ENTC 4350

1
ENTC 4350

• Ventilators

2

• Sometimes the outcome of the pulmonary function
  tests may dictate a need for assisted breathing,
  which is provided by the use of ventilatory
  assist devices or respirators.
• The most critical test in determining whether a
  patient requires respirator therapy is the blood
  gas situation, particularly regarding oxygen.

3

• Excessive or inadequate CO2 can be compensated by
  drugs but there is no substitute or compensation
  for low oxygen levels.

4

• In extreme cases, when the usual methods of
  oxygen enrichment and ventilatory assistance are
  inadequate or cannot be used, artificial blood
  oxygenation may be required.

5

• The techniques of anesthesiology are related to
  those of ventilatory assistance
• In fact, ventilatory assistance may be required
  as the anesthesiologist maintains the surgical
  patient on that razors edge between life and
  death.

6
ASSISTED BREATHING

• Under normal conditions, breathing is pretty much
  of an automatic thing.
• The rib cage moves outward, the lungs expand, and
  air enters the newly opened lung spaces.
• Oxygen diffuses through the alveoli into the
  blood, and carbon dioxide diffuses out.

7

• As an electrical analog to the pulmonary
  mechanism, we can think of the lungs as a
  capacitor that is filled with electricity passing
  through a resistance network from a voltage
  source

8

• The pressure of the atmosphere is analogous to
  the voltage source or battery, and the nose.
  mouth, trachea, bronchi, and so on provide the
  resistances.
• If every thing is normal, the atmosphere provides
  plenty of pressure, the tracheobronchial tree
  does not induce significant resistance, and the
  resuIting system is a low-impedance source that
  delivers adequate air to the lungs.

9

• If we climb a mountain, the atmospheric pressure
  drops.
• This, in effect, is like weakening the battery or
  raising its output impedance. and we feel
  altitude sickness.

10

• The same thing can occur if the air passages are
  blocked the body yells about a shortage of
  oxygen.
• Once again, a system which normally has low
  output impedance has been converted to a
  high-impedance system.

11
SOURCES OF RESPIRATORY DIFFICULTY

• Some of the more common problems include the
  following
• Deterioration of lung tissue may interfere with
  the gas-exchange processes in the lung.
• Infection or mucus can obstruct the trachea or
  bronchial tubes and reduce the air flow through
  these passages.
• Neural or muscular control of the nh cage may be
  impaired.
• Lung diseasese.g.. pulmonary fibrosis or
  pneumoniamay reduce the lung elasticity and
  produce stiff lung.

12

• The term compliance is often used in cases of
  stiff lung, and the thing to remember is that
  compliance is the opposite of stiffness.
• A normal lung has good elasticity, it is
  compliant.

13

• A stiff lung is just the opposite.
• That is why compliance measurements are an
  important guide to patient health.

14

• All these problems are to some extent related to
  aging, and, as our population grows older, we can
  expect to see more widespread use of
  breathing-assist devices.
• Problems 1 and 2 are examples of what happens
  when the resistance in the circuit goes up less
  current flows.

15

• Problems 3and 4 are of a different nature.
• In these, the input impedance of the system,
  i.e., the lung, increases because the lung cannot
  expand to the proper volume.

16

• With a transducer or a recorder, high input
  impedance was what we wanted, but in this case,
  we must get a maximum quantity of air into the
  lung to provide for O2 diffusion and CO2 removal.
  
• The lung as a capacitor has become smaller, and
  that is bad news.
• In terms of our example, pulmonary compliance is
  analogous to increased electrical capacitance.

17
MECHANICAL VENTILATORY ASSIST DEVICES

• The objective here is to force air, oxygen or a
  controlled mixture of the two into the patients
  lungs.
• Too little air, or oxygen. will cause lung
  deterioration too much will be just as bad.

18

• The earliest mechanical breathing system was the
  famous (or infamous) iron lung.
• This boiler-shaped device applied a negative
  pressure to the outside of the chest to induce
  rib cage expansion it then reversed and applied
  a positive pressure to compress the lungs and
  force air out.

19

• A simple (simple to discuss, that is) system of
  this type can simulate normal breathing for an
  indefinite period of time.

20

• The older, boiler-type systems had to be provided
  with glove ports so that the nurses could perform
  necessary patient services.
• They were clumsy, and bedsores were a continual
  problem.

