The document presents an overview of basic physics principles relevant to anaesthesia, including flow types, the Hagen-Poiseuille equation, and key effects such as Bernoulli's and Venturi. It highlights the clinical implications of laminar and turbulent flow in airways, factors affecting flow resistance, and the importance of surface tension and viscosity. Additionally, it discusses relevant physiological concepts such as Reynolds number and the role of surfactants in lung function.

1. Basic Physics and Measurement
in Anaesthesia
Presented by:-DR.KALYAN REGMI
Moderator: DR. ANIL PRASAD NEUPANE
Assistant professor,Department of
anesthesiology,PAHS,Pokhara

2. •Flows: Definition, types, clinical aspects
•Hagen-Poiseuillie equation
•Surface tension
•Laplaces law
•Bernoulli effect
•Viscosity
•Venturi effect
•Entrainment ratio
•Coanda effect
OBJECTIVE

3. • Flow is defined as the quantity of a fluid, i.e. a
gas or a liquid, passing a point in unit time
• where F = mean flow
• Q = quantity (mass or volume)
• t = time
FLOW

4. TYPES OF FLOW

5. LAMINAR FLOW
• Fluid moves in a steady manner and there are no
eddies or turbulence.
• Laminar flow normally present in smooth tubes
at low rates of flow.
• The velocity of flow is highest in the centre.

7. • Flow is directly proportional to pressure under
conditions of laminar flow
The ratio of pressure to flow is a constant
known as resistance R of the apparatus or tube
concerned

8. Measurement of flow resistance

9. TURBULENT FLOW
• Flow in which fluid is no longer flows in a
smooth fashion but swirls in eddies.
• Laminar flow may change to turbulent flow if
a constriction is reached which results in the
increasing fluid velocity.
• In turbulent flow the resistance is higher for
the same laminar flow

11. • For turbulent flow in tubes that are rough on the
inside it is found that the flow is approximately
proportional to the square root of the pressure

12. •The factors affecting turbulent flow are :-
1.Vicosity of fluid
2.Diameter of tube
3.Density of fluid and
4. Linear velocity of fluid

13. • These factors may all be combined to give an index
known as Reynolds number, which is calculated as
follows

14. •If Reynolds number exceeds about 2000, then
turbulent flow is likely to be present.
•If Reynolds number is below 2000, the flow is likely
to be laminar.

15. Clinical aspects of flow
•Changes between laminar and turbulent flow
depends on the velocity of gases,which in turn
depends on the volume flow and thus on the
diameter of tubing and airways.
•In patients airway,gases are humidified, warmed (34-
37ᵒ C) and contains carbondioxide. The overall effect
of these is reduction in density of gases and rises in
critical flow.

16. Critical flows for anesthetic gaseous
mixture of 60% nitrous oxide, oxygen
and air (20ᵒ C and patients airway)

17. •As breathing is cyclical, with peak flows over 50 lit
per min(turbulent flow during peak flow), laminar
flow at other times in respiratory cycles
•Bronchi and smaller air passages in lungs: slower
flow and laminar in lower respiratory tract.

18. Hegan- Poissuilles equation:
•Represents relationship between flow and
factors affecting flow.
• Applied for laminar flow.
Qᵒ = πPd4/128 ηl
Where, Qᵒ = flow
P = pressure across the tube
d = diameter of the tube
η = viscosity
l = length of the tube
π/128 = constant

20. Flow Rate (ml/min) 270ml 180ml 80ml 54ml 33ml 20ml
Length(mm) 45mm 45mm 45mm 32mm 25mm 19mm

22. Fig. Central venous catheter Fig . Peripheral venous catheter

23. SURFACE TENSION
• Surface tension is a result of the attraction
between molecules across the surface of a
liquid.
• In the surface water molecules are attracted
downwards and sidewise only.(air fluid
interface)

25. LAPLACES LAW

26. •surfactants are compounds that lower the surface
tension of a liquid.
•Surfactant molecules are synthesized by type II
pneumocytes (cuboidal cells lining alveoli) and kept in
clusters called lamilar bodies and secreted as tubular
myelin.
•The alveoli in the lungs are lined with fluid
which,comprising mainly water, has a significant
surface tension.
Surfactant

27. • Alveoli collapse
• Smaller sized alveoli collapses into large
sized alveoli causing ventillation perfusion
mismatch
• Tendency of edema formation in alveoli
• Surface area for gaseous exchange
decreases
Major problems with surface tension in
lungs

29. VISCOCITY
•Viscosity is a measure of the frictional forces
acting between the layers of the fluid as it flows
along the tube.
•Flow and viscosity are inversely proportional.

30. Higher
viscosity
Lower flow
Increased
risk of
vascular
occlusion

31. BERNOULLI’S PRINCIPLE :
States that when a gas flowing through a tube,
encounters a constriction, at that point, the pressure
drops and velocity increases.

32. VENTURI EFFECT :
•consequence of the Bernoulli’s principle
•The entrainment of the air from the surroundings due
to fall in the pressure at the point of constriction is
called venturi effect.

33. •Suction: injectors working on this venturi effect may
be used to provide suction, either water or gases
being used as the driving fluid.
•Nebulisers
•Venturi masks
Uses of venturi effect

34. • A Venturi is a simple design of valve that provide control
oxygen supplied through a narrow port which allows room
air to be drawn in from atmosphere
• The rate of flow generated by this Venturi principle and
Venturi mask may be equal to peak inspiratory flow of
patient.
Venturi masks

37. ENTRAINMENT RATIO
The entrainment ratio is defined as the ratio of
entrained flow to driving flow.

38. COANDA EFFECT
:
•If a constriction occurs at a bifurcation, due to
increase in velocity and reduction of pressure, the
fluid/air tends to stick to the side of the branch
causing maldistribution.

39. APPLICATION:
• Mucus plug at the branching of tracheo-bronchial
tree may cause maldistribution of respiratory gases.
•Unequal flow may result because of atherosclerotic
plaques in the vascular tree

40. TAKE HOME MESSAGE
• Reynold number less than 2000 indicate laminar flow
• Fluid resuscitation is done via wide bored peripheral
catheter
• Surfactant helps to prevent alveoli collapses
• Gas/fluid flow from higher pressure to low pressure area
• Gas/ fluid following through tube when encounter
constriction at that point pressure drop and velocity
increases.

41. THANKYOU