The Thermodynamics of Life and Fluctuations of Critical Illness
Life is an open thermodynamic system by which energy in the form of food and O2 is consumed from, and waste products from lungs, skin, urinary and gastrointestinal tracts are excreted to our environment. In the continuum of open thermodynamic systems from linear, fixed, near-to-equilibrium systems like crystals, to non-linear, far-from-equilibrium, chaotic systems like weather, we are situated in the transition zone, ordered, but poised on the edge of chaos. The amount of energy we consume, our metabolic rate, determines our distance from thermodynamic equilibrium. The freedom we have to change that distance and remain healthy is limited. The low metabolic rate during sleep is probably as close as we get to equilibrium and remain healthy, and maximal exercise is the farthest away. This mobile thermodynamic state applies to individual cells and organ systems as well as our bodies. Our cells are in a phase transition between liquids and solids and can move within the transition zone becoming more liquid-like or more solid-like as determined by their effective temperature.
Disease can take us too far in either direction. Ischemia, hypoxia, anemia, heart failure, cardiogenic and haemorrhagic shock all result in inadequate energy supplies that takes us too close to equilibrium. Inflammation takes us too far. Fluctuations in physiologic parameters may be the key to understanding our thermodynamic state in disease. Normally all homeostatically controlled parameters fluctuate indicating our ability to adapt and survive. If we move too close to thermodynamic equilibrium, we become more crystalline and the fluctuations diminish, we become unable to adapt, and survival is at risk. In myocardial ischemia, the normal variability in heart rate diminishes. In intestinal ischemia, the electrical activity of bowel smooth muscle decreases. As coma deepens, the respiratory rate becomes more regular. If we are too far from equilibrium, we become more weather-like and fluctuations increase taking us to new states beyond the protection of homeostasis. In asthma the variations in respiratory resistance to airflow are 10-105 times greater than normal. Blood pressure variability is increased by hypertension as is intestinal motility in inflammatory diarrhea. Perturbations are amplified and can reverberate throughout the whole body. Sepsis is an example and MODS may represent the severe damage that can result. Careful analysis of measurable fluctuations can provide valuable diagnostic and prognostic information in the intensive care unit.