21

• The more modern systems are mounted on the chest
  wall, and in some cases, the patients can sit up
  or even walk for limited periods of time.
• One major problem with many breathing-assist
  systems is their need for electrical power.
• Any loss of power or other breakdown is
  immediately life-threatening.

22

• Some systems have provisions for emergency
  operation, and it is well to find and practice
  using them before they are needed.
• Frayed and damaged electrical wires or a smell of
  hot insulation is a sign that something is
  definitely wrong.

23

• If you can reach the drive motor on the machine,
  make it a practice to touch it now and then to
  get a feeling for its normal temperature.
• If the motor seems warmer than usual, it is time
  to call the maintenance department.

24

• If your unit has no emergency power supply, you
  will have to be prepared to ventilate patients by
  hand (or mouth) in case of power outage.
• It is also vital that you know just what alarms
  the machine provides for various problems.

25

• The more common breathing-assist units use a mask
  that fits over the patients nose and mouth.
• There are two types of machines the
  constant-pressure and the constant-volume units.

26

• The constant-volume or volume-regulated
  respirator delivers a given volume of air
• The constant-pressure or pressure-regulated
  respirator delivers air until a predetermined
  pressure is reached.
• Both units should allow a measurement of the
  actual volume delivered and the proper gauges to
  measure the pressure applied to the lungs.

27

• Both of these systems are triggered by the
  patient upon inhalation.
• Patient inspiration pulls a small vacuum on one
  side of a diaphragm
• The diaphragm then moves and closes a switch to
  the machine.
• On some units, the switch can be set to trigger
  on every breath, on every third or fourth breath,
  or on any breath in which the patient does not
  inspire a sufficient quantity of air.

28

• If natural breathing stops, the machine will
  operate continuously at some fixed rate.
• For patients who have no normal breathing reflex,
  the machine is set to cycle at some 12 to 15
  breaths per minute.

29
Constant-Pressure Machines

• The constant-pressure respirator delivers air (or
  air plus oxygen) to the patient until the
  pressure reaches some preset level, at which time
  the pressure is released.
• The volume delivered by a constant-pressure
  machine will vary with the patients position.

30

• Any sputum or other plug in the airway system can
  stop the machine before it delivers a significant
  volume of air.
• This problem is particularly serious when the
  patient resents the machine the patient can
  effectively prevent the delivery of air by
  blocking the flow with his tongue.

31

• In fact, some patients will actually eject the
  mouthpiece.
• An air flow sensor to detect this loss of contact
  should be available, and if you have one, test
  it.

32

• Most sensors are of the heated thermistor type
• If air flow stops they overheat and sound an
  alarm.

33

• Another problem with the constant-pressure units
  is poor control of the oxygen-to-air ratio.
• If the O2 level goes too high, it will result in
  patient distress.

34

• In most cases, the constant-pressure machines are
  used for intermittent assistance only.
• Regular checking of the oxygen-to-air ratio is
  important.

35

• The fact that many constant-pressure machines
  operate on a purely pneumatic basis from gas
  bottles and do not need electrical power makes
  them suitable for use during patient transfer or
  in situations when the electrical power is
  uncertain.

36
Constant-Volume Machines

• The constant-volume machine is designed to
  deliver a preset quantity of air by means of a
  piston and cylinder.
• The machine will, within limits, raise the
  pressure as high as is necessary to deliver the
  proper quantity of air.

37

• Constant-volume systems have the advantage of
  delivering a known amount of gas regardless of
  the patients position.
• Airway obstructions and patient resistance can be
  overridden by the constant-volume system, but
  good drainage must be provided to insure that
  sputum is not forced down into the lung passages.

38

• Constant-volume systems are more complex than
  constant-pressure systems, but they allow better
  control over gas delivery and they are, in
  general, more suitable for long-term
  applications.

39

• Most constant-volume units provide a spirometer
  to allow the nurse to see how much air the
  patient is taking in on an unassisted breath.
• The volume delivered by the machine can then be
  adjusted to provide more ventilation if
  necessary.

40

• Many constant-volume machines have alarms for a
  variety of emergencies, such as loss of power,
  mucus blockage, breakage or kinks in the tubing,
  and so on.
• If you turn off the alarms while aspirating the
  patient, be sure to turn them on again when the
  machine is reactivated.
• Failure to do this has resulted in some nasty
  accidents.

41
0ther Features of Breathing-Assistance Systems

• There are other breathing-assistance systems that
  provide positive pressure during the patients
  normal inspiration.
• In many cases, the added air or oxygen can serve
  as a carrier for water vapor or for drugs
  dispersed by means of a nebulizer.

