Physics of Anaesthesia Made Easy

Physics is an attempt to describe the fundamental laws of world around us. As anesthesiologists we deal with liquids and gases under pressure at varying temperature and volume. These inter relationships are simple, measurable and their understanding ensures a safe outcome for the patient. For the safe and efficient use of anesthesia apparatus, a basic knowledge of fundamental physics is must for a clear concept of their working principle. We have tried to simplify the basic physics related to anesthesia in a simplified way through the review article. Introduction Basic Concepts Units of Measurements (Table 1) Table 1: Units of measurements. Basic SI Units Derived Units Units not in SI system length (meter) temp (degrees Celsius) pressure (mmHg) mass (kilogram) force (newton) pressure (cmH2O) time (second) pressure (pascal/ bar) pressure (standard atmosphere) current (ampere) energy (electron volt) energy (calorie) temp (kelvin) power (watt) force (kilogram weight) luminous intensity (candela) frequency (hertz) amount of substance (mole) Volume (lliter) Simple Mechanics a) kilopascal = 7.5mmHg. b) 1 Bar = 750mmHg c) 1 kilopascal = 10.2cmH2O d) 1 std atmosphere = 101.325kPa e) 1 calorie = 4.18J f) 1-kilogram weight = 9.8N g) Pounds / inch2(PSI) -Atmospheric Pressure (1 PATM=14.7PSI) h) 1 Bar = 100kPa = Atmospheric pressure at sea level [1]. Pressure a) Force = mass x acceleration = kgms-2 = Newton b) Pressure = Force/Area c) 1 Pascal = I Newton acting over 1m2 Gauge pressure is defined as pressure which is measured when unknown pressure is measured relative to atmospheric pressure [2]. This pressure is used in measuring: a) Blood pressure b) Airway measurements In order for fluid to pass out of the barrel of the syringe the same pressure must be developed in the syringe. a) For a 20ml syringe (diameter 2cm) – pressure generated is 100kPa; even this is 6 times more than SBP of 16kPa (120 mmHg). So, during Biers block, pressure in the vein during rapid injection can exceed systolic pressure, particularly if a vein adjacent to the


Units of Measurements
)  Gauge pressure is defined as pressure which is measured when unknown pressure is measured relative to atmospheric pressure [2]. This pressure is used in measuring: a) Blood pressure b) Airway measurements In order for fluid to pass out of the barrel of the syringe the same pressure must be developed in the syringe.
a) For a 20ml syringe (diameter 2cm) -pressure generated is 100kPa; even this is 6 times more than SBP of 16kPa (120 mmHg).
So, during Biers block, pressure in the vein during rapid injection can exceed systolic pressure, particularly if a vein adjacent to the cuff is present.

Fluid Mechanics
Flow is defined as amount of fluid or gas passing in unit time.
Flow becomes laminar to turbulent after Reynold number (defined

Laminar Flow
Flow moves in a steady state with no turbulence or eddies. Flow is greatest in the mid center and zero in peripheral wall. Hagen Poiseuille Equation is used to determine laminar flow, defined as:

Turbulent Flow
Turbulent flow denotes a situation in which the fluid flows in an unpredictable manner with multiple eddy currents which are not parallel to the sides of the tube through which they are flowing. b) Heliox (a mixture of 21% helium and 79% oxygen) is used to reduce density and thereby improve the flow and is used in respiratory tract obstruction. Helium is much less dense than nitrogen, which constitutes 79% concentration of air. In patients with upper airway obstruction, flow is through an orifice and hence more likely to be turbulent and dependent on the density of the gas passing through it. Therefore for a given pressure gradient (patient effort), there will be a greater flow of a low density gas (heliox) than a higher density gas (air).
c) There is laminar flow during quiet breathing which becomes turbulent during coughing and speaking thereby resulting in breathlessness or dyspnea.
d) According to, Hagen -Poiseuille's Law. flow is laminar at low flows in the flow meter, while at higher flows, the law applicable to turbulent flow is applicable [3].

