Knowledge of the patient

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• Knowledge of the patients ability to take in
  oxygen and get rid of carbon dioxide is an
  important factor in patient care.
• Some of the tests involved are done at the
  bedside, others require that blood or urine
  samples be taken to a laboratory, while in other
  cases, the patient will have to go to a pulmonary
  laboratory for tests.

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• In every case. we are concerned with the
  following questions
• Can the patient inhale and exhale a sufficient
  quantity of air at the proper rate?
• Are the gasesi.e.. oxygen (O2) and carbon
  dioxide (CO2)moving across the lung membranes at
  the proper rate?
• Is the proper balance of O2 and CO2 being
  maintained in the blood?

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• The data required to answer these questions are
  obtained by pulmonary function testing and blood
  gas analyses.

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• PULMONARY FUNCTION TESTING

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Lung Capacities and Volumes

• If the lungs are expanded to their maximum
  volume, we refer to the volume involved as the
  total lung capacity, or TLC.
• If the patient is asked to empty the lungs as
  much as possible, the remaining volume is called
  the residual volume (RV).
• The difference between the TLC and the RV is
  called the vital capacity (V C).

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• These volumes and some typical numerical values
  are shown in the figure.

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• The measurement of these volumes is of interest
  as a determinant of patient condition.
• Note that normal breathing does not involve
  maximum lung effort.
• The so-called resting tidal volume (RTV) is
  therefore used to measure the flow of air in and
  out of the lung under resting conditions.

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• The tidal volume is usually taken as a more
  normaI indication of the patients ability to
  breathe.

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Measurement of Rate of Respiration

• The preceding type of information is of interest
  in determining the patients lung volume and the
  ability of the chest muscles to expand and
  compress the lungs.
• However, it provides no data on the rate at which
  the patient can breathe, or on the amount of
  oxygen that actually passes from the lungs into
  the blood.

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• The rate at which the patient can breathe can be
  measured by a number of tests, one of which is
  called the forced vital capacity or FVC test.
• The patient takes a deep breath and blows it out
  as rapidly as possible.
• The quantity of air expired in some given length
  of time (say, 10 seconds) can be used to evaluate
  the degree of restriction or obstruction of lung
  function.

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• Restriction refers to the result of any
  interference with the bellows action of the lung
  itself, e.g., by fluid accumulation or fibrosis
• Obstruction in the passages leading to the lungs.
  
• If. there is a question about whether either or
  both effects are present, the test is repeated
  after the administration of a bronchodilating
  agent.

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• The measurement of the impedance of the chest by
  means of attached electrodes is sometimes used in
  the determination of the rate of respiration.
• This method is also used to determine the amount
  of chest expansion.
• Some of the older systems used devices that went
  around the chest at the pressure of expansion or
  contraction would be registered via a pressure
  gauge or mercury manometer.

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• More commonly, the heater-thermistor system is
  used in determination of respiration rate.
• A number of other related tests are used to
  determine the rate at which patient can inspire
  air.

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• The major matter of interest is the determination
  of the problem is, i.e.,
• Whether it is a
• restriction,
• obstruction, or
• both.

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• Some specific tests and the acronyms used to
  designate them are

Description of Test Name of Test Acronym
Largest volume measured on complete expiration after the deepest inspiration without forced or rapid effort. Vital capacity VC
Vital capacity performed with expiration as forceful and rapid as possible Forced vital capacity FVC
Volume of gas exhaled over a given time interval during the performance of forced vital capacity test Forced expiratory volume (qualified by subscript indicating the time interval in seconds e.g. 10 indicates an interval of 10 seconds) FEVt FEV10
FEV expressed as a percentage of the forced vital capacity. Percentage expired (in t seconds) FEV
Average rate of flow for a specified portion of the forced expiratory volume test (usually between 200 and 1200 ml) Forced expiratory flow FEF
Average rate of flow during the middle half of the forced expiratory volume test Forced midexpiratory flow FEF
Volume of air that a subject can breathe with voluntary maximal effort for a given time. Maximal voluntary ventilation MVV
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Other Pulmonary Functions

• The term volume is used for a paramrter that is
  measured as a function of time, whereas the term
  capacity refers to a measurement that does not
  involve time.
• For example, the vital capacity is the largest
  volume measured on complete expiration after
  complete inhalation, regardless of how long these
  take thus, no time parameter is involved.
• The forced expiratory volume, on the other hand,
  is the total volume of air the patient expires in
  some fixed period of time, e.g., 10 seconds the
  time factor is very important for FEV evaluations.

