NRNP 6566 ff
FINAL EXAM ff
STUDY GUIDE ff
Week 6 and 7 ff ff ff
1. Interpret arterial blood gases (ABG). Differentiate alkalosis/
ff ff ff ff ff ff
acidosis and respiratory / metabolic
ff ff ff ff ff
,2. Identify a ventilation – perfusion mismatch and how to treat it
ff ff ff ff ff ff ff ff ff ff
If there is a mismatch between the alveolar ventilation and the
ff ff ff ff ff ff ff ff ff ff
alveolar blood flow, this will be seen in the V/Q ratio. If the V/Q
ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ratio reduces due to inadequate ventilation, gas exchange within
ff ff ff ff ff ff ff ff ff
the affected alveoli will be impaired. As a result, the capillary partial
ff ff ff ff ff ff ff ff ff ff ff ff
pressure of oxygen (pO2) falls and the partial pressure of carbon
ff ff ff ff ff ff ff ff ff ff ff
dioxide (pCO2) rises.
ff ff ff
To manage this, hypoxic vasoconstriction causes blood to be
ff ff ff ff ff ff ff ff
diverted to better ventilated parts of the lung. However, in most
ff ff ff ff ff ff ff ff ff ff ff
physiological states the hemoglobin in these well-ventilated
ff ff ff ff ff ff ff
alveolar capillaries will already be saturated. This means that red
ff ff ff ff ff ff ff ff ff ff
cells will be unable to bind additional oxygen to increase the pO2.
ff ff ff ff ff ff ff ff ff ff ff ff
As a result, the pO2 level of the blood
ff ff ff ff ff ff ff ff ff
, remains low, which acts as a stimulus to cause hyperventilation,
ff ff ff ff ff ff ff ff ff
resulting in either normal or low CO2 levels.
ff ff ff ff ff ff ff ff
A mismatch in ventilation and perfusion can arise due to either
ff ff ff ff ff ff ff ff ff ff
reduced ventilation of part of the lung or reduced perfusion.
ff ff ff ff ff ff ff ff ff ff
Ventilation/perfusion mismatch — Mechanical ventilation can ff ff ff ff ff
alter two opposing forms of ventilation/perfusion mismatch (V/Q
ff ff ff ff ff ff ff ff
mismatch), dead space (areas that are overventilated relative to
ff ff ff ff ff ff ff ff ff
perfusion; V>Q) and shunt (areas that are underventilated relative
ff ff ff ff ff ff ff ff ff
to perfusion; V<Q). By increasing ventilation (V), the institution of
ff ff ff ff ff ff ff ff ff ff
positive pressure ventilation will worsen dead space but improve
ff ff ff ff ff ff ff ff ff
shunt.
ff
Increased dead space — Dead space reflects the surface area ff ff ff ff ff ff ff ff ff
within the lung that is not involved in gas exchange. It is the sum of
ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
the anatomic plus alveolar dead space. Alveolar dead space (also
ff ff ff ff ff ff ff ff ff ff
known as physiologic dead space) consists of alveoli that are not
ff ff ff ff ff ff ff ff ff ff ff
involved in gas exchange due to insufficient perfusion (ie,
ff ff ff ff ff ff ff ff ff
overventilated relative to perfusion). Positive pressure ventilation
ff ff ff ff ff ff ff
tends to increase alveolar dead space by increasing ventilation in
ff ff ff ff ff ff ff ff ff ff
alveoli that do not have a corresponding increase in perfusion,
ff ff ff ff ff ff ff ff ff ff
thereby worsening V/Q mismatch and hypercapnia.
ff ff ff ff ff ff
Reduced shunt — An intraparenchymal shunt exists where there ff ff ff ff ff ff ff ff
is blood flow through pulmonary parenchyma that is not involved in
ff ff ff ff ff ff ff ff ff ff ff
gas exchange because of insufficient alveolar ventilation. Patients
ff ff ff ff ff ff ff ff
with respiratory failure frequently have increased intraparenchymal
ff ff ff ff ff ff ff
shunting due to areas of focal atelectasis that continue to be
ff ff ff ff ff ff ff ff ff ff ff
perfused (ie, regions that are underventilated relative to perfusion).
