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Flashcards in 16 Respiratory Failure Deck (19)
1

Oxygenation failure

  • Occurs when...
  • Can be caused by...
  • PAO2 and A-a gradient
  • PaCO2
  • Causes of hypercapnia

  • Occurs when...
    • Lung disease causes a drop in the PaO2 and arterial hemoglobin saturation (SaO2)
    • This may result from the generation of abnormal V/Q ratios (including intra-pulmonary shunting) and/or the impairment of gas diffusion
  • Can be caused by...
    • Any lung disease, regardless of whether it primarily affects the airways, the lung parenchyma, or the pulmonary circulation
  • PAO2 and A-a gradient
    • Since lung disease does not alter any of the components of the alveolar gas equation, the calculated PAO2 is unchanged and the A-a gradient increases
  • PaCO2
    • In patients with oxygenation failure, an appropriate increase in VE mediated by central chemoreceptors maintains a normal PaCO2
  • Causes of hypercapnia
    • Abnormal degree of V/Q mismatching 
    • Diffusion impairment

2

Ventilation failure

  • Occurs when...
  • PaCO2 vs. VCO2, VE, and VD

  • Occurs when...
    • Neuromuscular or chest wall disease causes a primary drop in VE that prevents the respiratory system from maintaining a normal PaCO2
  • PaCO2 vs. VCO2, VE, and VD
    • PaCO2 α VCO2 / (VE – VD)
    • For a given level of CO2 production (VCO2) and dead space ventilation (VD), PaCO2 varies inversely with VE
    • As VE falls, PaCO2 rises

3

Ventilation failure

  • Minute ventilation
  • Inadequate VE can also be caused by...

  • Minute ventilation
    • Product of respiratory rate and tidal volume
    • Can be reduced by any disease that...
      • Decreases central respiratory drive
      • Interferes with the transmission of neural signals from the brain to the respiratory muscles
      • Reduces respiratory muscle strength
  • Inadequate VE can also be caused by...
    • Disorders such as morbid obesity and severe kyphoscoliosis, which markedly reduce chest wall compliance and increase the pressure that must be generated by the respiratory muscles

4

Ventilation failure

  • The diseases that precipitate ventilation failure
  • PaO2 vs. PaCO2
  • A-a gradient

  • The diseases that precipitate ventilation failure
    • Do not affect the lungs themselves
    • Do not alter V/Q ratios or gas diffusion
  • PaO2 vs. PaCO2
    • This means that the average alveolar PO2 (PAO2) and the measured PaO2 fall together with the rise in PaCO2, as predicted by the alveolar gas equation
    • PAO2 = (PB – PH2O) x FIO2 – PACO2 / R
  • A-a gradient
    • Since the PAO2 and PaO2 decrease by the same amount, ventilation failure does not change the difference between them (the A-a gradient)

5

Oxygenation-ventilation failure

  • This type of respiratory failure
  • Pathophysiology 
  • Causes

  • This type of respiratory failure
    • Combines the features of oxygenation and ventilation failure
  • Pathophysiology
    • Largely the same as for pure oxygenation failure
    • The difference is that the underlying lung disease causes such a profound abnormality in lung compliance and/or resistance that the respiratory system either...
      • Cannot maintain a normal VE
      • Cannot increase VE to compensate for the increased dead space ventilation produced by V/Q mismatching
  • Causes
    • Any disease causing oxygenation failure can also cause oxygenation-ventilation failure
    • The most common causes
      • ARDS
      • Cardiogenic pulmonary edema
      • COPD
      • Severe acute asthma

6

Management of respiratory failure

  • Establish a patent airway
    • This can usually be accomplished by appropriate head positioning or insertion of an oral or nasal airway
    • Endotracheal intubation may be required
  • Maintain adequate oxygenation
    • Supplemental oxygen is administered to keep the SaO2 > 90%
  • Maintain sufficient ventilation
    • Ventilation must be maintained at a level that provides a safe arterial pH
    • If necessary, spontaneous breathing can be assisted by mechanical ventilation
  • Treat the underlying cause of respiratory failure

7

Oxygen therapy

  • Supplemental oxygen delivery systems can be divided into two groups 
  • Group 1
  • Group 2

  • Supplemental oxygen delivery systems can be divided into two groups
    • Group 1: those that deliver a fractional oxygen concentration (FO2) of 1.0
    • Group 2: those that deliver a variable, clinician-set FO2
  • Group 1: those that deliver a fractional oxygen concentration (FO2) of 1.0 
    • Includes nasal cannulae, simple masks, and non-rebreather masks
  • Group 2: those that deliver a variable, clinician-set FO2 
    • Includes venturi masks and aerosol masks
    • Devices use a jet of pressurized O2 to entrain sufficient room air (venturi effect) to deliver the selected FO2 to the patient
      • The lower the selected FO2, the more air is entrained, and the higher the total flow delivered to the patient
      • Venturi masks deliver an FO2 between 0.28 and 0.50
      • Aerosol masks usually deliver an FO2 between 0.35 and 1.0

