Entropy Generation and Exergy Destruction During and After Weaning from Mechanical Ventilation in Patients with Respiratory Failure

Abstract

BACKGROUND: Mechanical ventilation is a useful supportive treatment for patients with respiratory failure who are not able tomaintain the level of ventilation required to maintain the oxygenation and carbon dioxide elimination. Mechanical ventilation is oftenlife-saving, but it also has risks. Thermodynamic analyses are used to test the feasibility of processes leading to a better understandingof the system’s overall performance. Energy losses (entropy) and the destruction of maximum useful work (exergy) leading to reducedrespiratory work of breathing efficiency, can be calculated by thermodynamic analysis of the respiratory muscles.OBJECTIVE: To determine the entropy generation, exergy destruction, and glucose consumption during and after weaning frommechanical ventilation in patients with respiratory failure by thermodynamic analysis.METHODS: In this study, a human respiratory system during and after weaning from mechanical ventilation modeledthermodynamically using the first and second laws of thermodynamics. The work of breathing data adapted from the literature. Massand energy analyzes are carried out according to the 1st law of thermodynamics, while entropy generation is calculated according to the2nd law of thermodynamics which enables us to measure energy losses. In this thermodynamic model, the body temperature wasconsidered at 37 °C, and the surrounding air condition was taken at 25 °C.RESULTS: Exergy destructions during and after weaning from mechanical ventilation were calculated as 2.23x10-2and 1.75x10-2kJ/min, respectively. Entropy generation by the patients through the breathing cycle was 7.48x10-5(kJ/K)/min during mechanicalventilation while 5.89x10-5(kJ/K)/min after weaning from mechanical ventilation, respectively. The glucose consumed for work ofbreathing in patients during and after weaning from mechanical ventilation was calculated as 0.58-0.45 mmol/min, respectively.CONCLUSION: After weaning from mechanical ventilation, the patients have significantly decreased entropy generation, exergydestruction, and glucose consumption, indicating to the improvements in the structure of respiratory mechanics and diaphragmperfusion. The reductions in entropy generation and exergy destruction after weaning from mechanical ventilation also indicates anincrease in the mechanical efficiency of the respiratory muscles. According to the results of the energy balance analysis, the decreasein muscle energy requirement, was determined and the calculations found that the patient used 1.3 times more glucose during mechanicalventilation than after weaning from mechanical ventilation for work of breathing. In this study, the thermodynamic approach was usedto determine the benefit of mechanical ventilation. More powerful work and multidisciplinary data are needed to progress reliableprocedures.