Ventilator Monitoring parameters – Advance

In continuation to basic monitoring parameters, below are major advance ventilation monitoring parameters

 iPEEP – (intrinsic (Unintended)positive end-expiratory pressure)

iPEEP, or intrinsic positive end-expiratory pressure, measures unintentional positive pressure at the end of expiration. Ideally, this pressure should be zero. It’s caused by air trapped in the alveoli due to incomplete lung emptying. Active patients may introduce inaccuracies in its measurement. When iPEEP is present, it can lead to lung damage (volutrauma or barotrauma). In inactive patients, iPEEP can add an extra breathing workload.

iPEEP, or air trapping, can occur if the expiratory phase is too short, which can happen under these conditions:

• Excessive delivered tidal volume
• Inadequate expiratory time or high respiratory rate
• High circuit impedance or airway blockage during expiration
• Low peak expiratory flow

Managing iPEEP involves adjusting ventilator settings, like extending the expiratory time, reducing tidal volume, or using controlled mechanical ventilation to allow complete lung emptying.

WOB – (Work of Breathing)

WOB measures the total work done during the breathing process one minute. It is the addition of WOB done by Patient and WOB done by ventilator.

WOBpat – (Work of Breathing Patient)

WOBpat calculates the work done by the patient during breathing one minute. It is calculated by monitoring tidal volume, change of pressure in patient lung and respiratory rate to calculate Elastic work of breathing and resistive work of breathing as below –

Elastic work is associated with the stretching and recoiling of the lung and chest wall.

It is calculated as

WOBe = (0.5) x (tidal volume) x (change in pressure during inspiration)

Resistive work is related to the resistance encountered by the airflow through the airways and breathing circuit.

WOBr = (resistance) x (tidal volume) x (respiratory rate) 

So total work of breathing by patient = WOBe+WOBr

WOBvent – (Work of Breathing Ventilator)

WOBvent represents the work done by the ventilator during breathing assistance in one minute.

Calculating it can be a bit more complex as it involves the energy required to drive the ventilator and overcome the resistance of the breathing circuit. The ventilator’s work of breathing is not typically a value that is directly measured but can be estimated or calculated in some scenarios.

1. Work to Generate Ventilation (Wg):

• This component represents the energy or work done by the ventilator to generate the required tidal volume.
• It’s influenced by factors like the set tidal volume, inspiratory flow rate, and the pressure applied by the ventilator.

Wg = (Tidal Volume) x (Ventilator Inspiratory Pressure).

2. Work to Overcome Circuit and Airway Resistance (Wr_v):

• This component accounts for the energy expended to overcome resistance in the breathing circuit and patient’s airways.

Wr_v = (Resistance) x (Tidal Volume) x (Respiratory Rate).

3. Total Ventilator Work of Breathing (WOB_v):

• The total work of breathing for the ventilator is the sum of Wg and Wr_v.

WOB_v = Wg + Wr_v.

WOBimp – (Work of Breathing Imp)

WOBimp is the work of breathing done by the patient to overcome the suction valve, tubing, and humidifier during spontaneous breathing. It involves considering the resistive component of the work. It is calculated using the following formula:

WOB_imp = (Resistance) x (Tidal Volume) x (Respiratory Rate)

RSBI – (Rapid Shallow Breathing Index)

RSBI assesses a patient’s ability to breathe effectively by calculating the ratio of respiratory rate to tidal volume. It is a simple measurement calculated by dividing the total breathing frequency (fTotal) by the exhaled tidal volume (VTE). Dyspnoeic (breathless) patients tend to have a higher RSB because they take quicker, shallower breaths compared to non-dyspnoeic patients, who have a lower RSB. Clinically, RSB is often used to determine a ventilated patient’s readiness for weaning from mechanical ventilation. However, it is only relevant for spontaneously breathing patients weighing over 40 kg, and it’s calculated based on the last 25 breaths, with RSB shown if at least 80% of these breaths were spontaneous.

RCexp – (Expiratory Time Constant)

RCexp measures the time constant for the expiratory phase and it is calculated by multiplying resistance and compliance.  This represents the rate at which the lungs empty as follows –

Actual Expiratory time      % emptying

1 x RCexp                      63%

2 x RCexp                      86.5%

3 x RCexp                      95%

4 x RCexp                      98%

Normal values in intubated adult patients:

• Short, < 0.6 seconds: restrictive disease (ARDS, atelectasis, chest wall stiffness)
• Normal, 0.6 to 0.9 seconds: normal compliance and resistance, or combined decreased compliance and increased resistance
• Long, > 0.9 seconds: obstructive disease (COPD, asthma), bronchospasm, ET tube obstruction, or incorrect positioning

Use RCexp to set the optimum TE (Goal: Te ≥ 3 x RCexp):

• With passive patients: Adjust Rate and I:E
• With active patients: Increase Psupp and/or Exp% to achieve a longer Te
• These actions may reduce the incidence of iPEEP.