Principles of Mechanical Ventilation

1
Principles of Mechanical Ventilation

• RET 2284
• Module 6.0 Ventilator Management
• - Improving Ventilation/Oxygenation

2
Improving Ventilation / Oxygenation

• The first 30 60 minutes following initiation
  of ventilation are generally spent evaluating
  vital signs, breath sounds, ventilator
  parameters, lung compliance and resistance, the
  artificial airway, and documenting patient
  response to therapy
• After that initial phase, the RT is often
  concerned with improving ventilation and
  oxygenation and managing the patient-ventilator
  system

3
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• A change in will often be needed when a
  patient is first placed on mechanical ventilation
  to correct for respiratory alkalosis or acidosis
  this is facilitated by making a change in VT or
  rate (f)

4
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Methods of Changing Ventilation Based on PaCO2
  and pH
• If it is appropriate to keep rate (f) constant
  and change VT, the equations is as follows
• Desired VT Known PaCO2 x Known VT Desired
  PaCO2

5
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Methods of Changing Ventilation Based on PaCO2
  and pH
• If it is appropriate to keep VT the same and
  change rate (f), then the equations is as
  follows
• Desired f Known PaCO2 x Known f Desired PaCO2

6
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Respiratory Acidosis
• Volume and Pressure Ventilation Changes
• When PaCO2 is elevated (gt45 mm Hg) and pH is
  decreased (lt7.35), respiratory acidosis is
  present and VA is not adequate
• Causes
• PE, Pneumonia
• Airway disease (e.g., severe asthma attack)
• Pleural abnormalities (e.g., effusions)
• Chest wall abnormalities
• Neuromuscular disease
• CNS problems

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7
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Respiratory Acidosis
• Volume and Pressure Ventilation Changes
• Guideline
• ?VT to 8 12 mL/kg ideal body weight (based on
  patients pulmonary problem)
• Maintain plateau pressure lt30 cm H2O
• If VT is already high and/or Pplateau are already
  high, then f should be increased
• Read example 1, 2 and 3 Respiratory Acidosis,
  Increasing VT, page 259 260 (Pilbeam)

8
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Respiratory Alkalosis
• Volume and Pressure Ventilation Changes
• When PaCO2 is decreased (lt35 mm Hg) and pH
  increases (gt7.35), then respiratory alkalosis is
  present and alveolar ventilation is excessive
• Causes
• Hypoxia with compensatory hyperventilation
• Parenchymal lung disease
• Medications
• Mechanical ventilation
• CNS disorders
• Anxiety
• Metabolic disorders

9
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Respiratory Alkalosis
• Volume and Pressure Ventilation Changes
• Guideline
• Volume ventilation ?f, and if necessary, ?VT
• Pressure ventilation ?f, and if necessary,
  ?pressure
• Read example 1 and 2 Respiratory Alkalosis,
  Decreasing the rate, page 261 (Pilbeam)

10
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Metabolic Acidosis and Alkalosis
• Treatment of metabolic acidosis and alkalosis
  should focus on identifying those metabolic
  factors that can cause these acid-base
  disturbances

11
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Metabolic Acidosis and Alkalosis
• Metabolic Acidosis
• Causes
• Ketoacidosis (alcoholism, starvation, diabetes)
• Uremic acidosis (renal failure to excrete acid)
• Loss of bicarbonate (diarrhea)
• Renal loss of base following administration of
  carbonic anhydrase inhibitors (e.g., Diamox)
• Overproduction of acid (lactic acidosis)
• Toxin ingest that produce acidosis (salicylate,
  ethylene glycol antifreeze, methanol

12
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Metabolic Acidosis and Alkalosis
• Metabolic Acidosis
• Treatment should first deal with the cause of the
  acidosis
• Secondly, assess the need to reverse the acidemia
  with some form of alkaline agent

