16 Respiratory Failure

Question 1

Oxygenation failure

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

Answer 1

• 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

Question 2

Ventilation failure

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

Answer 2

• 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

Question 3

Ventilation failure

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

Answer 3

• 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

Question 4

Ventilation failure

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

Answer 4

• 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)

Question 5

Oxygenation-ventilation failure

• This type of respiratory failure
• Pathophysiology
• Causes

Answer 5

• 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

Question 6

Management of respiratory failure

Answer 6

• 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

Question 7

Oxygen therapy

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

Answer 7

• 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

Question 8

Oxygen therapy

Answer 8

• 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

Question 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…

Answer 9

• 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

Question 10

Oxygen therapy

• PAO2 and PA-aO2 vs. FIO2

Answer 10

• 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

Question 11

Mechanical ventilation

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

Answer 11

• 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

Question 12

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

• General
• Volume

Answer 12

• 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

Question 13

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

• General
• Pressure
• Volume

Answer 13

• 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

Question 14

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

Answer 14

• 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

Question 15

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

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

Answer 15

• 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

Question 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…

Answer 16

• 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

Question 17

Mechanical ventilation and oxygenation failure

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

Answer 17

• 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

Question 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…

Answer 18

• 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

Question 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…

Answer 19

• 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