ff ff ff ff ff ff ff ff ff
Treating atelectasis with positive pressure ventilation can reduce
ff ff ff ff ff ff ff ff
intraparenchymal shunting by improving alveolar ventilation,
ff ff ff ff ff ff
thereby improving V/Q matching and oxygenation.
ff ff ff ff ff ff
This is particularly true if PEEP is added. (See "Positive end-
ff ff ff ff ff ff ff ff ff ff
expiratory pressure (PEEP)" and "Measures of oxygenation and ff ff ff ff ff ff ff
mechanisms of hypoxemia", section on 'V/Q mismatch'.)
ff ff ff ff ff ff ff
3. Be able to calculate an Aa gradient. Be able to interpret an Aa gradient.
ff ff ff ff ff ff ff ff ff ff ff ff ff
The alveolar to arterial (A-a) oxygen gradient is a common
ff ff ff ff ff ff ff ff ff
measure of oxygenation ("A" denotes alveolar and "a" denotes
ff ff ff ff ff ff ff ff ff
arterial oxygenation). It is the difference between the amount of
ff ff ff ff ff ff ff ff ff ff
the oxygen in the alveoli (ie, the alveolar oxygen tension [PAO2])
ff ff ff ff ff ff ff ff ff ff ff
and the amount of oxygen dissolved in the plasma (PaO2):
ff ff ff ff ff ff ff ff ff ff
A-a oxygen gradient = PAO2 - PaO2
ff ff ff ff ff ff
PaO2 is measured by arterial blood gas, while PAO2 is calculated
ff ff ff ff ff ff ff ff ff ff
using the alveolar gas equation:
ff ff ff ff ff
PAO2 = (FiO2 x [Patm - PH2O]) - (PaCO2 ÷ R)
ff ff ff ff ff ff ff ff ff ff
, where FiO2 is the fraction of inspired oxygen (0.21 at room air),
ff ff ff ff ff ff ff ff ff ff ff
Patm is the atmospheric pressure (760 mmHg at sea level), PH2O
ff ff ff ff ff ff ff ff ff ff ff
is the partial pressure of water (47 mmHg at 37ºC), PaCO2 is the
ff ff ff ff ff ff ff ff ff ff ff ff ff
arterial carbon dioxide tension, and R is the respiratory quotient.
ff ff ff ff ff ff ff ff ff ff
The respiratory quotient is approximately 0.8 at steady state, but
ff ff ff ff ff ff ff ff ff ff
varies according to the relative utilization of carbohydrate, protein,
ff ff ff ff ff ff ff ff ff
and fat.
ff ff
The A-a gradient calculated using this alveolar gas equation may
ff ff ff ff ff ff ff ff ff
deviate from the true gradient by up to 10 mmHg. This reflects the
ff ff ff ff ff ff ff ff ff ff ff ff ff
equation's simplification from the more rigorous full calculation and
ff ff ff ff ff ff ff ff ff
the imprecision of several independent variables (eg, FiO2 and R).
ff ff ff ff ff ff ff ff ff ff
The normal A-a gradient varies with age and can be estimated from
ff ff ff ff ff ff ff ff ff ff ff
the following equation, assuming the patient is breathing room air:
ff ff ff ff ff ff ff ff ff ff
A-a gradient = 2.5 + 0.21 x age in years
ff ff ff ff ff ff ff ff ff
The A-a gradient increases with higher FiO2. When a patient
ff ff ff ff ff ff ff ff ff
receives a high FiO2, both PAO2 and PaO2 increase. However, the
ff ff ff ff ff ff ff ff ff ff ff
PAO2 increases disproportionately, causing the A-a gradient to
ff ff ff ff ff ff ff ff
increase. In one series, the A-a gradient in men breathing air and
ff ff ff ff ff ff ff ff ff ff ff ff
100 percent oxygen varied from 8 to 82 mmHg in patients younger
ff ff ff ff ff ff ff ff ff ff ff ff
than 40 years of age and from 3 to 120 mmHg in patients older
ff ff ff ff ff ff ff ff ff ff ff ff ff ff
than 40 years of age [5].