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• Another determination of importance is that of
  airway resistance, which is the ratio of pressure
  to the rate of air flow.
• You can think of this flow (current) induced by a
  pressure (voltage) through a resistance R (Ohms
  law again).

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• Lung compliance is a measure of the change in
  lung as a function of a change in lung pressure.
• Poor lung compliance is a sign of the condition
  known as stiff lung.

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The Spirometer

• The apparatus most often used for pulmonary tests
  is the water spirometer consists of a cone or
  bell that is designed to ride up and down in a
  cylinder of about 10cm diameter.
• The bell is counterweighted to keep the pressure
  inside the cylinder at atmospheric level, the
  water provides a seal, and the bell moves up and
  down in response to the patients inhalation and
  expiration.
• The vertical motion of the bell is recorded on a
  moving drum that is covered with calibrated chart
  paper. The rotation of the drum provides a time
  scale, and the resultant chart is called a
  spirogram

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A typical spirometer is shown in the figure.
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• The water spirometer can serve as a good example
  of the impedance concept.
• The tube leading from the patients mouth to the
  spirometer is a source of resistance, and the
  spirometer itself consists of a volume that must
  expand to contain the expired air.
• If the resistance of the tube is toohigh or if
  there are leaks, the output impedance of the
  sourcei.e., that of the patient plus the
  tubewill be too high.

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• The total amount of air in the spirometer will be
  less than it should be, and the time required for
  the patient to exhale will be much too long (it
  may be considered equivalent to pumping up a tire
  with a pump that has a small and leaky hose).
• If the patients respiratory system has high
  airway resistance, this will raise the output
  impedance of the source
• this will appear as a deviation from the normal
  FEV1 values and as such is valuable for
  diagnosis.

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• The point here is that any leaks in the hose or
  in the patients mouth fitting may show up as a
  clinical problem on the spirogram and lead to a
  false diagnosis.

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• The water spirometer is a bulky instrument that
  is not well suited for in-the-bed measurements.
• For such applications or for mass screening
  tests, it is common to employ the waterless
  spirometer.
• This unit is held in the patients mouth, and, as
  inhalation and exhalation occur, the time and
  rate of air flow are measured by one of a variety
  of flowmeters (the heated thermistor is one such
  device).

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• The rate of flow (liters per minute), multiplied
  by the time during which air flow occurs, yields
  the volume (liters).

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• Airway resistance has been noted as a cause of
  reduced flow during FEV measurements.
• To separate this effect from any problem that
  might exist in the lungs themselves, it is common
  practice to measure both the rate of flow with a
  spirometer and the intraalveolar pressure in a
  body plethysmograph at the same time.

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• A high alveolar pressure in conjuction with a
  reduced flow would be a sign of excessive airway
  resistance.
• The body plethysmograph is used for a number of
  other tests, including those for lung compliance
  and airway resistance, but the details of its
  operation are best left to specialized books on
  respiratory testing.

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Oxygen-Carbon Dioxide Exchange

• Another test of respiratory function is the
  measurement of the ability of the lungs to pass
  oxygen (O2) and carbon dioxide (CO2).
• In one such test, the patient breathes a mixture
  of air and carbon monoxide (CO).
• Carbon monoxide is used because it passes easily
  through the lung membrane and because no normal
  reserve of CO exists in the body to interfere
  with the measurements.
• The CO level is not high enough to cause any
  patient injury.

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• The CO normally passes very rapidly through the
  lungs and is absorbed by the blood.
• In the test, the level of CO in the exhaled air
  is measured by the respiratory technician and
  compared with a standard value.
• If the exhaled air is high in CO, the patients
  ability to exchange gasesincluding O2 and CO2is
  impaired.

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Helium Washout Test

• A test of the physical condition of the lungs
  involves having the patient breathe a mixture of
  air and helium until an equilibrium mixture of
  helium has been distributed to all areas of the
  patients lungs.
• Helium does not pass through lung tissue, and the
  only loss of this gas will occur by expiration.