42

• Some dispute exists about the value of
  intermittent positive-pressure breathing (IPPB)
  systems and their use should be controlled by the
  policy of the hospital.
• Some patients use positive-pressure units for
  home therapy, and again, the rationale for this
  is the physicians decision.

43

• Another factor in the employment of
  breathing-assistance systems is the use of
  differential inspiration-expiration periods.
• Usually, the time allowed for expiration is twice
  that allowed for inspiration.

44

• This additional time is needed for the patients
  muscular system to exhale the air that the
  constant-volume or constant-pressure machine has
  forced into the lungs.

45

• A feature of these systems is the application of
  periodic, extra large breaths.
• This procedure is called sighing the patient.

46

• Sighing is thought to be necessary to prevent
  deterioration of the lung tissue, which may lead
  to atelectasis.
• The exact mechanism of atelectasis is not yet
  under- stood, and there are various opinions
  about the need for sighing.

47

• We suggest you consult a local pulmonary
  specialist if the question arises.

48
Power source Electric blower Inspiratory flow rate control Delivery tube from blower to plastic cylinder Inlet valve to plastic cylinder Outlet valve from plastic cylinder Sensitivity control Inspiratory signal lamp J. Inlet valve to bellows K. Outlet valve from bellows M. Rubber bellows N. Plastic cylinder P. Humidifier chamber R. Foam of bubbles S. Heater with thermoswltch T. Non-rebreathing valve W. Patient connection
49

• Some breathing-assistance machines maintain a
  small positive pressure on the patients
  respiratory system at all times.
• This is called positive-expiratory pressure
  (PEP), and it seems to be an important factor in
  weaning the patient off the respiratory-assist
  system.

50

• The proper pressure level (about 25 mm Hg) should
  not be exceeded
• Too great a pressure would excessively increase
  the work of exhalation.

51
Oxygen Enrichment Methods and Problems

• Many machines provide facilities for oxygen
  enrichment and humidification as necessary.
• If significant quantities of O2 are used,
  humidification is always necessary.

52

• Humidity plays an important role in loosening
  sputum.

53

• When O2 is used with assisted breathing, the
  apparatus for controlling the mixture of O2 and
  air should be carefully checked.
• Just because the mixer worked last year does not
  mean it will work tomorrow.
• Some systems use an O2 sensor, whereas others mix
  predetermined volumes of O2 and air.

54

• Excessive O2 can be dangerous, especially to the
  newborn.
• It is worth emphasizing the hazard of excessive
  O2
• Experiments with normal individuals indicate that
  an O2 level of 35 to 55 can be tolerated
  indefinitely, but with 100 O2 severe pain and
  respiratory disturbances occur after 6 to 30
  hours of exposure.

55

• If your breathing-assistance machine is driven by
  compressed O2 with a venturi for air mixture,
  there is no way of knowing exactly what the
  O2-to-air ratio is.
• In many cases where compressed O2 was used to
  drive the machine, the gas inhaled by the patient
  was found to be over 40 O2 .

56

• This suggests that compressed air should be used
  to drive the machine and the O2 be added as
  necessary.
• Periodic checks with an O2 analyzer might well be
  in order.

57
Weaning the Patient from the Machine

• This becomes a problem whenever the patient gets
  lazy and does not put out enough effort to
  breathe without the machine.
• In this case, you have to be kind but firm and
  adjust the machine so that it only triggers on
  every third or fourth breath.

58

• Some machines can be set to trigger only if the
  patient fails to inspire enough air.
• In many cases, the machine is simply disconnected
  and the patient is encouraged, or forced, to walk
  about.

59
Humidity Control

• As ambient air enters the nasal passages, it is
  humidified to avoid drying the trachea and lung
  tissue.
• Under normal circumstances, the nasal passages
  produce an adequate amount of moisture to handle
  the driest of air.

60

• This may not be the case when assisted breathing
  is employed, because commercial bottled air or O2
  is sold in bone dry condition to avoid
  condensation and rusting of the tanks.
• If a patient is to breathe bottled gas for any
  period of time, it should be humidified with a
  bubbler to avoid excessive drying of nasal tissue.

