Blood Gases: Pathophysiology and Interpretation

TintinalliChapter 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

Minute Ventilation 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 Anatomic dead space occurs in the trachea,

bronchi, and bronchioles Alveolar dead space (high V/Q mismatch)

occurs when ventilation of the alveolar-capillary 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 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: 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

A-a Gradient 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:PaO2 corresponds to SaO2

60mm Hg 90%

50mm Hg 80%

40mm Hg 70%

30mm Hg 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

Factors Affecting Oxyhemoglobin Dissociation 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

Factors Affecting Oxyhemoglobin Dissociation

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

Factor Affecting Oxyhemoglobin Dissociation Temperature

With a rise in blood temperature, hemoglobin releases oxygen more readily, which increases the PO2 in the plasma

Factors Affecting Oxyhemoglobin Dissociation 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

Factors Affecting Oxyhemoglobin Dissociation 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

PaO2/FIO2 Ratio 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 Factors that affect the pulse ox

effectiveness 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

Questions1. 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 polishB. Ambient fluorescent lightingC. HypothermiaD. CarboxyhemoglobinE. All will adversely affect the pulse ox readings

3. Which is false regarding oxyhemoglobin dissociation?A. As temperature rises, hemoglobin binds oxygen with more affinityB. Exercise shifts the dissociation curve to the right, releasing oxygen more readilyC. Hemoglobin binds oxygen with more affinity in an alkylotic stateD. During sepsis, the curve shifts left, causing a decrease in the PaO2

Questions4. ARDS is defined as bilateral diffuse infiltrates, normal PCWP, appropriate mechanism, and a

PaO2/FIO2 ratio of what?A. Below 500B. Below 400C. Below 300D. Below 200E. Below 100

5. True or FalseMinute ventilation is the respiratory rate multiplied times the tidal volume

Answers1. C2. E3. A4. D5. T