Transcript Blood Gases: Pathophysiology and Interpretation

Blood Gases:
Pathophysiology and
Interpretation
Tintinalli
Chapter 26
Sept. 1, 2005
Definitions
Ventilation: Is a function of the rate
and depth of breathing and
determines the clearance of carbon
dioxide from the body
 Oxygenation: Diffusion of oxygen
from the lungs to the bloodstream
for subsequent delivery to the
tissues
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Minute Ventilation
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Total amount of new air moved in and out
of the airways and lungs each minute
Equals tidal volume multiplied by the
respiratory rate
Normal tidal volume is 7mL/kg, or 500mL
in adult
Normal rate is 12 breaths/minute
Normal minute ventilation is 6L/min
Dead Space
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Anatomic dead space occurs in the
trachea, bronchi, and bronchioles
Alveolar dead space (high V/Q mismatch)
occurs when ventilation of the alveolarcapillary is normal but perfusion is absent
The combined dead space is physiologic
dead space and is about 30% of the tidal
volume
ARDS and COPD can increase dead space
to 60%
Dead space over 60% typically requires
intubation
Partial Pressures
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Normal atmospheric pressure is 760mm
Hg
Partial pressure of H20 is 47mm Hg and is
subtracted from atmospheric pressure
(760-47=713)
Remaining gases are:
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Nitrogen 79% (563mm Hg)
Oxygen 21% (149mm Hg)
CO2 0.04% (0.3mm Hg)
Alveolar Gas Equation
For each mL of O2 leaving the
alveolus, 0.8 to 1.0 mL of CO2 enters
it
 This is the respiratory quotient (RQ)

A-a Gradient
Determines the degree of lung
function impairment
 The A-a gradient is the partial
pressure of alveolar oxygen minus
the partial pressure of arterial
oxygen (PAO2-PaO2)
 Normal is 2-10mm Hg or 10 plus
one tenth the person’s age
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A-a Gradient
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PAO2=(PB-PH2O)(FIO2)-PaCO2/RQ
PAO2=(760-47)(0.21)-40/0.08
PAO2=100mm Hg at sea level in room air
PaO2 in a normal, healthy adult in room
air at sea level is 90-100mm Hg
So, the PAO2-PaO2 is 100 minus 90, or
about 10mm Hg
A-a Gradient
PAO2-PaO2 of 20-30mm Hg on room
air indicates mild pulmonary
dysfunction, and greater than 50mm
Hg on room air indicates severe
pulmonary dysfunction
 The causes of increased gradient
include intrapulmonary shunt,
intracardiac shunt, and diffusion
abnormalities

PaO2
Factors affecting the PaO2 include
alveolar ventilation, FIO2, altitude,
age, and the oxyhemoglobin
dissociation curve
 Relation between PaO2 and SaO2:
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PaO2
60mm
50mm
40mm
30mm
Hg
Hg
Hg
Hg
corresponds to
SaO2
90%
80%
70%
60%
Alveolar Ventilation
During hyperventilation, the PaCO2
falls and the PaO2 rises
 If the PaCO2 falls by 1mm Hg, the
PaO2 rises by about 1.0-1.2mm Hg
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Factors Affecting
Oxyhemoglobin Dissociation
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pH:
The more acidic the blood, the more
readily hemoglobin gives up oxygen
and the higher the PaO2
 With alkalosis, hemoglobin binds more
tightly to oxygen
 A rise or fall in pH of 0.10 causes a fall
or rise in the PaO2 of about 10%,
respectively
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Factors Affecting
Oxyhemoglobin Dissociation
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Partial pressure of CO2
CO2 entering blood from tissues shifts
the curve to the right
 Oxygen is displaced from the
hemoglobin and delivers oxygen at a
higher PO2 than normal
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Factor Affecting
Oxyhemoglobin Dissociation
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Temperature
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With a rise in blood temperature,
hemoglobin releases oxygen more
readily, which increases the PO2 in the
plasma
Factors Affecting
Oxyhemoglobin Dissociation
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Exercise
With exercise, muscles release large
amounts of CO2 and acids
 Muscle temperature can rise 3-4oC
 Combined, these shift the
oxyhemoglobin dissociation curve to
the right, which releases O2 more
readily
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Factors Affecting
Oxyhemoglobin Dissociation
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2,3-Diphosphoglycerate (2,3-DPG)
With prolonged hypoxia over several
hours, 2,3-DPG quantities increase,
which shifts the dissociation curve to
the right
 If the concentration of 2,3-DPG falls,
such as in banked blood or sepsis, the
curve shifts left and the PaO2 falls
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PaO2/FIO2 Ratio
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To estimate the impairment of
oxygenation, calculate the PaO2/FIO2 ratio
Normally, this ratio is 500-600
Below 300 is acute lung injury*
Below 200 is ARDS*
*Along with diffuse infiltrates, normal
PCWP, and appropriate mechanism
Pulse Oximetry
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Factors that affect the pulse ox
effectiveness
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Impaired local perfusion (hypothermia,
vasopressors)
Ambient light (fluorescent)
Nail polish (particularly blue)
Abnormal hemoglobin
Very high PO2
Carboxyhemoglobin falsely raises readings
Methemoglobin falsely lowers the readings
Questions
1. A SaO2 of 90% corresponds to a PaO2 of:
A. 40%
B. 50%
C. 60%
D. 70%
E. 80%
2. Which of the following affects the pulse oximetry readings?
A. Blue nail polish
B. Ambient fluorescent lighting
C. Hypothermia
D. Carboxyhemoglobin
E. All will adversely affect the pulse ox readings
3. Which is false regarding oxyhemoglobin dissociation?
A. As temperature rises, hemoglobin binds oxygen with more affinity
B. Exercise shifts the dissociation curve to the right, releasing oxygen more readily
C. Hemoglobin binds oxygen with more affinity in an alkylotic state
D. During sepsis, the curve shifts left, causing a decrease in the PaO2
Questions
4. ARDS is defined as bilateral diffuse infiltrates, normal PCWP, appropriate mechanism, and a
PaO2/FIO2 ratio of what?
A. Below 500
B. Below 400
C. Below 300
D. Below 200
E. Below 100
5. True or False
Minute ventilation is the respiratory rate multiplied times the tidal volume
Answers
1.
2.
3.
4.
5.
C
E
A
D
T