ICU Oxygenation & Ventilation Monitoring

Objectives - Review oxygenation and ventilation monitoring used in the intensive care unit, including oxygen content calculations, oxygenation indices, dead space, and capnography.

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Oxygenation Monitoring

Core Oxygenation Values

Arterial-Venous Oxygen Content Difference

C(a-v)O2 measures oxygen consumption by the tissues.

Formula:

C(a-v)O2 = CaO2 - CvO2

Interpretation:

• Normal value is 4 to 5 vol%
• Used in the Fick equation to calculate cardiac output or oxygen consumption
• Requires an ABG and a Swan-Ganz catheter
• C(a-v)O2 increases when CvO2 decreases

Arterial Oxygen Content

CaO2 is the best measurement of oxygen delivered to the tissues and the best index of oxygen transport. It estimates oxygen carried by hemoglobin and oxygen dissolved in plasma.

Formula:

CaO2 = (Hb x 1.34 x SaO2) + (PaO2 x 0.003)

Interpretation:

• Normal value is 17 to 20 vol%, or mL/dL
• Most oxygen is carried on hemoglobin
• A small amount is dissolved in plasma

Mixed Venous Oxygen Content

CvO2 is the total amount of oxygen carried in mixed venous blood, measured in mL O2/100 mL blood.

Formula:

CvO2 = (Hb x 1.34 x SvO2) + (PvO2 x 0.003)

Key points:

• Blood is drawn from the pulmonary artery through a balloon-tipped, flow-directed Swan-Ganz catheter
• Normal value is 14 vol%, with a range of 12 to 16 vol%
• CvO2 decreases when cardiac output decreases
• SvO2 also decreases when cardiac output decreases

Alveolar Air Equation

PAO2 calculates the partial pressure of oxygen in the alveoli.

Formula:

PAO2 = (Pb - PH2O) x FiO2 - PaCO2 / 0.8

The normal value varies directly with FiO2 and barometric pressure.

A-a Gradient

The A-a gradient measures the difference between alveolar PO2 and arterial PO2.

Formula:

A-aDO2 = PAO2 - PaO2

Interpretation on 100% oxygen:

The normal value varies directly with the patient's FiO2.

P/F Ratio

The P/F ratio is the ratio of PaO2 to FiO2.

Formula:

P/F ratio = PaO2 / FiO2

Interpretation:

Oxygenation Index

The oxygenation index, or OI, measures the ventilatory support required to provide the observed oxygenation.

Formula:

OI = Paw x FiO2 x 100 / PaO2

Interpretation:

• Normal OI is less than 10
• Increasing OI indicates the patient's condition is worsening
• ECMO is recommended for newborns with OI greater than 40

Shunt Equation

The shunt equation measures the portion of cardiac output that is shunted and does not participate in gas exchange.

Formula:

Qs/Qt = (A-aDO2 x 0.003) / ((A-aDO2 x 0.003) + C(a-v)O2)

Key points:

• Arterial and mixed venous blood gases are drawn and analyzed
• A-aDO2 and C(a-v)O2 are used
• Normal value is 3% to 5%

Murray Lung Score

The Murray Lung Score is a composite score that quantifies lung status across injury features, including gas exchange, radiographic findings, and mechanics.

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Ventilation Monitoring

PaCO2

PaCO2 is the partial pressure of carbon dioxide dissolved in plasma.

The source prompts review of normal PaCO2 values and asks how respiratory therapists can regulate CO2.

Dead Space Types

Anatomical dead space is estimated as 1 mL/lb ideal body weight.

Diseases that may increase dead space include:

• Pulmonary embolism
• Emphysema
• Shock
• Obstruction of pulmonary vessels by masses
• Pulmonary hypertension

Dead Space to Tidal Volume Ratio

VD/VT is the percentage of tidal volume that is dead space. It represents ventilation without perfusion.

Formula:

VD/VT = (PaCO2 - PECO2) x 100 / PaCO2

Interpretation:

• Normal value is 20% to 40%
• Up to 60% may occur in ventilator patients
• PECO2 is the average PCO2 of exhaled air measured by capnograph
• An increase in VD/VT indicates a dead-space-producing disease such as pulmonary embolus

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Capnometry and Capnography

Definitions

Capnometry is a numeric display of CO2 measurements from the airway.

Capnography is a graphic display of CO2 measurements from the airway.

Indications and Contraindications

Indications:

• Verification of artificial airway placement
• Assessment of pulmonary circulation and respiratory status
• Optimization of mechanical ventilation

Contraindications:

• No absolute contraindications

Equipment Types

Colorimetric CO2 ranges:

The source states normal EtCO2 is above 5% and uses the reminder that purple to yellow color change supports tube placement.

Limitations of colorimetric CO2:

• False positive yellow color if contaminated with acidic substances such as gastric acid
• No reading if clogged with secretions or broken
• CO2 may not be detected in low cardiac output states such as shock, cardiac arrest, or CPR with inadequate compressions

Steps for Use

• Verify physician's order
• Assemble equipment and supplies
• Wash hands
• Calibrate monitor according to manufacturer instructions
• Connect monitor to the patient's airway or as close to the airway as possible

PetCO2 is recorded as the highest value. It may be expressed as percent of exhaled gas or as mm Hg. The source lists 5% and 38 mm Hg as normal examples.

Waveform Interpretation

Capnography is used for diagnosis and ventilator troubleshooting.

Analyze:

• Height
• Frequency
• Rhythm
• Baseline
• Shape

Normal capnography has three phases:

• Phase I: end inspiration
• Phase II: beginning expiration
• Phase III: alveolar plateau

It also has two angles:

• Alpha angle
• Beta angle

Increasing CO2 suggests ventilation is worsening. Decreasing CO2 suggests hyperventilation.

ETCO2 During CPR

ETCO2 during CPR helps evaluate chest compression effectiveness. Cardiac compressions transport CO2 to the alveoli, so EtCO2 correlates with cardiac output.

Key points:

• Goal is to keep EtCO2 as high as possible
• Effective compressions should result in EtCO2 of 10 to 20 mm Hg
• Sudden rise in EtCO2 is the earliest indicator of return of spontaneous circulation
• When the heart restarts, cardiac output and perfusion increase, rapidly increasing exhaled CO2

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