Physics for Anesthesiologists

Anesthesia has evolved over the previous decades; however, the basic concepts are still applicable in day to day anesthesia instruments and apparatuses. The understanding of the clinical relevance of physical principles for liquids and gases under pressure and at varying temperatures and volumes is crucial to ensure a safe outcome for the patient.

For instance, intravenous fluids flow via laminar flow, and flow is determined by the Hagen-Poiseuille Equation. In laminar flow, the flow rate is directly proportional to the pressure divided by the flow. This means that the flow is greater through a shorter and wider cannula as compared to a cannula, which is long and small when the same pressure is applied to it. This is the reason why shorter and wider cannula such as 16 or 18 G cannulas are preferred during resuscitation. [1]

The Hagen-Poiseuille Law is a physical law that gives the pressure drop in an incompressible and Newtonian fluid where the pressure drop is governed by the length and viscosity, and flow rate is inversely proportional to the radius to the fourth power. Gas flow is typically categorized as laminar or turbulent. Laminar flow is smooth and orderly, with particles all moving through a line, parallel to the wall; in this case, the flow is fastest at the center and slows down towards the walls. Most importantly, in laminar flow systems, the Hagen-Poiseuille Law applies.

Turbulent flow is disorderly and occurs at constrictions, bends, valves and irregularities. In this case, the gas particles move in all directions, including sidewise, and the flow is equal across the diameter of the tube. Resistance in the flow of gases occurs by a pressure drop as fluid moves through a tube, circuit, or airway. This pressure drop is a consequence of overcoming resistance; it is typically expressed as a pressure drop per-flow rate or as centimeters of water per liter per second cm – i.e. H2O/L/sec.[1]

The turbulent flow requires more power than laminar flow for a given flow rate, which means a higher energy consumption by the respiratory muscles. Rapid breathing typically generates a turbulent flow. The oxygen demands of the muscles associated with respiration can outstrip the extra oxygen supply gained by faster breathing. This outstripping of demand over the supply of oxygen can lead to hypoxia; in patients with respiratory diseases, the over-stimulation of respiration by medical interventions can cause a danger of hypoxia as a result. [2] For a given pressure gradient, there is a greater flow of a low-density gas than a higher density gas. Thus, the use of helium, which has similar viscosity but lower density, is used to relieve resistance due to turbulent flow. Heliox, a mixture of 21% helium and 79% oxygen, is used in cases of respiratory tract obstruction to reduce density and thereby improve the flow.[2]

Considerations with the fundamental physics behind every anesthetic instrument are essential since the solubility mechanics of anesthetic gases, resistance, and flow rates evaluations are essential factors to be considered in the choice of anesthesia, pressure monitoring, airway management, and pre-operatory evaluation. For instance, during emergence from nitrous oxide anesthetic, rapid elimination of nitrous oxide from the lungs dilutes other alveolar gases, producing alveolar “diffusion hypoxia.”  The rapid uptake of high concentrations of nitrous oxide at the induction of inhalational anesthesia produces an increase in alveolar concentrations of oxygen and the accompanying volatile anesthetic agent. This process is known as the Fink Effect. The effect is caused by the concentrating effect of nitrous oxide uptake on the partial pressures of the other gases in the alveolar mixture. [3] Nonetheless, this effect only lasts a couple of minutes, and hypoxia can be avoided by increasing the fractional inspired oxygen concentration when recovering from N2O anesthesia.[4] As this example shows, a basic understanding of physical laws is necessary for optimize patient safety.

[1] Lumb AB, Nunn JF. Nunn’s Applied Respiratory Physiology Elsevier/Butterworth Heinemann, 2005  501:152-153.

[2] Gupta B, Gupta L, Physics of Anaesthesia Made Easy. Glob J Anes & Pain Med 1 (2)-2019. GJAPM.MS.ID.000107. DOI: 10.32474/GJAPM.2019.01.000107. Glob J Anes & Pain Med Volume 1 – Issue 2

[3] Hardman, JG, Hopkins PM, Struys MMRF. Oxford Textbook of Anaesthesia 1995 368. DOI https://doi.org/10.1016/C2009-0-49651-X

[4] Hemmings HC, Hopkins PM. Foundations of Anesthesia: Basic Sciences for Clinical Practice. Elsevier Health Sciences, 2006 0323037070, 9780323037075 903 pages