Handbook of Pulmonary & Critical Care Medicine SK Jindal
INDEX
Page numbers followed by f and t refer to figure and table
A
Abdominal
and miliary tuberculosis 95
tuberculosis 100
Abnormal gas exchange 40
Acid base status 41
Acidosis 41
Acquired immunodeficiency syndrome 81
Acute
miliary TB 103
respiratory distress syndrome 103
Adenosine deaminase enzyme 60
Adequate sputum disposal, cough hygiene and 85
Administrative measures 84
Adventitious sounds 26
Adverse drug reactions 111
Air contamination, removal of 39
Airway
hyperresponsiveness 35
morphology 13
resistance 35
Algorithm for blood gas analysis 42
Alkalosis 41
Allergic bronchopulmonary aspergillosis 21
Alterations in gas exchange 37
Alveolar-arterial oxygen gradient 40
Ambient temperature and pressure 8
Amoxicillin 82
Amplicor MTB test 63
Amplified MTB direct test 63
Anion gap 43
Antibody detection based diagnosis 63
Antitubercular drugs 72
Applied respiratory physics 1
Arterial blood gas 37
analysis 37, 38
and acid-base balance 37
interpretation 39
sampling 38
Atmospheric pressure 3
Auscultation 26
B
Bacillus Calmette-Guérin 62
vaccine 86, 91
Bactec culture medium 59
Bacteriophage-based tests 63
Basic algorithm for spirometry interpretation 31f
BCG vaccination, efficacy of 92
BD probetec MTB test 63
Bedside measurement of lung functions 27
Behçet disease 27
Bernoulli's principle 6, 6f
Beyond microscopy, diagnosis of TB 63
Blood gas, simple algorithm for 43
Blood-brain barrier 100
Body temperature and pressure saturated 8
Bone and joint tuberculosis 97
Boyle's law 4
Breastfeeding 113
Breath sounds 26
Breathing, control of 36
Bronchodilator responsiveness 32
Bronchogenic carcinoma 20
Broncholiths 54
C
Capping specimen 39
Capreomycin 80
Cardiopulmonary exercise testing 35
Causes of drug resistance 108
inadequate treatment of tuberculosis 108t
Cell wall structure 46
Centers for disease control and prevention 87
Central nervous system tuberculosis 95, 99
Charles' law 5
Chemoprophylaxis 87
Chemoprophylaxis and MDR-TB 90
in children 89
in HIV patient 88
in special situations 89
Chemotherapy of tuberculosis 66
supervision of 70
Chest pain 21
radiology 57
examination of 25
Chingleput trial 92
Chronic
case 69
obstructive pulmonary disease 20, 28
Clarithromycin 82
Clavulanate 82
Clofazimine 81
Clubbed fingers 24f
Clubbing 24
Cobas
amplicor MTB test 63
Taqman test MTB 63
Community-acquired pneumonia 54
Compensated acid base disorders 44
Complications of BCG vaccination 93
Constraints on use of UV lamps 86
Control of
breathing 36
ventilation 18
Cough 20
hygiene and adequate sputum disposal 85
Counseling, pretreatment evaluation and 109
Cryptic miliary TB 104
Cyanosis 24
Cycloserine 80
D
Delta gap 44
Designing an appropriate regimen 110
Detection of latent tuberculosis infection 87
Diagnosis of
antibody detection based 63
beyond microscopy 63
drug-resistant TB 108
latent tuberculosis 63
pulmonary tuberculosis 57
tuberculosis 56, 61
Disease,
Behçet 27
Osler-Weber-Rendu 27
Drugs,
antitubercular 72
group 5 81
Duration of preventive therapy 89
Dyspnea 22
E
Efficacy of BCG vaccination 92
Elastic properties of chest wall and lung-chest 13
Environmental controls 85
Enzyme linked