13
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Metabolic Acidosis and Alkalosis
• Metabolic Acidosis
• These patients are often struggling to lower
  their PaCO2 to compensate for the metabolic
  acidemia. As a consequence, these patients are
  at risk for developing respiratory muscle fatigue
• If the patient is losing the struggle to maintain
  high with spontaneous breathing, assisted
  ventilation may be necessary to avoid respiratory
  failure. It is then appropriate to keep the pH
  (7.35 7.45)

14
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Metabolic Acidosis and Alkalosis
• Metabolic Alkalosis
• Causes
• Loss of gastric fluid and stomach acids
  (vomiting, nasogastric suctioning)
• Acid loss in the urine (diuretic administration)
• Acid shift into the cells (potassium deficiency)
• Lactate, acetate, citrate administration
• Excessive bicarbonate loads (bicarbonate
  administration)

15
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Metabolic Acidosis and Alkalosis
• Metabolic Alkalosis
• Treatment involves correcting the underlying
  cause and reversing those factors leading to the
  alkalosis. In severe cases, carbonic anhydrate
  inhibitors, acid infusion, and low bicarbonate
  dialysis my be required
• Only in rare circumstances does partial
  respiratory compensation of metabolic alkalosis
  occur PaCO2 will usually not rise higher than
  55 mm Hg (Remember that as the CO2 rises, the
  PaO2 falls)

16
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Mixed Acid Base Disturbances
• Combined Respiratory Alkalosis and Metabolic
  Acidosis
• Read case studies Pilbeam, pg. 262 263
• Combined Respiratory Acidosis and Metabolic
  Alkalosis
• Read case study Pilbeam, pg. 263

17
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Increased Physiological Dead Space
• If pure respiratory acidosis persists even after
  alveolar ventilation has been increased, the
  patient may have a problem with increased dead
  space
• Causes
• Pulmonary emboli
• Low cardiac output ? low pulmonary perfusion
• High alveolar pressure (PEEP) ? ? pulmonary blood
  flow
• Air trapping ? ? pulmonary perfusion

18
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Increased Metabolism and Increased CO2 Production
• Read case study Pilbeam, pg. 264
• Metabolic rate and VCO2 are increased in the
  following patients
• Fever
• Sepsis
• Burns
• Multiple trauma and multiple surgical procedures
• Hyperthyroidism
• Seizures

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19
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Increased Metabolism and Increased CO2 Production
• In these patients is increased and WOB is
  elevated
• Treatment Options
• Increase machine rate to ?WOB may cause
  auto-peep
• Add pressure support for spontaneous breaths to
  ?WOB through ET and circuit
• Switch to PC-CMV, use sedation to ?WOB

20
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Intentional Iatrogenic Hyperventilation
• Definition
• Deliberate hyperventilation in patients with
  acute head injury and increased intracranial
  pressure (ICP)
• Hyperventilation reduces PaCO2 which causes
  vasoconstriction of cerebral blood vessels and
  decreases blood flow to the brain and is believed
  to lower increased intracranial pressure ICP

21
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Intentional Iatrogenic Hyperventilation
• Current therapy guideline for head injuries with
  increased ICP do not recommend prophylactic
  hyperventilation (PaCO2 lt25 mm Hg) during the
  first 24 hours - may cause cerebral ischemia and
  cerebral hypoxemia

22
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Intentional Iatrogenic Hyperventilation
• Hyperventilation may be needed for brief periods
  when acute neurological deterioration is present
  and ICP elevated
• Mild hyperventilation (PaCO2 30 35 mm Hg) may
  be used for longer periods in a situation in
  which increased ICP is refractory to standard
  treatment
• The practice of iatrogenic hyperventilation
  still remains controversial

23
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Permissive Hypercapnia (PHY)
• Definition
• Deliberate limitation of ventilatory support to
  avoid lung overdistention and injury of lung
• ARDS
• Status asthmaticus
• PaCO2 values are allowed to rise above normal
• 50 150 mm Hg
• pH values are allowed to fall below normal
• 7.10 7.30
• Most researchers agree pH 7.25 is acceptable