Critical Flow
Critical flow for a typical anesthetic gas has approximately the same numerical value as the diameter of the airway concerned. If these are stored at high temperatures, pressures will raise causing explosions. b) Adiabatic changes: it is a change which doesn't involve transfer of heat (Q) or matter into and out of a system, so that Q = 0, and such a system is said to be adiabatically isolated.
Clinical Relevance: When a valve of an oxygen cylinder is opened suddenly, oxygen will rush into high pressure hose or stem of oxygen regulator and on reaching the end of hose, adiabatic process might occur. That suggests that local pressure is much

Solubility Mechanics
Solubility Saturated vapor pressure is defined as the partial pressure exerted by vapour in the equilibrium state is achieved at the surface between vapor of the liquid and liquid itself when a liquid is placed in a closed container. SVP is associated with Henry's law [4].
Henry's Law states that at a temp, the amount of a given gas dissolved in a given liquid is directly proportional to the partial pressure of the gas in equilibrium with the liquid.   c) Heliox mixture of helium and oxygen, is a lighter gas, hence is used in airway obstruction to improve diffusion and gas exchange.

Fick's Law of Diffusion:
The rate of diffusion of a gas across a membrane is directly proportional to the membrane area (A) and the concentration gradient (C 1 -C 2 ) across the membrane and inversely proportional to its thickness (D). This results in effects known as the "concentration effect" and the second gas effect. When a constant concentration of an anesthetic such as sevoflurane is inspired with nitrous oxide, the alveolar concentration of sevoflurane is accelerated due to nitrous oxide, because alveolar uptake of the latter creates a potential sub atmospheric intrapulmonary pressure that leads to increased tracheal inflow.

Rate of diffusion a
Diffusion Hypoxia: Nitrous oxide diffuses faster from the alveoli at the end of anesthetic exposure, as N 2 0 diffuses faster into the alveoli thereby diluting the gases leading to fall in oxygen saturation, also known as diffusion hypoxia, therefore 100% oxygen is required at the end of surgery to avoid diffusion hypoxia.

Osmolarity
It is defined as the sum total of the molarities of the solutes in

Energy Mechanics
Heat Capacity: Heat Capacity is defined as the amount of heat required to raise the temperature of a given object by 1 kelvin.

Specific Heat Capacity
Specific Heat Capacity defined as the amount of heat required to raise the temperature of 1kg of a substance by 1 kelvin. (J /kg/

Clinical Relevance
Normal body temperature is 36 degrees Celsius and basal heat production is 80 W(J/Sec) Shivering increases heat production by 4fold (ie 320W, with extra 240W= 14.4kJ/min) 245kJ needed to increase temp by 1 degree (total heat capacity = 3.5x70kg), so patient has to shiver for approximately 245/14.4=17min to produce this extra heat.

Bernoulli's Principle
It is defined by the law of conservation of energy. Flowing liquid possess 2 types of enrgy-potential and kinetic energy. If there is a constriction in tube, there is increase in kinetic energy, there is subsequent fall in potential energy, to conserve the total energy [4].

The Venturi Effect
Venturi effect was named after famous Italian physicist, Giovanni Battista Venturi (1746-1822). It is the effect by which the introduction of a constriction to fluid flow within a tube causes the velocity of the fluid to increase, therefore, the pressure of the fluid to fall. By measuring the change in pressure, the flow rate can be determined, as in various flow measurement devices such as venturi masks, venturi nozzles and orifice plates.
a) The Venturi effect may be observed or used in the following: b) The capillaries of the human circulatory system, where it indicates aortic regurgitation. c) Injectors used to add chlorine gas to water treatment chlorination systems.

