WORK AND POWER IN ANAESTHESIA PHYSICS BY MOHAMED ANWER RIFKY

1. WORK AND
POWER
MOHAMED ANWER RIFKY

2. ENERGY CHANGES
Work, Energy and Power
MEASURING THE WORK OF INSPIRATION
POWER OF BREATHING
EFFECT OF HYPERVENTILATION IN PATIENTS
WORK One joule of work is done when a force of one newton
moves its
point of application one metre in the direction of the
force.
1 joule of work is performed as illustrated in
the Fig.
Mechanical energy>>converted into potential
energy.
Shortening of the muscle multiplied by the mean force exerted is the mechanical work
performed.
WORK OF A VENTILATOR DURING INSPIRATION A constant-pressure generator type of ventilator >> force and distance moved,
(V = D x A)
work done =
PV.
The work done by the ventilator = 300
mJ
.Most ventilators give a smoother build-up of pressure as indicated in the
Fig>>shaded area
enclosed by the loop>>the area is more complex and uses the process of integration
for…. During inspiration about half the mechanical energy used is stored as potential energy in the elastic tissues
of the lung and the chest wall>>for work of expiration. The remaining half >> overcoming airway resistance
and in moving the air
and
tissues.
A ventilator of a constant-pressure generator and the patient's lungs are represented by a syringe in which movement of the plunger is opposed by the stretching of
an elastic band(diaphragm, elasticity of the lung tissues and chest wall-with resistance to the flow).PL.&PM .
The pressure generated by the elastic band is atmospheric at the
start of
inspiration and increases linearly with volume >>tracing C to D >>slope >>compliance ,and the work done against the elastic band is 1/2PV
pressure gradient across the airway resistance is 0.6 kPa,which decreases to zero at the end of inspiration and so the average pressure
difference is again 1/2P and the energy expended in moving the gas is as 1/2PV.
WORK OF EXPIRATION
WORK OF INSPIRATION IN SPONTANEOUS BREATHING
Cycling device is open to the atmosphere and the PRESSURE>> to zero . The work expended is consequently 1/2PV and is used in
overcoming the airway resistance in the model.
Inspiration >> contraction of the diaphragm draws down>>15 mm>>reduction of pressure
within the chest-The syringe >>two plungers with fluid between them (intrapleural space),
while the right-hand
plunger represents the diaphragm and the left-hand plunger represents the lung tissues moved- A plot of volume against intrapleural pressure for a typical inspiration
is shown-the stippled area >> gives the work expended in overcoming airway resistance and the triangular shaded>> the stored elastic energy which is used during
expiration-0.5 litre inspired when the average pressure in the coupling fluid is 0.6 kPa,>>300 mJ of work-Chest expansion (not shown= 10 to 40% of the inspired tidal
volume- Energy for the contraction
>> adenosine triphosphate (ATP)- 02 + 2H20 + 4e = 4(OH)- + Energy- In the respiratory muscles 10% of the chemical energy is turned into mechanical energy and 90%
into heat.
The total work of inspiration is more difficult to measure in a spontaneously breathing patient- volume inspired
>> pneumotachograph &Intrapleural pressure changes >>the pressure in the lower oesophagus by a catheter
with a
balloon-The greater the expiratory resistance the greater the bowing of the line(graph)-The cross-hatched area to the left of this dotted line represents the expiratory
work done against airway and lung-tissue resistance while the balance of the stored energy in the triangular area is converted into heat in the tissues- We excluded
the work of movement of the chest wall and also the oesophageal pressure reading scan does not give absolute accuracy.
Power is the rate of working and is measured in watts, 1 watt being 1 joule per second. W = J s- 1 300 mJ x 16/60
s -1 =80 mW
EFFICIENCY OF RESPIRATORY MUSCLES
The respiratory muscles are only 10% efficient in producing mechanical energy ,as rest of the energy >> heat. Thus, the
actual would be ten times greater than 80 mW -i.e. about 800 mW. Requirements for breathing would be 1% of the total
metabolism.(80 W).
The power of breathing depends on the type of flow- Energy E = P x V & Power È = P x V 1-Pl prop. V>> Power in laminar flow ÈL prop.
VL2. 2-PT prop.V2 >>Et prop. Vt3. The flow normally present in the respiratory and circulatory systems the has a very small kinetic energy.
Hyperventilation>> the power of breathing increases (cube of the gas flow)-stimulation of ventilation in respiratory
disease >> hypoxia.
The time-constant -product of compliance and resistance-Patients with a low time-constant >>rapid expiratory rate
whereas a high constant is associated with a slower rate
LUNG COMPLIANCE AND AIRWAY RESISTANCE
THE EFFECT OF THE
TYPE OF FLOW
With special reference to ventilation and cardiac
output.
**(1 m3 = 103 litre).

3. WORK OF MYOCARDIAL
CONTRACTION
The graph of the changes in cardiac pressure and volume >>work of myocardial contraction .The pressure and volume axes are reversed from those given
when ventilation was discussed the left ventricular volume increased 60 ml during diastolic filling, then the pressure increased from 0 to 16 kPa (0-120 mmHg)
during isovolumetric contraction. isovolumetric relaxation are indicated and the area of the loop represent the work done if he heart rate is 60, then the power of
the left ventricle of the heart is about 60 joules per minute, i.e. one watt-continuous ventricular pressure>> intraventricular cardiac catheter, and volume
measurements may be deduced from ultrasonic flow or cardiography measurement, cineangiography or other imaging techniques.
POWER OF THE HEART
The power of the right heart can be similarly calculated as follows. If the
mean pulmonary artery pressure is 2.4 kPa (18 mmHg) above the central
venous pressure,
If the mean arterial pressure is 12 kPa (90 mmHg), the pulmonary
venous pressure 0 kPa
EFFECT OF BLOOD PRESSURE AND CARDIAC OUTPUT ON THE
WORK OF THE HEART
A high cardiac output >>thyrotoxicosis, anaemia or exercise increases the work load of
the heart. >>may lead to heart failure.