Advanced Blood Gas Analysis

Robert L. Joyner, Jr., PhD, RRT

Associate Professor of Health Sciences

Director, Respiratory Therapy Program

Arterial Blood Gases Analysis

When caring for critically ill patients it is essential to be able to evaluate changes in blood gas values and determine how they relate to:

Alveolar ventilation

Dead space, shunt

Tissue oxygenation, metabolism

Ventilation/perfusion imbalance

Methods for Evaluating Arterial Blood Gas Data

Evaluate alveolar ventilation (V_A) and its relation to changes in the alveolar oxygen (P_AO₂) and alveolar carbon dioxide (P_ACO₂) tension

Evaluate the relation of alveolar ventilation to arterial carbon dioxide tension (PaCO₂) and carbon dioxide clearance (V_CO2)

Assess changes in PaCO₂ affecting arterial oxygen tension (PaO₂)

Examine pH changes associated with changes in PaCO₂ and bicarbonate (HCO₃^-)

Widely accepted normals for the clinical setting:

pH           7.35 – 7.45

PaCO₂     35 – 45 mmHg

PaO₂        80 – 100 mmHg

HCO₃^-      22 – 26 mEq/L

BE           ±2 mEq/L

SaO₂        96 – 100%

Levels of hypoxemia will be defined as:

PaO₂ 60 – 79 mmHg is mild hypoxemia.

PaO₂ 40 – 59 mmHg is moderate hypoxemia.

PaO₂ < 40 mmHg is severe hypoxemia.

Important Point #1

Physical Correlation is a Must!!

Blood gas data must always be examined in regard with what effort it is taking the patient is exerting to sustain that blood gas.

Example #1

In comparing two patients’ (Patient A & Patient B) blood gas values you find they both have a PaCO2 of 40 mm Hg.

Upon physical exam you find patient A has a minute ventilation of 5 L/m and patient B has a minute ventilation of 40 L/m.

Do these patients have similar lung disease?

What additional information does the physical exam provide that the blood gas values in isolation did not?

Blood gas values must always be examined In the light of what it takes to get that patient to that blood gas.

Example #2

In comparing two patients’ (Patient A & Patient B) blood gas values you find they both have a PaO2 of 100 mm Hg.

Upon physical exam you find patient A if on 5 L/m oxygen by nasal cannula and patient B is on 100% O₂ by high flow mask.

Do these patients have similar lung disease?

What additional information does the physical exam provide that the blood gas values in isolation did not?

Prediction Is Key

When using mechanical ventilation to support a life, the care giver must be able to predict the consequences of a change in ventilator settings.

If you are guessing you are dangerous!!

Changes in P_AO₂ and P_ACO₂ Associated With Changes in Alveolar Ventilation

Normal alveolar ventilation is 4 – 5 L/m.

Alveolar Ventilation, PaCO₂, and VCO₂

How much CO₂ is being produced versus how well it is being removed by the lungs is described by:
- CO₂ production must be in milliliters per minute and alveolar ventilation must be in liters per minute.

Alveolar Ventilation, PaCO₂, and VCO₂

Example Calculation

What is the PaCO₂ when the VCO₂ is 475 mL/min and V_A is 4.5 L/min?

Changes in PaCO₂ Affecting PaO₂

Alveolar Air Equation
As PaCO₂ rises the P_AO₂ will fall and vice versa.
In the absence of shunt, theoretically P_AO₂ = PaO₂, and rising PaCO₂ with cause a reduction in PaO₂.
Rule of Thumb: as the PaCO₂ increases by 1 mmHg, the PaO₂ will decrease by 1.25 mmHg.

Changes in PaCO₂ Affecting PaO₂

Example Calculation

Assuming the barometric pressure (P_b) is 760 mmHg, fractional inspired oxygen (F_IO₂) is 0.21, and P_H2O is 47 mmHg, what is the P_AO₂ if:

PaCO₂ is 20 mmHg
PaCO₂ is 40 mmHg

Changes in PaCO₂ Affecting PaO₂

Under normal physiological conditions (because of normal venous admixture), the pressure of alveolar oxygen is always greater the pressure of arterial oxygen.
This is defined as the alveolar – arterial oxygen difference or P(A-a)O₂.
Normal P(A-a)O₂ is 5 mmHg at age 20, breathing room air, and increases each decade after 20 by 4 mmHg.

Changes in pH, PaCO₂, and Bicarbonate

As the arterial partial pressure of CO₂ increases, the level of acid available in the blood also increases.
The relationship between pH, PaCO₂ and bicarbonate can be described with the Henderson-Hasselbalch equation:

Changes in pH, PaCO₂, and Bicarbonate

¨ The Henderson-Hasselbalch equation can be rearranged to express the concentration of [H⁺] or bicarbonate.