61
Role of Carbon Dioxide

• Carbon dioxide displays an interesting aspect of
  lung physiology in that the air entering the
  lungs must have a low level of CO2 (below 1) in
  order for the CO2 to diffuse out of the blood.
• If the CO2 level rises above 2, or about 10 mm
  Hg, there will be changes in the patients
  behavior.

62

• At the 3 level, the disturbance is severe, and
  stupor and death ensue at the 6 CO2 level.
• This dramatic effect of CO2 is due to the direct
  influence of this gas on respiration and heart
  rate.

63

• An increase in the CO2 level in the lungs is a
  signal to the body that a more rapid metabolic
  rate is required.
• This allows CO2 to be used as a respiratory
  stimulant, but only up to a point.

64

• If the CO2 level goes above about 4, there will
  be patient distress, regardless of how much O2 is
  available.

65

• The CO2 level is one reason why medical gases are
  handled differently than the gases used for
  welding.
• If a tank of welding oxygen is 3 CO2, the buyer
  might get a lower price, and the gas would be
  quite suitable for welding.

66

• In hospital use, however, this could be a
  tragedy.
• Other impurities that are kept out of
  breathing-grade gases are oil vapor (because it
  causes lung problems) and carbon monoxide.
• The effects of carbon-monoxide poisoning are so
  serious and so well known that no discussion is
  necessary.

67

• The fact that CO2 normally stimulates the
  respiratory process is the basis for the tendency
  to use CO2 whenever there is depression of the
  respiratory process.
• The danger here is that such patients are already
  suffering from excess CO2 or hypercapnia, and the
  respiratory control center has become immune to
  this stimulus.

68

• At this point, the lack of O2, or hypoxia, is the
  only thing that maintains the breathing process.
• Administration of CO2 will not be of any help,
  because the response center is already saturated,
  and the sudden admission of high concentrations
  of O2 is equally dangerous, because it will
  remove the only stimulus for breathing (hypoxia)
  and respiration will cease.

69

• Patients with hypercapnia, or, as it is sometimes
  called, carbon-dioxide narcosis, must be brought
  out very carefully.
• Excessive loss of CO2, or hypocapnia, may be due
  to a variety of factors associated with
  hyperventilation.

70

• In this case, CO2 is administered in spite of the
  fact that it is usually considered a respiratory
  stimulus.
• The patients blood level of CO2 must be raised
  to the proper level, and this is usually best
  done by CO2 administration.

71

• The point is that the effects of CO2 are not
  necessarily simple, and when dealing with
  problems of ventilation, both thought and
  knowledge are needed.

72
BLOOD OXYGENATION SYSTEMS

• We know that oxygen is carried by the blood to
  various parts of the body.
• The carrier system for O2 involves the formation
  of a chemical compound, oxyhemoglobin, which
  serves as the actual transport medium in the
  blood.

73

• When there is severe impairment or failure of the
  blood-gas exchange that normally occurs in the
  lungs, a blood oxygenation machine can be used.

74

• The blood is pumped from the body through a
  gas-exchange system. then to a heating or cooling
  system and, finally, through a bubble separator
  and filter and back to the body.

75

• The gas-exchange system provides for diffusion of
  O2 into the blood to produce oxyhemoglobin.
• It must also encourage CO2 (carbon dioxide) and
  CO to diffuse out of the blood.

76

• These processes can be accomplished by bubbling
  airor more commonly, oxygenthrough the blood
  while at the same time rotating a series of discs
  in the blood to increase the contact area between
  the gas and the blood.
• In some systems, a complex of narrow passages is
  produced by a membrane system.

77

• The membrane (a special plastic) permits O2 to
  diffuse into the blood while allowing CO and CO2
  to diffuse out.

78
The Bubble Problem

• The main problem with blood oxygenation systems
  is that O2 can enter the blood as a solution of a
  gas in a liquid.
• The danger of this gas-in-solution business is
  that the gas can come back out of solution and
  appear as bubbles.

79

• If the gas bubbles at the wrong time (e.g., after
  the blood has returned to the patient), there
  will be dire consequences.

80

• The physics of the bubble problem depends upon
  two effects
• Frothing of the blood in the gas-exchange system.
  
• This produces large bubbles that are easily
  removed by the usual bubble separation system.
• True solution problems based on the effects of
  Henrys law.
• This is by far the most serious problem.

81

• Henrys law states that the quantity of gas that
  dissolves in a fluid goes up as a function of the
  gas pressure in the system.
• In the older blood oxygenation systems, the O2
  pressure was held at a value that was high enough
  to insure the rapid diffusion of O2, but this
  meant that there was some dissolution of O2 in
  the blood.