immunosorbent assay 57
immunospot assay 63
Equation,
Hagen-Poiseuille 8
Henderson-Hasselbalch 41
ESP II culture system 60
Establish
safe radiology procedures for patients 85
separate rooms 85
Ethambutol 67, 78
Ethionamide/prothionamide 79
Evaporation 7
Examination of chest 25
Exercise
hypoxemia 40
testing 34
Expression of gas volumes and pressures 8
Extrapulmonary tuberculosis 60, 95
F
Fiberoptic bronchoscopy 59
Fine needle aspiration cytology 96
Flow of gases 8
Flu syndrome 76
G
Gas
exchange 39
alterations in 37
laws 4
solution and tension 6
Gases, flow of 8
Gastric washings 59
Gastrointestinal tract, tuberculosis of 100
Gay-Lussac's law 5
General
gas law 5
physical examination 23
Genitourinary tuberculosis 95, 102
Grading of activities of antituberculosis 67f
Graham's law 6
Gravity, role of 15
Group 5
drugs 81
H
Hagen-Poiseuille equation 8
Henderson-Hasselbalch equation 41
Henry's law 6
Hepatic tuberculosis 105
High-dose isoniazid 83
HIV and TB 55
Human immunodeficiency virus 75, 93
infection 68
Hypercapnea 41
I
Imipenem/cilastatin 82
Immune
reconstitution inflammatory syndrome 97
responses to tuberculosis 51
Indications and contraindications 28
Infection, route and spread of 47
In-house phage amplification tests 63
Inpatient care facilities 84
Inspection 25
Interferon gamma release assays 57, 63
Interpretation of arterial blood gas 44
Intravascular pressure 16
Isoniazid 67, 73
J
Joint tuberculosis 95
K
Kaposi's sarcoma 27
Klebsiella pneumoniae 21
L
Latent tuberculosis 57
infection, detection of 87
Law,
Gay-Lussac's 5
Graham's 6
Henry's 6
Leprosy 93
Lessons learnt from previous studies 110t
Line zolid 82
Loeffler's syndrome 27
Luciferase reporter phage assay 63
Lymph node tuberculosis 95
Lymphadenopathy 25
M
Management of tuberculosis 66
Matter, state of 1
MB/BACT system 60
Mechanisms of hemoptysis in tuberculosis 54t
Metabolic acidosis 43
Microarray technology 63
Microcolony detection on solid media 59
Microscopic observation of in broth culture 60
Mild hypoxemia 40
Miliary TB 55
tuberculosis 103
Miscellaneous physical findings 25
Mismatching of ventilation and perfusion 18f
Mixed acid-base disorder 44
Moderate hypoxemia 40
Molecular
diagnosis of tuberculosis 61
movement 1
tests for species identification 63
weight of oxygen 2
Molecule and compound 1
Movements of chest wall 25
Multidrug-resistant tuberculosis 107
Mycetoma in tubercular cavity 54
Mycobacterial cultures 59
genome 48
groups 46
growth indicator tube 59
identification 47
Mycobacterium avium 47, 48
bovis 91
intracellulare 47
tuberculosis 46, 48, 49, 53, 72, 86, 113
infection 49
N
Necrosis of pulmonary venules and capillaries 54
Need-based hospitalization of TB patients 84
Nontuberculosis mycobacteria 62
Normal anionic gap metabolic acidosis 43
Nucleic acid amplification detection based 63t
Nutritional support 112
O
Optimal TB test 64
Osler-Weber-Rendu disease 27
Osmosis 10
Other uses of BCG 93
Oxygen saturation SAO2/SPO2 39
Oxygenation 39
index 41
Oxyhemoglobin dissociation curve 40f
P
Palpation 25
Para-aminobenzoic acid 79
Paradoxical response 56
Partial pressure 3
Peak expiratory flow 32
Percussion 26
Pericardial tuberculosis 104
Permeability 10