Relationship between pH and hydrogen ion (H⁺) concentration

Changes in pH, PaCO₂, and Bicarbonate

¨ Calculation Example

– What is the bicarbonate when pH is 7.40 and PaCO₂ is 60 mmHg?

Determining Acute Vs. Chronic Hypercapnia

¨ The ratio between the change in hydrogen ion concentration and the change in PaCO₂ can be used to determine acute, chronic, or acute-on-chronic acid-base disturbances.

– In acute hypercapnia, (D[H⁺]/DPaCO₂) will be about 0.7.

– In chronic hypercapnia, the ratio will be about 0.3 or less.

– In acute-on-chronic hypercapnia, the ratio is between 0.3 and 0.7.

Determining Acute Vs. Chronic Hypercapnia

¨ In the following example determine if the acute acidosis, chronic acidosis, or acute-on-chronic acidosis.

– pH = 7.25 PaCO₂ = 92 mmHg HCO₃^- = 38 mEq/L.

• Acute-on chronic acidosis.

Changes in pH Caused by Changes in PaCO₂

¨ When starting at a PaCO₂ of 40 mmHg, for every 20 mmHg increase in PaCO₂, the pH decreases by 0.10 units.

Changes in Plasma Bicarbonate Caused by Changes in PaCO₂

¨ As CO₂ is added or removed from the blood it raises and lowers the bicarbonate level, respectively.

¨ For each 10 mmHg increase in PaCO₂, the bicarbonate increases 1 mEq/L.

¨ For each 10 mmHg decrease in PaCO₂, the bicarbonate decreases about 1.5 mEq/L.

Changes in Plasma Bicarbonate Caused by Changes in PaCO₂

¨ If hypoventilation or hyperventilation persists for 24 – 48 hours, normal kidney function will help correct the pH and compensate by retaining or excreting bicarbonate.

Changes in Plasma Bicarbonate Caused by Changes in PaCO₂

¨ As hypoventilation produces a respiratory acidosis, the kidneys compensate by retaining about 2 mEq/L of HCO₃^- for every 10 mmHg increase in PaCO₂.

¨ During hyperventilation the plasma becomes more alkalotic and is compensated in the kidneys by the excretion of bicarbonate, causing a 3 mEq/L decrease for every 10 mmHg decrease in PaCO₂.

Metabolic Changes in Bicarbonate and pH

¨ Changes in pH that are due to metabolic and not respiratory changes can be estimated by:

– a pH change of 0.15 will be approximately equal to a change in base of 10 mEq/L.

End

Advanced Blood Gas Analysis

Robert L. Joyner, Jr., PhD, RRT

Associate Professor of Health Sciences

Director, Respiratory Therapy Program

Arterial Blood Gases Analysis

When caring for critically ill patients it is essential to be able to evaluate changes in blood gas values and determine how they relate to:

Alveolar ventilation

Dead space, shunt

Tissue oxygenation, metabolism

Ventilation/perfusion imbalance

Methods for Evaluating Arterial Blood Gas Data

Evaluate alveolar ventilation (VA) and its relation to changes in the alveolar oxygen (PAO2) and alveolar carbon dioxide (PACO2) tension

Evaluate the relation of alveolar ventilation to arterial carbon dioxide tension (PaCO2) and carbon dioxide clearance (VCO2)

Assess changes in PaCO2 affecting arterial oxygen tension (PaO2)

Examine pH changes associated with changes in PaCO2 and bicarbonate (HCO3-)

Widely accepted normals for the clinical setting:

pH 7.35 – 7.45

PaCO2 35 – 45 mmHg

PaO2 80 – 100 mmHg

HCO3- 22 – 26 mEq/L

BE ±2 mEq/L

SaO2 96 – 100%

Levels of hypoxemia will be defined as:

PaO2 60 – 79 mmHg is mild hypoxemia.

PaO2 40 – 59 mmHg is moderate hypoxemia.

PaO2 < 40 mmHg is severe hypoxemia.

Important Point #1 Physical Correlation is a Must!!

Blood gas data must always be examined in regard with what effort it is taking the patient is exerting to sustain that blood gas.

Example #1

In comparing two patients’ (Patient A & Patient B) blood gas values you find they both have a PaCO2 of 40 mm Hg.

Upon physical exam you find patient A has a minute ventilation of 5 L/m and patient B has a minute ventilation of 40 L/m.

Do these patients have similar lung disease?

What additional information does the physical exam provide that the blood gas values in isolation did not?

Blood gas values must always be examined In the light of what it takes to get that patient to that blood gas.

Example #2

In comparing two patients’ (Patient A & Patient B) blood gas values you find they both have a PaO2 of 100 mm Hg.