Spectrophotometry-Basic Concepts a) Beers Law
Beer law states that amount of light absorbed is proportional to the concentration of the light absorbing substance.

b) Lamberts Law
Equal thicknesses absorb equal amounts of radiation. Amount of light absorbed is proportional to the length of the path that the light has to travel in the absorbing substance. Both laws say that the absorption of radiation depends on the amount of a particular substance. This fact has been utilized in pulse oximetry. c) More the Hb per unit area more is the light is absorbed. This property is described in a law in physics called "Beer's Law". While, longer the path the light has to travel, more is the light absorbed.
This property is described in a law in physics called "Lambert's Law.

Glob J Anes & Pain Med
35 a) For the flow of blood in a blood vessel, the ΔP is the pressure difference between any two points along a given length of the vessel. When describing the flow of blood for an organ, the pressure difference is generally expressed as the difference between the arterial pressure (PA) and venous pressure (PV).

Law of Laplace (Wall Stress):
Laplace Law states that for cylinders, T = Pr or P =T/r (e.g. Arteries) For sphere, P= 2T/r (e.g. Anesthesia Bag/ Heart) (Where T = wall tension, P = pressure of fluid within the cylinder/ sphere, r = radius); Tension may be defined as the internal force generated by a structure.

Clinical Relevance
a) In a failing heart -there is an increase in radius therefore a decrease in pressure, and failing heart is unable to increase T. b) In a normal heart, increase in radius is beacuase of increase in venous return, also there is increase in Tension according to Frank starling law. Therefore there is no change in pressure.
c) The management of stable angina is to reduce wall stress thereby decreasing myocardial oxygen demand.

Archimedes' Principle
Archimedes' principle is a law of physics fundamental to fluid mechanics. It says any object, wholly or partially immersed in a stationary fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object. a) Air Bubbles: According to Archimedes' principle, air bubbles always tend to go upward in any liquid, including saline, drugs, and blood. So, just keep up the cone of the syringe (exit path), bubbles can be removed by ejecting air by pushing the plunger of syringe. b) Cardiac Surgery: During cardiac surgery, de-airing is done before aortic de-clamping in order to prevent air bubbles from reaching the brain. If de-airing is performed through a ventriculotomy, the anesthesiologist is asked to place the patient in the Trendelenburg position, so that the venting site is located above and air expulsion is favored. c) Archimedes' principle helps cardiac anesthesiologists to prevent (or reduce) cerebral air embolism when air accidentally enters the circuits during cardiopulmonary bypass (CPB) by immediately placing the patient in steep Trendelenburg position.

Calculating the Duration of a N 2 O Cylinder
Just Now a new N 2 O cylinder is fitted to the Machine. How (Suppose the flow of N 2 O is 3Lt/m=180Lt/hr, so the cylinder will last for 1272/180= 07hr).

Unexpected Help from the Reservoir Bag
The reservoir bag in an anesthesia machine allows manual ventilation as well as a "visual" monitoring of spontaneous breathing. a) Thanks to Laplace's law, it can prevent barotrauma in case of malfunction or unintentional closing of the APL (adjustable pressure limiting) valve. In fact, in the presence of an overflow or a flow obstruction in the breathing system, the radius of the reservoir bag increases ( Figure 5) and, according to Laplace's law, the pressure inside it decreases (P=2T/R), thus preventing a dangerous rise in pressure in the entire breathing system and, consequently, in lungs. b) Accordingly, a reservoir bag which feels stiff should be replaced, since its wall tension (which we can define, similarly to surface tension γ, as the work required to extend the surface of an elastic membrane by a unit area will be higher, for the same radius (or its radius will increase by a lesser extent for the same value of wall tension), thus providing a lower "pressure relief".

Conclusion
Anesthesia has evolved very fast over last few decades but the basic are still same and applicable in day to day anesthesia instruments and apparatus. It is necessary to understand the basic physics behind every anesthetic instrument, so that it becomes easy to operate. Learning conceptual physics also helps to trouble shoot the problem associated with them.