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• After the equilibration period, the flow of
  helium is cut off, and the patient breathes pure
  air.
• During this period, the expired air is analyzed
  for helium, and the rate at which the helium is
  washed out is determined.

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• If the patient requires an excessively long time
  to wash out the helium, this is taken as a sign
  that certain areas of the lung are open but
  inactive, in the sense that no expansion or
  contraction of these portions occurs during
  breathing.
• This is often seen in emphysema, where the
  enlarged areas are totally ineffective for gas
  exchange.

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• If the physician suspects that a problem is
  specific to only one lung, he may ask for a
  bronchospirometric test.
• This involves passing a doublelumen catheter into
  the trachea.
• One catheter tip is passed into each of the
  bronchi. and a balloon at the end of the catheter
  is inflated to insure that all the air entering
  or leaving the lung passes through the catheter.
• Under these conditions, the gas flow,
  composition. and pressure can be measured for
  each of the lungs.

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DISTRIBUTION OF PULMONARY BLOOD FLOW

• The test involves the injection of a radioactive
  substance into the blood vessels leading to the
  pulmonary area.
• Postinjection scanning with radiation detectors
  provides a measure the blood flow to the lungs.

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BLOOD GAS AND pH ANALYSIS

• The gas content and the pH of the blood are often
  the earliest indicators of a change in patient
  condition.
• At one time, it was necessary actually to take
  blood samples to the laboratory for blood gas and
  pH testing.
• The laboratory measurement of blood gases and pH
  involves the use of special electrodes that
  provide an electrical output proportional to the
  fraction of a particular chemical species
  (hydrogen, carbon dioxide, oxygen, or whatever)
  in the blood.

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Blood Oxygen Measurements

• Arterial blood is almost always taken for oxygen
  analysis, and it is vital that the sample get to
  the laboratory before the oxygen level changes.
• If the patient is receiving oxygen therapy, this
  should be noted, because it will affect the
  physicians evaluation of the data on blood
  oxygen level.

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• ARTERIAL O2 TENSION (PO2) AND ARTERIAL SATURATION
  (SO2)
• The PO2 is a measure of the actual partial
  pressure of oxygen in the blood its normal range
  is around 95 to 100 mm Hg.
• When chronic pulmonary disease is present, the
  PO2 level can fall as low as 70 to 75 mm Hg
  without evidence of hypoxia.

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• Arterial saturation SO2, is the ratio of the
  actual oxygen content to the content that would
  exist if the blood were saturated with oxygen.
• Blood saturation will only occur if the patient
  breathes 100 O2 for some length of time.

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• The change in arterial O2 content when the
  patient breathes a gas mixture that is high in O2
  is often used as a measure of the patients
  ability to pass O2 across the pulmonary membrane.
  
• The correlation of PO2 and SO2 test data with
  other respiratory function data provides
  information for diagnostic purposes.

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OXYGEN MEASUREMENT TECHNIQUES

• In some cases, special electrodes designed to
  respond to a specific dissolved gas like O2 have
  been inserted in the arteries for continuous PO2
  measurements.
• Their use is not yet as common as either the
  method of taking laboratory samples or the ear
  Oximeter.

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• A major use of the Oximeter is in monitoring
  infants who were born prematurely or have
  respiratory problems.
• For the continuous bedside measurement of the
  blood oxygen or PO2 level, it is possible to use
  the ear-probe Oximeter, which determines the
  amount of O2 combined as oxyhemoglobin.

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• In this device, a quartz-iodine lamp is used to
  generate white light.
• The light is split into eight wavelengths in the
  red and infrared regions.
• This energy is passed through the pinna, or top
  part of the ear, and the absorption of light at
  each wavelength is measured.

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• The absorption of light by hemoglobin increases
  with wavelength (going from the red to the
  infrared), while the absorption of oxyhemoglobin
  decreases in the same optical region.

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• The Oximeter can determine the PO2 in the blood
  to within about 1 if the patient is in the
  normal range (95-100mm Hg).
• In the range of 70-75 mm Hg, its accuracy falls
  to about 3, but this is usually quite adequate.

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• Instrument response is almost instantaneous, and
  the device can be left in place for long periods
  of time.
• It is important that patients blood circulation
  be adequate
• If the blood flow to the ear is impaired, the
  device cannot be used.
• In this case, laboratory analysis techniques will
  be required.