82

• If the pressure was reduced even slightly, O2
  could come back out as bubbles, like removing the
  cap from a bottle of soda.
• This problem could be compounded if the blood is
  cooled before it enters the oxygenation system,
  as, say, in hypothermic therapy.

83

• Gases are more soluble in cooled blood, but they
  tend to come out of solution when the blood warms
  up.
• Boiled water tastes flat because it has no air
  dissolved in it.

84

• The blood oxygenation system always includes a
  bubble and froth remover, but it might not always
  get all the dissolved gas out.
• If this unit is inadequate,
• If the blood is too cold,
• If the residence time (i.e., the time the blood
  spends in the oxygenator) is too short, or
• If the O2 pressure in the oxygenator is too high,
  
• The bubbles will then reappear after the blood
  has entered the connecting tubes or, worse yet,
  the patients body.

85

• This has led to be a severe problem with many
  blood oxygenation systems.
• A useful technique for detecting bubbles in
  oxygenated blood is ultrasonic monitoring.

86
ANESTHESIOLOGY AND ASSOCIATED PROBLEMS

• In most hospitals, the giving of anesthetics is a
  medical specialty.
• The anesthesiologist is responsible for watching
  the patient, providing the proper dosage, and
  advising the surgeon of any changes in patient
  condition.

87

• There are three basic types of anesthesia
  systems
• open,
• partially open, and
• closed.

88

• The open system is the simplest one.
• The liquid anesthetic is dripped onto a gauze
  cone that covers that patients nose and mouth.
• The anesthetic vaporizes and is mixed with the
  air that enters the patients lungs.

89

• One advantage of this system is its insensitivity
  to excessive use of anesthetic
• It is almost impossible to give the patient too
  much.

90

• The disadvantages are
• The need for excessive quantities of anesthetic,
• Contamination of the operating room, and
• Possible fire or explosion hazards if ether is
  used.

91

• The partially open system is really a generic
  term for several kinds of apparatus.
• In most units, the anesthetic gases are provided
  in a bag reservoir so that they do not mix with
  air.

92

• As the patient breathes, he or she receives a
  controlled mixture of air and anesthetic.
• In some cases, the air will be enriched with O2
  or c to assist the patients respiration
  processes.
• If bottled gases are used, appropriate
  humidification is necessary to avoid patient
  injury.

93

• This system makes it convenient to use gases that
  are not liquids at room temperature.
• You would have great difficulty in dosing a
  patient with a gas like halothane via a nose
  cone.

94

• The partially open systems allow the gas mixture
  that leaves the patients lung to exhaust into
  the operating room.
• This provides for good control of the patients
  CO2 level, but there is some contamination of the
  OR.

95

• Some machines provide for partial rebreathing of
  the expired air, thereby saving anesthetic and
  reducing OR contamination.
• With this system, the patient must be watched
  carefully, because of the possibility that excess
  anesthetic will build up in the system.

96

• This is one place where an ear Oximeter or
  indwelling O2 sensor can be helpful
• Any change in the patients condition would be
  detected almost immediately.

97

• The last and most common systemthe closed
  systemprovides for 100 rebreathing of the
  air-anesthetic mixture.
• As the patient exhales, the excess CO2 and water
  vapor are absorbed in appropriate canisters.

98

• Any necessary O2 or additional anesthetic is
  added to the mixture before the gases are cycled
  back to the patient.
• This system prevents any contamination of the OR,
  but the canisters for water vapor and CO2 must be
  watched and changed regularly.

99

• The canisters usually contain a chemical
  indicator that tells the operator when a change
  is needed by turning color.
• The canisters also pick up CO2 and water from the
  ambient air whenever they have been opened.
• Most hospitals put on new canisters for every
  operation.

100

• If ventilatory assistance is needed during
  anesthesia, the constant-volume or
  constant-pressure machines discussed previously
  can be used.
• In emergencies, the rebreathing bag, which is
  used to catch the expired gas, can be used to
  ventilate the patient
• The operator periodically squeezes and then lets
  go of the bag.

101

• This activates the absorption and supply system,
  and it allows patient maintenance in case of
  machine failure.

102

• Improper gas mixtures must be watched for,
  because many anesthesia systems do not have
  integral gas analyzers.
• You just have to adjust the appropriate
  flowmeters and observe the patients condition.