pH, pCO2 and HCO3 in primary acid base 41t
Physical properties of gases 2
Pitfalls in arterial blood gas interpretation 45
Pleural effusion 97
Pneumonia, community-acquired 54
Polymerase chain reaction 63
Poncet's disease 98
Post-primary pulmonary tuberculosis 53
Post-tubercular bronchiectasis 54
Postvaccination reactions 92
Pott's
disease 97
paraplegia 98
Pregnancy 113
Pressure saturated, body temperature and 8
Pretreatment evaluation and counseling 109
Prevention of tuberculosis 84
Primary acid-base abnormality, types of 43
Pulmonary
circulation 15
driving pressure 16
function testing 28
mechanics 36
tuberculosis 53, 54
diagnosis of 57
Pulse oximetry 39
Purified protein derivative 57, 62, 88
Pyrazinamide 67, 77
Q
Quantiferon-TB gold 63, 87
Quinolones 78
R
Radial artery 38
Radiology, chest 57
Rasmussen's aneurysm 54
Rationale for recommended treatment regimens 69
Removal of air contamination 39
Resective thoracic surgery 112
Respiratory function and mechanics 11
Result of sputum smear 67
Revised national tuberculosis control program 90, 95, 96, 115
Reynold's number 9
Rifampicin 67, 75
Role of gravity 15
Route and spread of infection 47
Routine laboratory testing 56
Rupture of Rasmussen's aneurysm 54
S
Scar carcinoma 54
Secondary prophylaxis 89
Serum electrolyte measurement 39
Severe hypoxemia 40
Severity of disease 67
Simple algorithm for blood gas 43
Site of tubercular disease 67
Skin tuberculosis 103
Solubility 10
Spinal tuberculosis 97
Spirometry 28
Sputum
collection area 86
induction 59
microscopy 59
Standard or normal temperature and pressure 8
State of matter 1
Static lung volumes 33
Streptomyces griseus 76
Streptomycin 67, 76
Suitability for intermittent use 66
Superior vena cava 23
Supervision of chemotherapy 70
Syndrome,
acquired immunodeficiency 81
acute respiratory distress 103
immune reconstitution inflammatory 97
Loeffler's 27
Systemic examination 27
T
TB
diagnosis in HIV+ individuals 62t-HIV
coinfection 113t
patients, need-based hospitalization of 84
test, optimal 64
Temperature and heat 4
Tertiary prevention 90
Test,
amplicor MTB 63
amplified MTB direct 63
BD probetec MTB 63
Cobas amplicor MTB 63
Tests
for drug-resistance 63
bacteriophage-based 63
Thiacetazone 67, 81
Total and alveolar ventilation 14
Transmural pressure 16
Transporting blood sample for analysis 39
Treatment
after interruption or return 69
failure 68
of bone and joint tuberculosis 99
strategy 109
Tuberculin skin test 57, 63, 87
Tuberculosis 22
abdominal and miliary 95
and air travel 114
causes of inadequate treatment of 108t
chemotherapy of 66
diagnosis of 56
in elderly 55
molecular diagnosis of 61
of gastrointestinal tract 100
osteomyelitis 99
Prevention of 84
Tuberculous meningitis 95
Turbulence 9
Turbulent flow 8f
Types of primary acid-base abnormality 43
U
Ultraviolet germicidal irradiation 85, 86
Upper respiratory tract symptoms 22
V
Vaccination 91
Vaccine, Bacillus Calmette-Guerin 86, 91
Vaccines 86, 91
Vapor 7
pressure 7
Various
lung volumes and capacities 30f
stages in pathogenesis 50
Ventilation 11, 12
control of 18-perfusion 37
relationships 18
Vitamin D 113
Vocal resonance 27
Volume 2
W
Wright meter 33
×
Chapter Notes

