Work, Energy and Power Flashcards
What is work
Mechanical work is a form of energy. Other forms of energy include heat energy and chemical energy.
work is done whenever the point of application of a force moves in the direction of the force.
Work = Force x distance
SI unit: Joule
Define one joule
One joule of work is done when a force of 1 Newton moves its point of application one meter in the direction of the force
How much work is done if an apple weighing 102 grams is lifted one meter against gravity
1 Joule
W = force x distance
F = ma
F = 0.102kg x 9.81 m.s^-2 F = 1kg.m.s^-2 F = 1 N
W = F X D W = 1 N x 1 m W = 1 J
Describe from basic principles how the work of breathing is calculated
- Energy used to overcome elasticity of the lung
- Energy used to overcome airway resistance
Diagram with Mass M with downward Force F on top of the Bellows over Area A descending distance D
–> draw volume Pressure graph –> square (for perfect constant pressure generator)
P = F/A
W = F X D
V = D X A
So F = P x A and D = V/A
As W = F X D
So W = PA X V/A
So W = P x V (hence the area under the pressure volume curve.
As this is not a perfect square under real conditions, integral calculus can be used to calculate area under the PV curve which will calculate work done in Joules
1J = Pa.m^3 (And N.m) Add units to calculations above
Describe how energy is used during inspiration
- To overcome elastic energy of the lung which is then stored as potential energy (which overcomes airway resistance during expiration)
- To overcome airway resistance during inspiration
- Moving the air and tissues
Describe the muscles which do the work of inspiration and how they do this
They create a negative pressure between the pleural layers by expanding the volume of the thoracic cavity. Boyles law states that at constant temperature a change in volume is inversely proportional to the change in pressure. So as the volume of the thoracic cavity increases, the pressure decreases. This falls below atmospheric pressure and draws air into the lung parenchyma.
- Diaphragm - Downward displacement –> increase 3D length of thoracic cavity
- Diaphragm - Raises lower margin of ribs –> increase AP diameter
- External intercostals –> raise lower ribs –> increase AP diameter (bucket handle effect)
- Interscalene muscles –> raise upper ribs –> increase AP diameter.
Summarise the energy changes for a typical inspiration and expiration in terms of proportions of energy types produced
INSPIRATION
- Potential energy (150mJ)
- Air/tissue movement (overcoming airway and tissue resistance) (150mJ)
- Energy converted in muscle contraction to heat (2700 mJ)
EXPIRATION
1. The energy created during inspiration creates the potential energy (stored in lung tissue) required during expiration to over come the work required for airway and tissue resistance (during tidal breathing)
Describe the methods for measurement of work done during breathing for IPPV versus Spontaneous breathing
IPPV
–> Area under the pressure volume curve
(easy as pressures and volumes are easily measured and known
Spontaneous breathing
- -> More difficult as the pressures exerted by the respiratory muscles in the chest wall cannot be monitored directly
- -> Volumes are easily measured by a pneumotachograph but pressures exerted by respiratory muscles cannot be measured directly
Intrapleural pressure changes indicate pressure changes acting on the lung and can be measured in the lower oesophagus with a balloon tip catheter
Describe the limitations of measuring work of spontaneous breathing using a balloon tipped catheter in the lower oesophagus. Describe the different regions of the graph and what these represent
Limitations
- Inaccurate surrogate for intrapleural pressure
- Intrapleural pressure is not uniform and varies according to site and posture/position
- Excludes work of movement of chest wall (only diaphragm)
Graph:
Axes
Negative X - axis from -0.4 to - 0.8 kPa (PIntrapleural or lower oesophageal pressure)
Y axis - Volume
Shaded area below gradient (compliance line) is work during inspiratioshaded area above gradient is work during expiration
Dotted line triangle back to y-axis represents heat energy lost
Define Power
Power is the rate of working and is measured in Watts W
W = J.s^-1
Calculate mechanical power of breathing if 1 breath expends 300 mJ of energy and the respiratory rate is 16 breaths per minute
Power = Work
____
time
P = 0.3J/breath x 16 breaths
_______________
60 seconds
P = 80 mW
What is the efficiency of respiratory muscles in producing mechanical energy?
10%
90% lost as heat
So 80 mW mechanical energy is mechanical energy
But 800 mW is the total work done if heat energy is included
This means that the energy requirements for breathing are 800 mW whilst just 80 mW is needed for the mechanical movements.
What proportion of normal metabolic rate is required for breathing
Metabolism: 80 Watts
Breathing: 800 mW
So 1% of metabolic rate
How does turbulent versus laminar flow affect the power of breathing. What are the clinical implications of this
Energy = average pressure different x volume moved
Power = Pressure difference x Flow
Laminar flow
Pressure difference is proportional to flow
So Power is proportional to Flow squared
Turbulent flow
Pressure difference is proportional to flow squared
So power is proportional to Flow cubed
During hyperventilation, flow becomes turbulent and as power in turbulent flow is proportional to flow cubed, the work of breathing required is significantly higher.
At extremes O2 consumption by respiratory muscles during hyperventilation exceeds additional O2 transport achieved by the increase in this ventilation
Why does pharmacological stimulation of ventilation in respiratory disease carry the risk of worsening hypoxis
during hyperventilation, the work of breathing increases significantly due to turbulent as opposed to laminar flow. The work required to overcome the increase in airway resistance in turbulent flow is significant.
Laminar flow, the power is proportional to the flow squared. In turbulent flow the power required is proportional to the flow cubed.
This additional work required for breathing increases O2 consumption. When this increase in O2 consumption exceeds the additional O2 transport achieved from the hyperventilation, then the patient my develop worsening hypoxia in the face of increased ventilation (pharmacological)
