What useful information can be gained from respiratory pressure-volume loops in the management of the ICU patient?
The usefulness of the information depends on the awareness of the limitations of the technique.
PV loops require either steady state (super-syringe technique) or quasi-steady state techniques (slow constant flow) to minimise effects of flow characteristics on pressure. The use of non constant flow requires mathematical computerised correction of the curve.
Traditionally curves have been performed from zero PEEP, and are dependent on the recent ventilatory history.
Assumptions have been made that the change in slope at the lower end of the inspiratory curve
(lower inflection point) reflect recruitment of all/most/some of the collapsed and recruitable alveoli.
These assumptions are being questioned. This point/zone of inflection has been proposed to be used as a way of choosing a level of PEEP to allow recruitment or prevent derecruitment Despite some published literature seeming to support this approach, many limitations have been raised (eg. recent ventilatory history, variability due to underlying lung disease (primary versus secondary) presence of decreased compliance of abdominal or chest wall, greater importance of expiratory component of curve,etc.).
The Upper Inflection Point has been proposed as a way of detecting overdistension of the lung. Many published studies have suggested that this is an oversimplification, as many areas of lung may already be overdistended (eg. CT studies) before the UIP is reached. Setting the ventilator to prevent "overdistension» is possible but may not be clinically relevant.
Inflection points/zones on the descending part of PV curve may eventually become useful to titrate levels of PEEP to prevent de-recruitment.
The shape of the dynamic PV curve (and its deviation from-expected) may allow some degree of estimation of the magnitude of the patient's (as opposed to mechanical) work of breathing, or the presence of airway obstruction.
Interpretation of pressure-volume loops is dealt with elsewhere.
In summary, the following useful information can be derived from them:
- graphical representation of lung compliance
- estimation of lower inflection point
- estimation of pressure required for complete alveolar recruitment
- adjusting PEEP to this may pervent derecruitment
- estimation of pressure which causes alveolar overdistension
- adjusting plateau pressure to this may prevent VILI
- estimation of the work of breathing
- estimation of the degree of airway obstruction
Limitations of the loops are as follows:
- Poor representation of heterogenous lung pathology
- Inconsistent agreement among observers as to where the lower inflection point is