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Applied Respiratory PhysicsCHAPTER 1

SK Jindal,
VK Jindal
 
STATE OF MATTER
Matter can exist in three different forms in nature—solid, liquid or gas; although plasma, a fourth state of matter has also been identified under the extremes of temperature and pressure. Matter is either an element made from similar atoms, e.g. iron or compound made from two or more types of atoms, e.g. water (H2O) in nature. An element is a basic unit of matter, which retains the same properties on subdivisions by chemical or mechanical means. Oxygen and other gases, such as hydrogen, helium, chlorine and nitrogen are elements in nature. Atoms of different elements may form molecules or compounds. Hydrogen can exist in atomic or molecular form, whereas nitrogen and oxygen occurs in molecular form (N2 and O2).
 
Molecule and Compound
All substances consist of exceedingly small particles called molecules. There are about 1019 molecules in 1 ml of air under the normal conditions of temperature (T) and pressure (P). A molecule possesses the distinctive properties of the parent element or compound. A molecule is found to consist of two or more atoms of same kind or of different kinds. The number of molecules comprising a macroscopic quantity of a gas is enormous typically around 1023 molecules. The number of molecules and their velocity determine many properties of gases.
A compound is composed of two or more elements united chemically to form a substance different from the individual elements forming that compound. For example, carbon dioxide is a compound of carbon and oxygen. On subdivisions, the compound loses its properties and may resume those of the constituent elements. Both elements and compounds exist as molecules as the smallest component.
 
Molecular Movement
All the molecules of matter are in a state of incessant motion. This is known as Brownian movement and forms the basis of the kinetic theory of matter. This motion results from temperature—the higher the temperature, the larger the velocity of the molecules. At absolute zero temperature, the velocity of a classical molecule goes to zero. Molecules of a gas have great mobility and travel longer distances before colliding with other molecules. It is because of this mobility that a gas has no fixed shape and mixes readily with other gases.2
 
PHYSICAL PROPERTIES OF GASES
The air we normally breathe is a mixture of gases of which oxygen and nitrogen constitute the main bulk.
Breathing of additional or supplemental gases is required in abnormal situations. The physical properties of these gases influence the mechanics of normal breathing, as well as the therapeutic strategies. Equal volumes of gases at the same temperature and pressure contain equal number of molecules (Avogadro's Hypothesis).
 
Volume
Volume is the space occupied by a gas. The volume of a cuboid-shaped vessel is determined by the multiplication of internal length, width and height of the vessel. It is expressed in cubic centimeters (cc), cubic feet or liters (L), etc. (1 L ≡ 1000 cc or ml).
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A 10 cm cube has the volume equaling 1 L. Volume of a container of uniform cross-sectional area A and height is given by V = A h. The gas shall occupy the available volume irrespective of the amount (mass) of gas. For example, if a small vessel containing oxygen is emptied in a larger vessel, the entire volume of the larger vessel will be occupied by the same amount of gas.
 
Mass and Weight
Mass is the bulk or the total mass of number of molecules of the gas. In the above mentioned example, the mass of the gas in two cylinders shall remain the same, although the volume has changed. The number of molecules per unit of volume in the two vessels has changed, i.e. lesser number of molecules per unit volume in the larger vessel.
Weight is often used synonymously with mass. Weight is determined by the pull of gravity on mass (i.e. m × g). Since, the force of gravity on the earth is nearly constant, mass is equivalent to weight on the surface of the earth. In fact, weight is scaled to indicate mass and therefore both represent the same thing.
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Density
Density is expressed as the weight in the grams of one liter of a gas. Since the weight of 22.4 liters of a gas is that of a gram molecule of that gas, one liter of gas shall weigh molecular weight/22.4 gm. When expressed this way, the density will be measured in gm/liter. It is also measured in gm/cc, which will equal 1/1000 in value to that in gm/liter.
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The density of a gas is also expressed as relative to the density of air. Density of air shall vary depending upon its composition. For all practical purposes, it comprises of 1 volume of oxygen (about 20%) and 4 volumes of nitrogen (about 80%). At NTP, the density of air is (32 × 1/5 + 28 × 4/5)/22.4 = 1.3 gm/L. Therefore, the density of oxygen relative to that of air is 1.1. When expressed this way, it becomes unimportant, if the density of individual gases was calculated in gm/liter or gm/cc.3
 