24
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Permissive Hypercapnia (PHY)
• ?PaCO2 accompanied ?PaO2
• O2 administration must be provided and monitored
  closely
• ?PaCO2 stimulates the drive to breath
• Appropriate to provide sedation to patients in
  whom PHY is being employed

25
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Permissive Hypercapnia (PHY)
• Procedures for Managing PHY
• Allow PaCO2 to rise and pH to fall without
  changing mandatory rate or volume
• Sedate the patient
• Avoid high ventilating pressures
• Maintain oxygenation
• Reduce CO2 production
• Paralyze
• Cool
• Restrict glucose

26
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Permissive Hypercapnia (PHY)
• Procedures for Managing PHY
• Keep pH gt7.25
• Sodium bicarbonate
• Tris-hydroxiaminomethane (an amino buffer)
• Carbicarb (mixture of sodium carbonate and
  bicarbonate

27
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Permissive Hypercapnia (PHY)
• Contraindications and Effects of PHY
• Head trauma
• Intracranial disease
• Intracranial lesions

28
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Permissive Hypercapnia (PHY)
• Relatively contraindicated in the following
• Cardiac ischemia
• Left ventricular compromise
• Pulmonary hypertension
• Right heart failure

29
Improving Ventilation / Oxygenation

• Correcting PaCO2 Abnormalities
• Permissive Hypercapnia (PHY)
• The use of PHY is restricted to situations in
  which the target airway pressure is at its
  maximum and the highest possible rates are being
  used
• The risks of hypercapnia are considered by some
  to be preferable to the high Pplat required to
  achieve normal CO2 levels
• Read Case Study Pilbeam, pg. 265 266

30
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Adjusting FiO2
• Every attempt should be made to maintain the FiO2
  lt0.40 to 0.50 to prevent the complications of O2
  toxicity while keeping the PaO2 between 60 and 90
  mm Hg
• This goal is not always possible and sometimes a
  higher FiO2 is required
• The SpO2 can be used to titrate FiO2, with the
  goal of maintaining the SpO2 gt90
• The SaO2 on an ABG is used to establish the
  relationship with the current SpO2

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31
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Adjusting FiO2
• ABGs are obtained after mechanical ventilation is
  initiated and compared with FiO2 being delivered
  and the SpO2 to establish their relationships
• A linear relationship exists between PaO2 and
  FiO2 as long as VE, CO, Shunt, VD/VT remain
  fairly constant (cardiopulmonary status)

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32
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Adjusting FiO2
• Because of the linear correlation between PaO2
  and FiO2 the following equation can be used to
  select the desired FiO2 to achieve a desired
  PaO2
• Desired FiO2 PaO2 (desired) x FiO2 (known)
• PaO2 (known)

33
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Adjusting FiO2
• Exercise
• After being supported on a ventilator for 30
  minutes, a patients PaO2 is 40 mm Hg on an FiO2
  of 0.50. Acid-base status is normal and all
  other ventilator parameters are within the
  acceptable range. What FiO2 is required to
  achieve a desired PaO2 of 60 mm Hg?

34
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Adjusting FiO2
• Desired FiO2 PaO2 (desired) x FiO2 (known)
• PaO2 (known)
• Desired FiO2 (60 mm Hg) (0.50 FiO2)
• 40 mm Hg
• Desired FiO2 0.75

35
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Selection of FiO2 or Adjustment of Paw
• Maintaining an FiO2 gt60 may lead to
• O2 toxicity
• Absorption atelectasis
• Lower limits of target PaO2 is 60 mm Hg
• Lower limits of target SpO2 is 90

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36
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Selection of FiO2 or Adjustment of Paw
• When PaO2 remains very low on high FiO2,
  significant shunting, V/Q abnormalities , and/or
  diffusion defects are present - other methods to
  improve oxygenation, besides increasing FiO2,
  must be considered
• ?Paw
• PEEP
• HFOV
• APRV

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37
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Selection of FiO2 or Adjustment of Paw
• Paw can be used to increase the PaO2
• Factors that affect Paw during PPV
• PIP
• PEEP
• Auto-PEEP
• IE ratio
• Respiratory rate
• Inspiratory flow patterns