8

Oxygen therapy

  • Regardless of the device used, the fractional concentration of inspired oxygen (FIO2), i.e. the FO2 of the gas reaching the patient’s lungs, depends on...
    • The FO2 being delivered to the patient
    • The flow rate of the delivered gas
    • The patient’s spontaneous inspiratory flow rate
  • The more the patient’s inspiratory flow rate exceeds the delivered flow,...
    • The more room air will enter the lungs
    • The lower the actual FIO2 will be
  • For example, if 100% O2 is being delivered at 12 LPM to a patient with an inspiratory flow of 100 LPM,...
    • 88 LPM of room air must enter the patient’s lungs, thereby significantly lowering the FIO2

9

Oxygen therapy

  • Although FIO2 is an important determinant of arterial PO2, the effectiveness of supplemental oxygen also depends on...
  • The relationship between FIO2 and the resulting PaO2 varies with...
  • Even with severe V/Q imbalance, PaO2...
  • Shunt causes...
  • Based on Figures 1 and 2 and the knowledge that most patients have a combination of V/Q inequality, diffusion impairment, and shunt, it should be evident that...

  • Although FIO2 is an important determinant of arterial PO2, the effectiveness of supplemental oxygen also depends on...
    • The type and severity of the gas exchange abnormality
  • The relationship between FIO2 and the resulting PaO2 varies with...
    • The severity of the V/Q inequality
  • Even with severe V/Q imbalance, PaO2...
    • Increases appropriately when the FIO2 is high
    • This is because even very poorly ventilated alveoli fill with oxygen
  • Shunt, on the other hand, causes...
    • A more linear relationship between FIO2 and PaO2
    • A significant shunt causes refractory hypoxemia because a large proportion of alveoli are totally unventilated (rather than just poorly ventilated)
  • Based on Figures 1 and 2 and the knowledge that most patients have a combination of V/Q inequality, diffusion impairment, and shunt, it should be evident that...
    • It is impossible to accurately predict how much the PaO2 will change with an increase (or decrease) in FIO2

10

Oxygen therapy

  • PAO2 and PA-aO2 vs. FIO2

  • PAO2 and PA-aO2 vs. FIO2
    • Since PAO2 must increase linearly with FIO2 (remember the alveolar gas equation), it should also be evident that the PA-aO2 will change with FIO2
  • Figure 3 illustrates this relationship in the presence of varying V/Q inequality

11

Mechanical ventilation

  • The main indications for mechanical ventilation
  • "Invasive" ventilation
  • “Non-invasive” ventilation 

  • The main indications for mechanical ventilation
    • Significant respiratory acidosis resulting from an acute increase in PaCO2
    • Impending ventilation failure
    • Arterial hypoxemia that is refractory to supplemental oxygen
  • "Invasive" ventilation
    • Mechanical ventilation is most commonly provided following endotracheal intubation
  • “Non-invasive” ventilation
    • May be achieved by connecting the patient to a ventilator using a tight-fitting nasal or oral-nasal mask

12

Modes of mechanical ventilation:
Assist-control ventilatoin (A/C)

  • General
  • Volume

  • Most commonly used mode
  • Volume set mode
    • A physician-selected tidal volume is delivered during each mechanical breath
    • Provides a physician-set, mandatory number of mechanical breaths each minute
      • The patient is guaranteed to receive a minute ventilation equal to the product of the set tidal volume and the set respiratory rate
      • Because of this feature, A/C is used to support critically ill patients with acute respiratory failure
    • Although A/C provides a guaranteed respiratory rate, the total rate is determined by the patient
      • This is because the ventilator provides a mechanical breath every time the patient makes an inspiratory effort
  • Pressure-variable mode
    • Airway pressure (Paw) progressively increases, and a maximum or peak pressure is reached at the end of inspiration
    • At each point in time, the pressure generated by the ventilator is used to overcome both the viscous forces and the elastic recoil of the respiratory system
    • For the same set tidal volume, therefore, airway pressure may vary dramatically between patients

13

Mechanical ventilation and oxygenation failure:
Pressure support ventilation (PSV)

  • General
  • Pressure
  • Volume

  • A second, commonly used mode of mechanical ventilation
  • Pressure
    • On PSV, the physician selects a “pressure support level”
      • This is the pressure (in cmH2O) that will be maintained in the ventilator circuit and the lungs throughout inspiration
      • PSV, therefore, is a pressure-set mode of ventilation
      • Airway pressure is kept at a set, constant level rather than increasing throughout inspiration as it does in A/C
  • Volume
    • If pressure is set, volume cannot be, and PSV is a volume-variable mode

14

Mechanical ventilation and oxygenation failure:
Pressure support ventilation (PSV):
Three factors that determine the tidal volume delivered during a pressure support breath

  • The selected level of pressure
  • The compliance of the respiratory system
  • The amount of inspiratory effort exerted by the patient
    • Most important factor
    • Flow must initially be high to rapidly increase airway pressure to the selected level
    • Once the set pressure has been reached, the ventilator provides sufficient flow to maintain it

15

Mechanical ventilation and oxygenation failure:
Pressure support ventilation (PSV)

  • If the patient makes little inspiratory effort,...
  • If the patient actively inspires,...