Pressure
The gas molecules are always in a state of motion and constantly bombard the walls of the container. The force applied to (or acting upon) a unit area of the wall is called the gas pressure. The closer the molecules, the greater the number, which strikes each unit area and therefore the greater the pressure applied. Also, larger the velocity (or temperature), larger is the impact on the walls, leading to greater pressure.
Gas pressure is generally considered as that of a stationary gas when the pressure exerted is the same at any point in a gas container. The pressure is usually expressed in the millimeters of mercury (mm Hg) or Centimeters of water (cm H2O) or pounds per square inch (psi). One millimeters of mercury means force on a unit area (1 cm2) on which 1 mm of height of Hg is placed. The volume of height h cm on unit area is A × h = h cm3 and in mass is hd where d is the density of the liquid. The force on unit area due to this is hdg, which is also the pressure p. Thus, p due to a height h of a liquid of density d, p = hdg. The pressure due to 1 mm Hg can be calculated by putting h = 0.1 cm, d = 13.6 gm/cc and g = 980 cm/S2.
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Atmospheric Pressure
Atmospheric air is pulled to the earth by gravity and generates a force upon the surface of the earth, resulting in atmospheric pressure. Atmospheric pressure is the sum of pressures of all gases (e.g. N2, O2 and CO2) present in air. It is measured with the help of a glass tube filled with either mercury or water (manometer). The height of the column of mercury (or water) multiplied by its density is a measure of the atmospheric pressure. Standard pressure is measured at sea level and expressed in mm Hg (torr) or cm H2O.
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It is important to convert atmospheric pressure (≡ 76 cm of Hg) in CGS units. This is approximately 106 CGS units.
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It is relevant here to mention that the lungs are subject to atmospheric pressure all the time. Since, the alveoli are in direct communication with the atmosphere through the tracheobronchial tree, the alveolar pressure is the same as the atmospheric pressure. The changes in alveolar pressure during inspiratory and expiratory phases of respiration are relative to the atmospheric pressure. During inspiration when the alveolar pressure is −10 cm H2O, it implies the presence of atmospheric pressure—10 cm H2O (i.e. 1030 – 10 cm H2O). Similarly, when positive pressure is administered to a patient through a ventilator, the ventilator gauge pressure of 10 or 20 cm H2O refers to a total pressure of 1040 or 1050 cm H2O (i.e. atmospheric pressure + gauge pressure).
 
Partial Pressure
In a mixture of gases in a container, each gas exerts the same pressure, which it would if it alone occupied the container. There is no interference from the presence of other gas(es). The pressure exerted by each individual gas is called 4the partial pressure. The total pressure exerted by the mixture of gases is equal to the sum of the partial pressures of all the gases contained in the mixture (Dalton's law). The partial pressure is determined by the fraction of the concentration of the gas in the mixture.
The atmospheric air has a total pressure of 760 mm Hg when dry at sea level. The partial pressures of N2 (79%) and O2(21%) therefore, are as follows:
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Temperature and Heat
Temperature is the thermal state of a substance, which determines whether the substance will give or receive heat from another substance in contact. It is an indication of the level of molecular activity. Heat is the thermal energy of a substance, which can be given to or abstracted from it. Temperature is the measurement of heat.
Calorie is the unit of heat. It is defined as the quantity of heat required to raise the temperature of 1 gm of water by 1°C. As an example, if one calorie is required to raise the temperature of 1 gm of water by 1°C, 1000 calories will be required for 1000 gm of water. The caloric value of food is expressed by a larger heat unit, i.e. the kilocalorie (cal or kcal), which is equivalent to 1000 cal.
To raise the temperature to a given range, similar weights of different substances require different quantities of heat. The number of calories required to raise the temperature of 1 gm of that substance by 1°C is the specific heat of that substance.
The specific heat depends on the state of the matter— solid, liquid or gas. Specific heat of water is 1 (It follows from the definition of 1 calorie).
For gases, such as O2 and air, it is 0.0603 cal per cc, it is quite high when expressed per gm.
We can calculate the total quantity of heat required to raise the temperature of a given volume by multiplying the volume with specific heat and temperature rise, i.e. volume (cc) × specific heat (cal per cc) × temperature rise(°C).
 
THE GAS LAWS
There is a definite relationship between the gas properties described above. These relationships are described in different laws to understand the behavior of gases. These laws are valid for ideal gases only, where the assumption that the gas particles are very small and do not interact with each other is valid.
 
Boyle's Law
Pressure (P) of a gas is inversely proportional to its volume (V) provided the absolute temperature (T) of the mass of gas is kept constant. In other words, the product of pressure and volume remains constant. It follows immediately from the ideal gas equation,
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The application of this law in respiratory physiology is best exemplified in the use of body plethysmography to measure total lung capacity. It is also employed in many mechanical ventilators whereby the gas is driven into 5patient's lungs or into the cylinder of the ventilator by the upstroke and downstroke movements of the piston.
 
