Respiratory Therapy Blog

Monitoring Respiratory Mechanics during Mechanical Ventilation: Using the Ventilator as a Probe of Physiology - Dean Hess

Assistant Director of Respiratory Care, Massachustets General Hospital - Boston MA

The measurments that we normally take (Cs, R etc) treat the lung as a single unit. The lung is not a single unit.

Pplat = end - inspiratory alveolar pressure

And is determined by compliance and VT of the lung.

The amount of stretch of the alveoli is determined by the pressure across the alveolar wall (transpulmonary pressure) not plataeu pressure.

This causes a diffirent risk of ventilator induced lung injury at the same plataeu pressures.

Respiratory system compliance:
C = VT / Pplat - PEEP
Inorder for this to be accurate and meaningful, the VT needs to be adjusted for compressible volume and the PEEP to included autoPEEP.

Respiratory variations in CVP is related to pleural pressure changes. This can be used to calculate chest wall compliance or inspiratory muscle effort (in modes like PSV). The pressure support for a patient could be set by the CVP variations to unload the patient’s inspiratory efforts and work of breathing.

Techniques to Facilitate Spontaneous Breathing - Robert Campbell RRT

Ventilation and monitoring specialist, Respironics Inc.

Role of Sponateous breathing

• improved V/Q matching
  • 10% of VE resulting in a decrease in Qs/Qt of 15%
• reduced deadspace ventilation
• decreased intrathorcic pressure
• improved venous return / cardiac output

Diaphragmatic activity may recruit dependent areas of the lung.

Unloading of the respiratory muscles requires the ventilator to be in synchrony with the patient throughout the respiratory cycle. One must match the ventilator to the patient, not the patient to the ventilator.

Work of breathing during assisted breaths can equal that of spontaneous breathing.

PCV can improve patient synchrony by providing a rapid initial flow and variable flow throughout inspiration. This allows breath by breath variation.

Setting the inspiratory rise-time in PSV will effect when flow cycle to expiration happens. Therfeore it is important that when setting the expiratory trigger/sensitivity that the inspiratory rise-time is always set first.