised Intermittent Mandatory Ventilation (SIMV): Automode allows for some synchronisation with the patients effort.
- Controlled Mandatory Ventilation (CMV): Fully controlled by the machine
Pressure support (PS) or AKA Assisted spontaneous Breath (ASB) indications:
Self-breathing, to reduce ventilatory support and increase breath time.