Charles' Law
When pressure and mass of a gas are kept constant, the volume of the gas will vary directly with its absolute temperature. Again from gas equation,
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It is because of this reason that volumes measured with the help of lung function equipments (at room temperature) are a little lower than those at body temperature (37°C) and need to be corrected for the same. If the temperature of a container of a gas is lowered, the volume shrinks. Therefore, more gas can be stored in the same cylinder at a lower temperature.
 
Gay-Lussac's Law
Temperature and pressure of a gas are directly proportional when the volume and mass are kept constant.
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It implies an increase in pressure if the temperature is increased. For this reason safety valves are provided with devices using high pressure gases to vent high pressures in case there is an accidental heating.
 
The General Gas Law
Assume N ideal noninteracting molecules of a gas each of mass m are contained in a cube of volume V. They are in motion if the temperature is above 0°K (0°K = −273°C). If the temperature is T in Kelvin, the kinetic energy from each molecule is of the order of kT, where K is called Boltzmann constant (K = 1.38 × 10–16 CGS units). Because of this kinetic motion, the molecules of the gas keep bombarding the walls of the cube and exert pressure. The pressure increase results in extra energy (obtainable from force × distance or p x volume relation). In this way it is quite evident that one can equate the energy because of pressure PV as resulting from kinetic energy of N molecules, PV = NkT.
If N is expressed in No (Avogadro No.), PV = η NokT, where η = N/No. This is the famous gas equation valid for all ideal gases. is the number of moles of the gas, Nok is also called gas constant R. R ≃ 6 × 1023 × 1.38 × 10–16 erg/deg K
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The gas equation can be used to determine how the given initial state (P1, V1, T1) relates to (P2, V2, T2), some final state by using P1V1(T1= P2V2/T2).
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It may be stated here that under the conditions of lower temperature and high pressure the gas changes its state to liquid. This is because the gas molecules get attracted to each other (van der Waals forces) rather than being repelled. The higher pressure condenses the molecules and the lower temperature reduces their activity.6
The temperature at which the gas turns into liquid is the “critical temperature” of that gas. For oxygen, it is −116°C. A pressure of 50 atmospheres is required to liquefy oxygen at −116°C. To keep the oxygen in a liquid form at 1 atmosphere in a flask open to the atmosphere, the temperature is lowered to below −183°C. This principle forms the basis of the availability of oxygen in the liquid form for storage and ambulatory use.
 
Henry's Law
The amount of gas that enters into physical solution in a liquid is directly proportional to the partial pressure of the gas. For example, the greater the partial pressure of oxygen in the alveoli, the greater the solubility in plasma.
 
Graham's Law
The rate of diffusion (D) of a gas is inversely proportional to the square root of its density (d).
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Therefore, a light gas (such as helium) will diffuse at a faster rate than a heavier gas (such as oxygen).
 
Bernoulli's Principle
Flow of a gas through a partially obstructed tube can be described by Bernoulli's principle, i.e. the pressure required to produce flow is the difference in velocity at two points and the density of the gas (Fig. 1.1).
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Fig. 1.1: Bernoulli's principle
 
GAS SOLUTION AND TENSION
The amount of gas dissolved in a liquid is directly proportional to the pressure of the gas (Henry's law). It also varies with the temperature—lesser amount is dissolved at the same pressure if temperature is increased. A state of equilibrium is reached when no further gas dissolves in the liquid. This is a state of full saturation with the gas at a given temperature and pressure. The gas in solution is said to exert the same “tension” as the partial pressure of the gas over the liquid in equilibrium with it. For example, when the partial pressure of oxygen in alveoli is 100 mm Hg, the tension of O2 in alveolar capillaries is 100 mm Hg. At this pressure, 0.3 cc of oxygen at NTP dissolves in 100 cc of water. The weight of oxygen dissolved in 100 cc water is:7
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The amount of oxygen dissolved in plasma or water is the same (0.004 gm/100 ml). This is quite sufficient to supply all the oxygen necessary for the metabolism of the body.
 
