Saturday, April 22, 2006

Law of LaPlace

Saturday April 22, 2006
Law of LaPlace, PEEP and surfactant

Law of LaPlace tells us that "Pressure is always greater in smaller radius".

P = 2T/r

where P = pressure, T = tension and r = radius

So in lungs, smaller alveoli will have greater resistance for air to flow during inspiration because of higher pressure. We use PEEP to keep alveoli open during expiration (prevent derecruitment), as name says positive end-expiratory pressure. High tidal volume cause more shear force damage to smaller alveoli with each breath to overcome this pressure. Thats why, our present approach to ventilator management in ARDS is low tidal volume and optimum PEEP (See ARDSnet
Lower Tidal Volume/ Higher PEEP Reference Card).

Looking at same formula, other approach is to decrease Tension, by nature's method of applying surfactant. One study published in August 2004 looked into 'Effect of Recombinant Surfactant Protein C–Based Surfactant on the Acute Respiratory Distress Syndrome' and found no significant difference in terms of 28 days mortality or the need for mechanical ventilation but also showed that 'Patients receiving surfactant had a significant greater improvement in blood oxygenation during the first 24 hours of treatment than patients receiving standard therapy'. Actually literature suggests that "..Sufficient levels of PEEP will also help to prevent further loss of surfactant in still ‘healthy’ alveoli,"
Read Professor Lachmann's lecture -
Current status of lung protective ventilation in ARDS, discussing Law of LaPlace, surfactant and PEEP. (source: - European Association of Cardiothoracic Anaesthesiologists)

Effect of Recombinant Surfactant Protein C–Based Surfactant on the Acute Respiratory Distress Syndrome - Volume 351:884-892, Number 9, NEJM Aug. 26, 2004
Higher versus Lower Positive End-Expiratory Pressures in Patients with the Acute Respiratory Distress Syndrome - The National Heart, Lung, and Blood Institute ARDS Clinical Trials Network ,Volume 351:327-336, Number 4, NEJM, july 22, 2004