  • If the patient makes little inspiratory effort,...
    • Flow will fall exponentially
    • Inspiratory time will be short
    • Tidal volume will be relatively small
  • If the patient actively inspires, however,...
    • The ventilator, in order to maintain a constant airway pressure, must increase both flow and inspiratory time to match the demands of the patient
    • This causes tidal volume to increase

16

Mechanical ventilation and oxygenation failure:
Pressure support ventilation (PSV)

  • Advantages of PSV over A/C
  • Disadvantages of PSV
  • Because of these features, PSV is rarely used in patients with...

  • Advantages of PSV over A/C
    • PSV allows the patient to control both tidal volume and the rate of inspiratory flow
    • This is why most patients find PSV much more comfortable than A/C, which gives the patient no control over volume or flow rate
  • Disadvantages of PSV
    • Since tidal volume is, in part, effort-dependent, it may vary greatly from minute to minute in critically ill patients
    • During PSV there are no set or mandatory breaths
      • Every breath must be initiated by patient effort
      • If the patient does not trigger the ventilator, no breaths will be given
  • Because of these features, PSV is rarely used in patients with...
    • Acute respiratory failure
    • Instead it is usually used to determine whether a patient is able to breathe without assistance from the ventilator

17

Mechanical ventilation and oxygenation failure

  • Intubation and mechanical ventilation
  • Closed systems
  • Open systems
  • With these devices, room air...
  • Mechanical ventilators can reliably deliver...

  • Intubation and mechanical ventilation
    • Can also be used to treat patients with oxygenation failure
  • Closed systems
    • The ventilator-patient system is closed to the outside atmosphere
    • This means that all of the gas inspired by the patient has the FO2 set on the ventilator
  • Open systems
    • This is in contrast to open systems such as a nasal cannula and aerosol mask
  • With these devices, room air...
    • Is entrained to a variable degree, so the FIO2 is always less than the FO2 delivered to the patient
  • Mechanical ventilators can reliably deliver...
    • Any FO2 between 0.21 and 1.0

18

Positive end-expiratory pressure (PEEP)

  • What happens during inspiration in patients receiving mechanical ventilation
  • Positive end-expiratory pressure (PEEP)
  • Physiologic effects
  • These two changes are responsible for...

  • What happens during inspiration in patients receiving mechanical ventilation
    • Airway pressure falls to zero (atmospheric pressure) once inspiration ends
    • In these examples, airway pressure is positive only during inspiration
  • Positive end-expiratory pressure (PEEP)
    • The mechanical ventilator can also be set to provide positive pressure during expiration
  • Physiologic effects
    • Increases the volume remaining in the lungs at end-expiration
      • FRC
        • The point at which the elastic recoil of the lungs and chest wall are balanced and that airway and alveolar pressure at FRC are normally zero
      • Positive alveolar pressure at the end of expiration
        • Must increase the volume of gas in the lungs
    • The increase in alveolar pressure causes pleural pressure to increase
  • These two changes are responsible for...
    • Both the good and bad effects of PEEP

19

Positive end-expiratory pressure (PEEP)

  • In patients with extensive airspace (alveolar) filling (e.g. ARDS), the increase in FRC...
  • High levels of PEEP may cause...
  • PEEP vs. cardiac output
  • Blood flow to the right atrium is determined by...
  • By increasing pleural pressure, PEEP...
  • Remember that the amount of oxygen delivered to the tissues is dependent both on...

  • In patients with extensive airspace (alveolar) filling (e.g. ARDS), the increase in FRC...
    • Prevents the collapse of alveoli that were opened during positive pressure inflation
    • This reduces the volume of blood shunted through unventilated alveoli and improves PaO2
  • On the other hand, high levels of PEEP may cause...
    • Over-distention and rupture of alveoli resulting in extra-alveolar air (commonly referred to as “barotrauma”) or worsening alveolar edema
  • PEEP vs.cardiac output
    • PEEP may also decrease cardiac output
  • Blood flow to the right atrium is determined by...
    • The gradient between systemic venous pressure and right atrial pressure.
  • By increasing pleural pressure, PEEP...
    • Increases the pressure in the right atrium, thereby reducing the gradient for venous return
    • This may, in turn, decrease the output of the right (and left) ventricles
  • Remember that the amount of oxygen delivered to the tissues is dependent both on...
    • The saturation of hemoglobin (SaO2)
    • The cardiac output
    • PEEP may improve PaO2 and SaO2 but will cause tissue oxygen delivery to fall if it causes a significant drop in cardiac output

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