VAPORS
Vapor is defined as the gaseous state of a substance, which at room temperature and pressure is a liquid. On the other hand a gas at room temperature exists only in the gaseous state. Like any other gas, the molecules of a vapor are continuously in violent motion and bombard the walls of the container. The force exerted on each unit area is called the pressure of the vapor (Vapor pressure).
A vapor in a mixture of gases obeys the same laws as the gases. The partial pressure of the vapor in a mixture bears the same proportion to the total pressure as the volume, i.e. it depends on the percent (or fractional) concentration in the mixture. For example, the concentration of about 16% of water vapors in air at NTP, which is sufficient to saturate air with water vapors exerts a pressure of 16% of 760 mm Hg (47 mm Hg).
The presence of water vapors in air or oxygen is referred to as humidity. It is largely through the process of evaporation that the molecules of water (or any other liquid) evaporate into the overlying air (or any other gas in a container).
The molecules leave the liquid substance when their kinetic energy exceeds the surface tension of the liquid. If a liquid is kept in a closed container for long, a state of equilibrium is reached when the number of molecules returning to the liquid (condensation) is exactly equal to the number leaving it (evaporation). This is called the saturation-point. This is further dependent upon temperature; if the temperature increases, the number of molecules leaving the liquid also increase and the saturation point is raised, i.e. there is a greater amount of vapors in the same amount of gas. Reverse happens with a fall in temperature.
The air we breathe is normally humid due to the presence of water vapor. The actual amount of water vapor present in air is expressed as “relative humidity”, which is defined as the ratio of the amount of water vapor present in a given volume with the amount of water vapor, which the air (or the gas) is capable of holding at the given temperature, in the same volume.
The humidity of air varies with the atmospheric conditions. Once inhaled in the lungs, air gets fully saturated. The amount of water vapor required to saturate the alveolar air at body temperature and pressure is the body humidity.
The presence of water vapor in the inhaled air exerts its own partial pressure, and lowers the pressures of constituent gases of air—oxygen and nitrogen. Therefore, PN2 or PO2 is calculated as a proportion of the atmospheric pressure minus water vapor pressure, i.e. Patm – PH2O.
When fully saturated, PH2O of atmospheric air is equal to 47 mm Hg. Therefore, PO2= (Patm – PH2O) × 21% = (760 – 47) × 0.21 = 150 mm Hg. The rest, i.e. 760 – (150 + 47) would be approximately the PN2.8
 
EXPRESSION OF GAS VOLUMES AND PRESSURES
In view of the effects of temperature, pressure and humidity on all gases, these are expressed with reference to those conditions. Some of the common expressions are:
  1. Standard (or normal) temperature and pressure (STP) or NTP Temperature 0°C; Pressure 760 mm Hg.
  2. STPD: D indicates “dry” = complete absence of water vapor.
  3. Ambient temperature and pressure—dry or saturated (ATPD or ATPS). Ambient implies the room conditions.
  4. Body temperature and pressure saturated (BTPS): Body temperature (usually 37°C), ambient pressure and water vapor pressure (47 mm Hg).
Normally, gas volume measurements are made in the ambient conditions. Conversion is required to express the volume at BTPS or STPD. STPD is used for the uniformity of expression. This is done by multiplying with conversion factors. Tables of conversion factors from ATPS to STPD, STPD to BTPS, or BTPS to ATPS are available in most laboratories. Such corrections are also required to express volume of a gas (such as O2) produced in the laboratory. The volume is expressed at STPD, which is different than that produced at ATPS.
 
FLOW OF GASES
Flow is the movement of particles of a liquid or a gas from higher to lower pressure. It is expressed in the terms of volume per unit time, e.g. liters per minute or per second (L/min or L/sec). The movement of air into the lungs during inspiration and out into the atmosphere during expiration is accomplished by the flow of air through tracheobronchial tree. Similarly, oxygen flows from a container cylinder to the lungs or a ventilator through connecting tubes as long as there is a pressure difference.
The flow is described as laminar, if it is smooth and gas particles move along lines parallel to the walls of the tube. But it is turbulent if the lines of flow are irregular, broken up and disorderly (Fig. 1.2). Whether the flow is laminar or turbulent, it has to meet a certain resistance while moving from one to the other end of the tube. The laminar flow is described Hagen-Poiseuille equation, i.e. V = Πγ4ΔΡ)/(8ηl).
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Fig. 1.2: Turbulent flow
9
 