ff ff ff ff ff ff
Proper determinations of the A-a gradient require exact
ff ff ff ff ff ff ff
measurement of FiO2 such as when patients are breathing room
ff ff ff ff ff ff ff ff ff ff
air or are receiving mechanical ventilation. The FiO2 of patients
ff ff ff ff ff ff ff ff ff ff
receiving supplemental oxygen by nasal cannula or mask can be
ff ff ff ff ff ff ff ff ff ff
estimated and the A-a gradient approximated but large variations
ff ff ff ff ff ff ff ff ff
may exist and the A-a gradient may substantially vary from the
ff ff ff ff ff ff ff ff ff ff ff
predicted, limiting its usefulness. The use of a 100 percent non-
ff ff ff ff ff ff ff ff ff ff ff
rebreathing mask reasonably approximates actual delivery of 100 ff ff ff ff ff ff ff
percent oxygen and can be used to measure shunt.
ff ff ff ff ff ff ff ff ff
Why use the Aa gradient:
ff ff ff ff
The A-a Gradient can help determine the cause of
ff ff ff ff ff ff ff ff
hypoxia; it pinpoints the location of the hypoxia as
ff ff ff ff ff ff ff ff ff
intra- or extra- pulmonary.
ff ff ff ff
When to use the Aa gradient:
ff ff ff ff ff
Patients with unexplained hypoxia. ff ff ff
Patients with hypoxia exceeding the degree of their ff ff ff ff ff ff ff
clinical illness.
ff ff
FINAL EXAM ff
STUDY GUIDE ff
Week 6 and 7 ff ff ff
1. Interpret arterial blood gases (ABG). Differentiate alkalosis/
ff ff ff ff ff ff
acidosis and respiratory / metabolic
ff ff ff ff ff
,2. Identify a ventilation – perfusion mismatch and how to treat it
ff ff ff ff ff ff ff ff ff ff
If there is a mismatch between the alveolar ventilation and the
ff ff ff ff ff ff ff ff ff ff
alveolar blood flow, this will be seen in the V/Q ratio. If the V/Q
ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ratio reduces due to inadequate ventilation, gas exchange within
ff ff ff ff ff ff ff ff ff
the affected alveoli will be impaired. As a result, the capillary partial
ff ff ff ff ff ff ff ff ff ff ff ff
pressure of oxygen (pO2) falls and the partial pressure of carbon
ff ff ff ff ff ff ff ff ff ff ff
dioxide (pCO2) rises.
ff ff ff
To manage this, hypoxic vasoconstriction causes blood to be
ff ff ff ff ff ff ff ff
diverted to better ventilated parts of the lung. However, in most
ff ff ff ff ff ff ff ff ff ff ff
physiological states the hemoglobin in these well-ventilated
ff ff ff ff ff ff ff
alveolar capillaries will already be saturated. This means that red
ff ff ff ff ff ff ff ff ff ff
cells will be unable to bind additional oxygen to increase the pO2.
ff ff ff ff ff ff ff ff ff ff ff ff
As a result, the pO2 level of the blood
ff ff ff ff ff ff ff ff ff
, remains low, which acts as a stimulus to cause hyperventilation,
ff ff ff ff ff ff ff ff ff
resulting in either normal or low CO2 levels.
ff ff ff ff ff ff ff ff
A mismatch in ventilation and perfusion can arise due to either
ff ff ff ff ff ff ff ff ff ff
reduced ventilation of part of the lung or reduced perfusion.