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38
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Selection of FiO2 or Adjustment of Paw
• Paw is a major determinant of oxygenation in
  patients with ARDS
• ?Mean alveolar pressure ? oxygenation
• ?Alveolar recruitment ? oxygenation
• Typical method to increase Paw
• PEEP
• Other methods to increase Paw
• HFOV
• APRV

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39
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Selection of FiO2 or Adjustment of Paw
• Paw must be monitored closely to prevent
• Air trapping
• Overdistention
• Barotrauma (e.g. pneumothorax)
• ?Venous return
• ?CO

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40
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Goals of PEEP
• Enhance tissue oxygenation
• Maintain a PaO2 above 60 mm Hg, and SpO2 90 at
  an acceptable pH
• Restore FRC
• These goals my be accompanied by the opportunity
  to reduce the FiO2 to safer levels (lt0.50) as
  PEEP becomes effective
• Must maintain cardiovascular function and avoid
  lung injury

41
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Minimum or Low PEEP
• PEEP at 3 5 cm H2O to help preserve a patients
  normal FRC
• Therapeutic PEEP
• PEEP gt5cm H2O
• Used in the treatment of refractory hypoxemia
  caused by increased intrapulmonary shunting and
  V/Q mismatching accompanied by a decreased FRC
  and pulmonary compliance

42
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Optimal PEEP
• The level of PEEP at which the maximum beneficial
  effects of PEEP occur
• ?O2 transport
• ?FRC
• ?Compliance
• ?Shunt

43
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Optimal PEEP
• The level of PEEP is considered optimum because
  it is not associated with profound
  cardiopulmonary side effects
• ?Venous return
• ?CO
• ?BP
• ?Shunting
• ?VD/VT
• Barotrauma
• Volutrauma
• Accompanied by safe levels of FiO2

44
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Indications for PEEP Therapy
• Bilateral infiltrates on chest radiograph
• Recurrent atelectasis
• Reduced CL
• PaO2 lt60 mm Hg on high FiO2 of gt0.5
• PaO2/FiO2 ratio lt200 for ARDS and lt300 for ALI
• Refractory hypoxemia PaO2 increases lt10 with
  FiO2 increase of 0.2

45
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Specific clinical disorders that may benefit from
  PEEP
• ALI
• ARDS
• Cardiogenic PE
• Bilateral, diffuse pneumonia

46
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Application of PEEP
• Increased in increments of 3 5 cm H2O in
  adults, 2 3 cm H2O in infants
• Target acceptable PaO2/FiO2 ratio at a safe FiO2
• gt300 (e.g., PaO2 100, with FiO2 0.33
• (optimal, but not always realistic)

47
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Application of PEEP
• Patient Appearance
• Color, level of consciousness, anxiety a sudden
  deterioration may indicate cardiovascular
  collapse or pneumothorax
• Blood Pressure
• ?BP of 20 mm Hg systolic drop is significant
• Breath Sounds
• Barotrauma, e.g., pneumothorax

48
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Application of PEEP
• Ventilator Parameters
• VT, Flow, PIP, plateau pressure, VE

49
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Application of PEEP
• Static Compliance (CS)
• As PEEP progressively restores FRC, compliance
  should increase

50
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Application of PEEP
• Static Compliance (CS)
• Too Much PEEP ? Overdistention ? ?CS

51
Optimized Lung Volume Safe Window

• Overdistension
• Edema fluid accumulation
• Surfactant degradation
• High oxygen exposure
• Mechanical disruption
• Derecruitment, Atelectasis
• Repeated closure / re-expansion
• Stimulation inflammatory response
• Inhibition surfactant
• Local hypoxemia
• Compensatory overexpansion

Zone of Overdistention
Injury
Safe Window
Zone of Derecruitment and Atelectasis
Volume
Injury
Pressure
52
Application of PEEP
53
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Application of PEEP
• Arterial PO2, FiO2, and PaO2/FiO2
• The usual approach to the management of FiO2 and
  PEEP is to start with high FiO2 and incrementally
  decrease it as PEEP improves oxygenation