Resistance
Resistance is defined by the pressure difference under given conditions, between the entry and the exit points of a tube. The resistance is dependent on the tube length (l)and diameter. It is also directly proportional to the velocity of flow (∇) or rate in case of laminar flow. The flow is also viscosity (η) dependent and density-independent. When the flow is turbulent, the resistance rises for steeply. The laminar flow through a straight tube of uniform size is inversely proportional to the length (l) of the tube and directly to the fourth power of radius (r).
When the flow exceeds a “critical flow rate”, the laminar flow is replaced by the turbulent flow throughout the length of the tube. Turbulent flow is less efficient since the Δ Α varies directly with V2. Turbulent flow is density dependent and viscosity-independent. The critical flow varies directly with the internal diameter of the tube — the larger the diameter, the greater the flow. At a flow below the critical rate, local turbulence may occur as a result of irregularities in the pathways of the gas. During oxygen administration this may occur due to the constriction of kinking of the tubes.
Turbulence in a flowing system can also be predicted by Reynold's number. It is the ratio between inertial (density dependent, viscosity independent) and viscous (viscosity dependent, density independent) forces— a dimensionless number.
In case of pipes,
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Flow is “laminar” when the number is less than 2,000 and “turbulent” when it is more than 3,000. Turbulent flow is dominated by inertial forces producing random eddies and flow fluctuations. Between 2,000 and 3,000, the flow is transitional, i.e. neither fully laminar nor fully turbulent.
 
Flow Through Orifices
An orifice is a narrow opening of a tube. Unlike a tube, the diameter of the fluid pathway of the orifice exceeds the length. The greater the diameter compared to the length, the more does the opening approach the “ideal” orifice. The flow through an orifice depends on the diameter (or the cross-section area) of the orifice and the difference in pressures on either side of the orifice.
The intrinsic property of a liquid that influences its flow, which we earlier termed as resistance is called viscosity. It is attributed to the internal friction between different layers, which move at different speeds. While the laminar flow largely depends on viscosity, it is the density, which determines the flow when turbulent. The coefficient of viscosity is equal to the force per unit area necessary to maintain the unit difference of velocity between two parallel planes.
The flow through an orifice is at least partially turbulent. The lower the density, i.e. the lighter the gas, the greater is its volume flow for any given pressure difference on the either side of the orifice.
 
Diffusion
The molecules in a fluid (liquid or gas) unlike in a solid, move freely in all directions. The time taken by a molecule to travel a given distance depends 10upon its closeness to other molecules, and the intermolecular spaces. The gas molecules do not necessarily collide with the neighboring molecules when they move around or across a membrane.
Diffusion across a membrane is determined by the difference in concentrations between the two neighboring layers of the solution. The rate of diffusion is proportional to the gradient of concentration, i.e. the change of concentration per unit length in the direction of diffusion (Fick's law).
Diffusion also depends on molecular movement. The rates of diffusion of gases at similar partial pressures through a porous membrane are inversely proportional to the square roots of their molecular weights (Graham's law). The molecular weight of oxygen and CO2 being 32 and 44 respectively, the diffusion ratio will be 1.2.
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It implies that oxygen diffuses 20% faster than CO2 through a dry, porous membrane. It is because of the solubility of CO2 in water of the moist alveolar membrane that the CO2 diffusion is higher than O2.
Solubility: Another factor, which determines diffusion across a wet film is the solubility of the gas. The rate of diffusion is directly proportional to the solubility of the gas in the fluid.
Permeability: A membrane is permeable, if it allows the particular molecules to pass through, i.e. across the membrane. Permeability of different membranes to the molecules of different substances (solid, liquid or gases) is variable.
Osmosis: Osmosis is the migration of molecules of a solvent across a membrane. The pressure, which stops the transfer of molecules is called the osmotic pressure of the solution. The osmotic pressure of a solution depends only on the number of dissolved particles per liter and not on the nature of the substances, which is dissolved.
The diffusion mechanisms also depend upon the pressures, volume and filtration. In respiratory system, the diffusion of gases and fluids across the alveolar membrane are critically important for normal gas-exchange functions of the lung and in the maintenance of a fluid balance.