ff ff ff ff ff ff ff ff ff ff
Ventilation/perfusion mismatch — Mechanical ventilation can ff ff ff ff ff
alter two opposing forms of ventilation/perfusion mismatch (V/Q
ff ff ff ff ff ff ff ff
mismatch), dead space (areas that are overventilated relative to
ff ff ff ff ff ff ff ff ff
perfusion; V>Q) and shunt (areas that are underventilated relative
ff ff ff ff ff ff ff ff ff
to perfusion; V<Q). By increasing ventilation (V), the institution of
ff ff ff ff ff ff ff ff ff ff
positive pressure ventilation will worsen dead space but improve
ff ff ff ff ff ff ff ff ff
shunt.
ff
Increased dead space — Dead space reflects the surface area ff ff ff ff ff ff ff ff ff
within the lung that is not involved in gas exchange. It is the sum of
ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
the anatomic plus alveolar dead space. Alveolar dead space (also
ff ff ff ff ff ff ff ff ff ff
known as physiologic dead space) consists of alveoli that are not
ff ff ff ff ff ff ff ff ff ff ff
involved in gas exchange due to insufficient perfusion (ie,
ff ff ff ff ff ff ff ff ff
overventilated relative to perfusion). Positive pressure ventilation
ff ff ff ff ff ff ff
tends to increase alveolar dead space by increasing ventilation in
ff ff ff ff ff ff ff ff ff ff
alveoli that do not have a corresponding increase in perfusion,
ff ff ff ff ff ff ff ff ff ff
thereby worsening V/Q mismatch and hypercapnia.
ff ff ff ff ff ff
Reduced shunt — An intraparenchymal shunt exists where there ff ff ff ff ff ff ff ff
is blood flow through pulmonary parenchyma that is not involved in
ff ff ff ff ff ff ff ff ff ff ff
gas exchange because of insufficient alveolar ventilation. Patients
ff ff ff ff ff ff ff ff
with respiratory failure frequently have increased intraparenchymal
ff ff ff ff ff ff ff
shunting due to areas of focal atelectasis that continue to be
ff ff ff ff ff ff ff ff ff ff ff
perfused (ie, regions that are underventilated relative to perfusion).
ff ff ff ff ff ff ff ff ff
Treating atelectasis with positive pressure ventilation can reduce
ff ff ff ff ff ff ff ff
intraparenchymal shunting by improving alveolar ventilation,
ff ff ff ff ff ff
thereby improving V/Q matching and oxygenation.
ff ff ff ff ff ff
This is particularly true if PEEP is added. (See "Positive end-
ff ff ff ff ff ff ff ff ff ff
expiratory pressure (PEEP)" and "Measures of oxygenation and ff ff ff ff ff ff ff
mechanisms of hypoxemia", section on 'V/Q mismatch'.)
ff ff ff ff ff ff ff
3. Be able to calculate an Aa gradient. Be able to interpret an Aa gradient.
ff ff ff ff ff ff ff ff ff ff ff ff ff
The alveolar to arterial (A-a) oxygen gradient is a common
ff ff ff ff ff ff ff ff ff
measure of oxygenation ("A" denotes alveolar and "a" denotes
ff ff ff ff ff ff ff ff ff
arterial oxygenation). It is the difference between the amount of
ff ff ff ff ff ff ff ff ff ff
the oxygen in the alveoli (ie, the alveolar oxygen tension [PAO2])
ff ff ff ff ff ff ff ff ff ff ff
and the amount of oxygen dissolved in the plasma (PaO2):
ff ff ff ff ff ff ff ff ff ff
A-a oxygen gradient = PAO2 - PaO2
ff ff ff ff ff ff
PaO2 is measured by arterial blood gas, while PAO2 is calculated
ff ff ff ff ff ff ff ff ff ff
using the alveolar gas equation:
ff ff ff ff ff
PAO2 = (FiO2 x [Patm - PH2O]) - (PaCO2 ÷ R)
ff ff ff ff ff ff ff ff ff ff
, where FiO2 is the fraction of inspired oxygen (0.21 at room air),
ff ff ff ff ff ff ff ff ff ff ff
Patm is the atmospheric pressure (760 mmHg at sea level), PH2O
ff ff ff ff ff ff ff ff ff ff ff
is the partial pressure of water (47 mmHg at 37ºC), PaCO2 is the
ff ff ff ff ff ff ff ff ff ff ff ff ff
arterial carbon dioxide tension, and R is the respiratory quotient.