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BLOOD CARBON DIOXIDE MEASUREMENTS

• The employment of special indwelling CO2 sensors
  has been investigated, but they are not yet in
  general use.
• Laboratory tests on blood samples include the
  measurement of arterial CO2 tension (PCO2) and
  the CO2 combining power of plasma or serum.
• This latter test is usually performed on venous
  blood (which is why in blood sampling, both
  arterial and venous blood may have to be taken),
  and it serves as a measure of the patients
  alkali reserve.

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pH Measurement

• Once again, the blood pH can be measured by
  indwelling catheters, but it is usually done in
  the laboratory.
• Respiratory acidosis, or excess acid in the blood
  (low pH), may be caused by a high level of CO2 in
  the blood due to inadequate alveolar ventilation.
  

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• Metabolic acidosis occurs when there is excess
  production of organic acids or a sugar imbalance,
  as in diabetes.
• In metabolic acidosis, the body will attempt to
  compensate by means of hyperventilation to remove
  CO2 from the blood.
• Compensation may lower the blood acid level, but
  it does not solve the primary problem the excess
  of nonvolatile, organic acids.

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• In many cases, this test is a reliable indicator
  of metabolic disturbances.

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• The measurement of pH is a good example of a
  simple idea that requires a good deal of
  electronics before it can be used in practice.
• It depends upon the fact that blood and in fact
  all body solutions contain ions (charged atoms of
  hydrogen, calcium, carbonate, etc.).

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• If two electrodes made of two different materials
  are inserted into the solution one electrode will
  become positive with respect to the other.
• The ions respond to the resultant electrostatic
  field, the positive ions move to the negative
  electrode and the negative ions to the positive
  electrode.

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• The resultant voltages are characteristic of the
  ions involved and their number
• If we measure the hydrogen ions we call the
  result pH.
• If we measure the oxygen ions we call the result
  PO2 and so on.

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• All of this sounds easy but in practice we note
  first that these electrode systems have a very
  high output impedance.
• This means we have to measure their voltages with
  a meter having an even higher input impedance.

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• Another factor here is that different electrodes
  are used to measure different ions
• The pH electrode sees only hydrogen ions,
• the oxygen electrode is sensitive to oxygen ions.
  
• All of these gadgets are subject to damage and
  blinding by fibrin or other deposited material.

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• The instructions for cleaning, storage, and
  operation should be carefully observed.
• Good data require good instruments.

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• Respiratory alkalosis, or a high level of alkali
  in the blood (high pH), may be due to
  hyperventilation that produces a deficit of CO2.
• This is usually compensated by the kidney, which
  releases bicarbonate to the blood to yield more
  CO2.

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• Metabolic alkalosis may be due to excessive
  intake of alkaline salts, a deficit of potassium,
  or a loss of organic acids.
• In this case, the patient will be encouraged to
  hypoventilate and retain CO2 to restore the
  acid-base balance.

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CORRELATION OF PULMONARY FUNCTION TESTS WITH
RESPIRATORY ABNORMALITIES

• A plot of maximum exhaled volume versus time will
  indicate a reduced vital capacity in patients
  with stiff lung.
• Airway obstruction does not reduce the vital
  capacity, but it will extend the time required
  for complete exhalation.

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• Some diseases, e.g., atrophic emphysema, may
  increase lung compliance and produce an
  excessively large volumetric flow during
  exhalation.
• Hypertrophic emphysema, or chronic obstructive
  pulmonary edema, reduces the contractability of
  the lungs so that they become permanentIy
  enlarged
• In this case, a reduced volumetric flow is
  observed.

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• Atelectasis, which is lung collapse in adults or
  incomplete lung expansion at birth, is manifested
  by a decrease in total lung capacity.
• In some cases, the lung capacity will increase if
  the lungs are momentarily inflated by the
  application of a positive pressure. i.e., a
  pressure above atmospheric pressure.

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• A decrease in lung diffusion, as manifested by
  impaired gas exchange, may indicate a collagen
  disease or the blocking of pulmonary capillaries
  by emboli.

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• In many cases, a patient will have more than one
  respiratory problem, which will make the
  diagnosis more complicated.