Mechanical ventilation

Terms you need to know

1.       PaO2: Oxygen saturation in arterial blood  (N = 80-100)

2.       PaCO2: Carbon Dioxide saturation in arterial blood (N = 35 – 45)

3.       FiO2: Fraction of inhaled O2 (N = 21% i.e. the fraction of O2 in atmosphere air which we inhale)

4.       PEEP: Positive End Expiratory Pressure – The pressure needed at the end of expiration to keep the alveoli open.

5.       RR: Respiratory rate (N = 12-16)

6.       TV: Tidal Volume (N = 6-8 ml/kg = approx. 500 ml)

Things to remember:

1.       PaO2 is affected by changes in FiO2 and PEEP.

·         In a mechanically ventilated patient, PEEP can be kept at 5 cm of water. If the patient’s PaO2 is still low, increasing PEEP will help to increase PaO2. So, PEEP can be increased to 8-10 cm of water.

·         In a mechanically ventilated patient, FiO2 can be started at 80-100%. However, a FiO2 above 50-60% can cause oxygen toxicity. Hence, we always try to reduce the FiO2 in such a way that PaO2 is still maintained.

·         So, if a question shows a lower value of PaO2, do not adjust the RR or TV, focus on the PEEP and FiO2. A patient with ARDS would benefit by increasing the PEEP if adequate oxygenation is not being maintained with a high FiO2. 

2.       PaCO2 is affected by changes in RR and TV

·         In a mechanically ventilated person, if the PaCO2 is still high, increasing the RR or TV will help in decreasing the PaCO2.

PRESSURES

1.       Peak airway pressure = Resistive pressure  +  Plateau Pressure

2.       Resistive Pressure = flow * Resistance

3.       Plateau Pressure = elastic pressure + PEEP

Plateau Pressure = Equilibrium pressure reached if expiratory tubing is occluded. Surrogate for end-inspiratory alveolar pressure. Measured during inspiratory hold maneuver, when airflow = 0 (i.e. resistive pressure is 0). It is inversely proportional to the compliance of the lung.

During the inspiratory hold, when the ventilator tube is occluded no airflow takes place in the tubes of the ventilator and the patient’s trachea and bronchi. At this time, the relatively higher pressure in the patient’s airway will drive the air into the alveoli, which has lower pressure.

Hence, the proximal airway pressure = intra-alveolar pressure (equilibrium)

Peak Pressure = Maximum pressure in the proximal airway at the end of inspiration. It is a sum of the resistive pressure (due to tube/pipes of the mechanical ventilator + pressure in trachea/bronchi (human tubes)) and plateau pressure (pressure in the alveoli).

INCREASE IN PEAK PRESSURE can be due to two reasons

1.       Increase in plateau pressure : Pneumothorax, pulmonary edema, atelectasis, pneumonia (where the alveoli cannot expand hence decreased compliance)

2.       Normal plateau pressure, but increase in the resistive pressure: Occluded tubing, biting on Endotracheal tube, mucus plug, foreign body, extrinsic airway compression.

Practice Question:

A patient with ARDS has the following values. What will be your next step?

TV=370

RR= 22

PaCO2 = 30

PEEP = 5

FiO2 = 70

PaO2 = 54 

Ans: PaO2 is less than normal, hence we only focus on PEEP and FiO2. Since FiO2 is quite high at 70 (we prefer it to be below 60 ), increasing the PEEP from 5 to 8 or 10 would be the next best step to improve the patient’s oxygenation.

Written by Amrin Kharawala