ff ff ff ff ff ff ff ff ff ff
The respiratory quotient is approximately 0.8 at steady state, but
ff ff ff ff ff ff ff ff ff ff
varies according to the relative utilization of carbohydrate, protein,
ff ff ff ff ff ff ff ff ff
and fat.
ff ff
The A-a gradient calculated using this alveolar gas equation may
ff ff ff ff ff ff ff ff ff
deviate from the true gradient by up to 10 mmHg. This reflects the
ff ff ff ff ff ff ff ff ff ff ff ff ff
equation's simplification from the more rigorous full calculation and
ff ff ff ff ff ff ff ff ff
the imprecision of several independent variables (eg, FiO2 and R).
ff ff ff ff ff ff ff ff ff ff
The normal A-a gradient varies with age and can be estimated from
ff ff ff ff ff ff ff ff ff ff ff
the following equation, assuming the patient is breathing room air:
ff ff ff ff ff ff ff ff ff ff
A-a gradient = 2.5 + 0.21 x age in years
ff ff ff ff ff ff ff ff ff
The A-a gradient increases with higher FiO2. When a patient
ff ff ff ff ff ff ff ff ff
receives a high FiO2, both PAO2 and PaO2 increase. However, the
ff ff ff ff ff ff ff ff ff ff ff
PAO2 increases disproportionately, causing the A-a gradient to
ff ff ff ff ff ff ff ff
increase. In one series, the A-a gradient in men breathing air and
ff ff ff ff ff ff ff ff ff ff ff ff
100 percent oxygen varied from 8 to 82 mmHg in patients younger
ff ff ff ff ff ff ff ff ff ff ff ff
than 40 years of age and from 3 to 120 mmHg in patients older
ff ff ff ff ff ff ff ff ff ff ff ff ff ff
than 40 years of age [5].
ff ff ff ff ff ff
Proper determinations of the A-a gradient require exact
ff ff ff ff ff ff ff
measurement of FiO2 such as when patients are breathing room
ff ff ff ff ff ff ff ff ff ff
air or are receiving mechanical ventilation. The FiO2 of patients
ff ff ff ff ff ff ff ff ff ff
receiving supplemental oxygen by nasal cannula or mask can be
ff ff ff ff ff ff ff ff ff ff
estimated and the A-a gradient approximated but large variations
ff ff ff ff ff ff ff ff ff
may exist and the A-a gradient may substantially vary from the
ff ff ff ff ff ff ff ff ff ff ff
predicted, limiting its usefulness. The use of a 100 percent non-
ff ff ff ff ff ff ff ff ff ff ff
rebreathing mask reasonably approximates actual delivery of 100 ff ff ff ff ff ff ff
percent oxygen and can be used to measure shunt.
ff ff ff ff ff ff ff ff ff
Why use the Aa gradient:
ff ff ff ff
The A-a Gradient can help determine the cause of
ff ff ff ff ff ff ff ff
hypoxia; it pinpoints the location of the hypoxia as
ff ff ff ff ff ff ff ff ff
intra- or extra- pulmonary.
ff ff ff ff
When to use the Aa gradient:
ff ff ff ff ff
Patients with unexplained hypoxia. ff ff ff
Patients with hypoxia exceeding the degree of their ff ff ff ff ff ff ff
clinical illness.
ff ff