54
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Application of PEEP
• Arterial to End-Tidal Carbon Dioxide Tension
  Gradient
• Normal P(a-et)CO2 gradient is 4.5 2.5 (Pilbeam)
• Is lowest when gas exchange units are maximally
  recruited without being overdistended
• If P(a-et)CO2 gradient increases minimal
  acceptable values, it signifies that too much
  PEEP has been added and is producing a drop in
  cardiac output and in increase in VD/VT

55
Application of PEEP
56
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Application of PEEP
• Arterial-to-Venous Oxygen Difference (C(a-v)O2)
  reflects O2 utilization by the tissues
• Normal value is 5 vol
• Increases in C(a-v)O2 with increases in PEEP may
  indicate hypovolemia, cardiac malfunction,
  decreased venous return to the heart, and
  decreased cardiac output from PEEP

57
Application of PEEP
58
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Application of PEEP
• Mixed Venous O2 Tension or Saturation
• Normal PvO2 3540 mm Hg
• (minimal acceptable is 28 mm Hg)
• Normal SvO2 75
• (minimal acceptable is 50)
• PEEP usually improves PvO2 and SvO2
• When PvO2 and/or SvO2 decrease, with a increase
  C(a-v)O2 increase, this indicates a decrease in
  cardiac output TOO MUCH PEEP

59
Application of PEEP
60
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Application of PEEP
• Cardiac Output
• Cardiac output provide key information about the
  bodys response to PEEP
• PEEP improves V/Q ? ?Oxygenation ? ?CO
• Too much PEEP ? Overdistention ? ?Venous return ?
  ?CO

61
Application of PEEP
62
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Application of PEEP
• Pulmonary Vascular Pressure Monitoring
• When using PEEP gt15 cm H2O, it is important to
  closely evaluate the patients hemodyamic status,
  which may require the placement of a pulmonary
  artery catheter
• If pulmonary artery occluding pressure (PAOP),
  also known as wedge pressure, rises markedly as
  PEEP is increased, the lungs may be overinflated
• On the other hand, when PEEP rises, PAOP may be
  markedly decreased because of pulmonary blood
  flow is reduced as a result of decreased venous
  return to the right side of the heart

63
Application of PEEP
64
Improving Ventilation / Oxygenation

• Data From a Patient with ARDS on MV 24 Hours
  after Admission
• VT 700 f 6 VE 6.6 FiO2 0.8
• PEEP BP HR PCWP CO CS PIP PaO2 PVO2
• 0 130/65 130 16 4.8 28 50 40 27
• 120/55 135 13 4.2 31 58 45 37
• 135/65 125 18 5.8 33 60 50 35
• 130/70 120 19 5.9 36 55 115 37
• 110/50 130 25 4.1 27 63 150 29
• Can you find the optimal PEEP level?

65
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Weaning From PEEP
• Patient should demonstrate an acceptable PaO2 on
  an FiO2 of lt0.40
• Must be hemodynamically stable and nonseptic
• Lung conditions should have improved
• ?CS, ?PaO2/FiO2 ratio
• Reduce PEEP in 5 cm H2O increments
• Evaluate SpO2 within 3 minutes to determine
  effect if it falls lt20 from previous PEEP
  level, the patient is ready to tolerate lower
  PEEP level. If SpO2 drops gt20 place PEEP at
  previous level

66
Improving Ventilation / Oxygenation

• Oxygenation Using FiO2 and PEEP
• Positive End Expiratory Pressure (PEEP)
• Weaning From PEEP
• Wait between reductions in PEEP and reevaluate
  the initial criteria. If the patient is stable,
  reduce PEEP by another 5 cm H2O. This may take 1
  hour or may require as long as 6 hours or more
• When the patient is at 5 cm H2O, an additional
  evaluation is necessary. If reducing the PEEP to
  zero result is a worsening of the patient, then
  it may be appropriate to leave the patient at 5
  cm H2O until extubation