What are indications for Mechanical Ventilation?
- clinical or laboratory signs that the patient cannot maintain an airway or adequate oxygenation or ventilation.
- include respiratory rate > 30/min
- inability to maintain arterial O2 saturation > 90% with fractional inspired O2 (Fio2) > 0.60
- PCO2 of > 50 mm Hg with pH < 7.25
A little about Lung Mechanics and Ventilators when you breathe normally, your lungs SUCK AIR Into the lung, this creates a NEGATIVE pressure gradient btn the lung and intrapleural spacewhen your breathing depends on a machine, the ventilator creates the pressure gradient artificially by BLOWING air into your lungs via POSITIVE pressure
Here are some interesting things about Pressure and the waveforms we look at on the ventilator:
- Peak airway pressure(PAP): measures pressure at the airway
It represents the total pressure needed to overcome the inspiratory flow resistance (resistive pressure), the elastic recoil of the lung and chest wall (elastic pressure), and the alveolar pressure present at the beginning of the breath
PAP= Presistive + Pelastic + Palveolar
- Resistive pressure: product of
- circuit resistance
- airflow: IN Mechanically Ventilated patient depends on
- ventilator circuit
- endotracheal tube
- patient’s airways
NOTE: even when these factors are constant, an increase in airflow increases resistive pressure.
- Elastic pressure : product of
- elastic recoil of the lungs
- chest wall (elastance)
- volume of gas delivered
NOTE: anything that makes it harder to distend the alveoli(ie stretch the lungs) will increase elastic pressure
- eg. Inc lung stiffness: pulmonary fibrosis
- eg. Chest wall/diaphragm restriction: tense ascites, obesity
- elasticity: measures lung recoil
- compliance: measures lung stretchability
- elasticity and compliance are inversely proportional to one another
- End-expiratory pressure: normally the same as atmospheric pressure.
- INTRINISIC PEEP/AUTO PEEP: when the alveoli fail to empty completely because of
- airway obstruction
- airflow limitation, or shortened expiratory time
NOTE: you can ADD PEEP to the Mechanically Ventilated patient , this is therapeutic PEEP
What are signs of BRONCHOSPASM in the Mechanically Ventilated patient?
- elevated resistive pressure (eg, > 10 cm H2O) suggests
- kinked ETT, ETT plugging
- intraluminal mass
- increased intraluminal secretions
- intrinsic PEEP suggests:
- airflow obstruction (eg, airway secretions, bronchospasm)