Upon physical exam you find patient A if on 5 L/m oxygen by nasal cannula and patient B is on 100% O2 by high flow mask.

Do these patients have similar lung disease?

What additional information does the physical exam provide that the blood gas values in isolation did not?

Prediction Is Key

When using mechanical ventilation to support a life, the care giver must be able to predict the consequences of a change in ventilator settings.

If you are guessing you are dangerous!!

Changes in PAO2 and PACO2 Associated With Changes in Alveolar Ventilation

Normal alveolar ventilation is 4 – 5 L/m.

Alveolar Ventilation, PaCO2, and VCO2

How much CO2 is being produced versus how well it is being removed by the lungs is described by:

CO2 production must be in milliliters per minute and alveolar ventilation must be in liters per minute.

Alveolar Ventilation, PaCO2, and VCO2

Example Calculation

What is the PaCO2 when the VCO2 is 475 mL/min and VA is 4.5 L/min?

Changes in PaCO2 Affecting PaO2

Alveolar Air Equation

As PaCO2 rises the PAO2 will fall and vice versa.

In the absence of shunt, theoretically PAO2 = PaO2, and rising PaCO2 with cause a reduction in PaO2.

Rule of Thumb: as the PaCO2 increases by 1 mmHg, the PaO2 will decrease by 1.25 mmHg.

Changes in PaCO2 Affecting PaO2

Example Calculation

Assuming the barometric pressure (Pb) is 760 mmHg, fractional inspired oxygen (FIO2) is 0.21, and PH2O is 47 mmHg, what is the PAO2 if:

PaCO2 is 20 mmHg

PaCO2 is 40 mmHg

Changes in PaCO2 Affecting PaO2

Under normal physiological conditions (because of normal venous admixture), the pressure of alveolar oxygen is always greater the pressure of arterial oxygen.

This is defined as the alveolar – arterial oxygen difference or P(A-a)O2.

Normal P(A-a)O2 is 5 mmHg at age 20, breathing room air, and increases each decade after 20 by 4 mmHg.

Changes in pH, PaCO2, and Bicarbonate

As the arterial partial pressure of CO2 increases, the level of acid available in the blood also increases.

The relationship between pH, PaCO2 and bicarbonate can be described with the Henderson-Hasselbalch equation:

Changes in pH, PaCO2, and Bicarbonate

¨ The Henderson-Hasselbalch equation can be rearranged to express the concentration of [H+] or bicarbonate.

Relationship between pH and hydrogen ion (H+) concentration

Changes in pH, PaCO2, and Bicarbonate

¨ Calculation Example

– What is the bicarbonate when pH is 7.40 and PaCO2 is 60 mmHg?

Determining Acute Vs. Chronic Hypercapnia

¨ The ratio between the change in hydrogen ion concentration and the change in PaCO2 can be used to determine acute, chronic, or acute-on-chronic acid-base disturbances.

– In acute hypercapnia, (D[H+]/DPaCO2) will be about 0.7.

– In chronic hypercapnia, the ratio will be about 0.3 or less.

– In acute-on-chronic hypercapnia, the ratio is between 0.3 and 0.7.

Determining Acute Vs. Chronic Hypercapnia

¨ In the following example determine if the acute acidosis, chronic acidosis, or acute-on-chronic acidosis.

Evaluate alveolar ventilation (V_A) and its relation to changes in the alveolar oxygen (P_AO₂) and alveolar carbon dioxide (P_ACO₂) tension

Evaluate the relation of alveolar ventilation to arterial carbon dioxide tension (PaCO₂) and carbon dioxide clearance (V_CO2)

Assess changes in PaCO₂ affecting arterial oxygen tension (PaO₂)

Examine pH changes associated with changes in PaCO₂ and bicarbonate (HCO₃^-)

PaCO₂ 35 – 45 mmHg

PaO₂ 80 – 100 mmHg

HCO₃^- 22 – 26 mEq/L

SaO₂ 96 – 100%

PaO₂ 60 – 79 mmHg is mild hypoxemia.

PaO₂ 40 – 59 mmHg is moderate hypoxemia.

PaO₂ < 40 mmHg is severe hypoxemia.

Important Point #1

Physical Correlation is a Must!!

Upon physical exam you find patient A if on 5 L/m oxygen by nasal cannula and patient B is on 100% O₂ by high flow mask.

Changes in P_AO₂ and P_ACO₂ Associated With Changes in Alveolar Ventilation

Alveolar Ventilation, PaCO₂, and VCO₂

How much CO₂ is being produced versus how well it is being removed by the lungs is described by:

CO₂ production must be in milliliters per minute and alveolar ventilation must be in liters per minute.

Alveolar Ventilation, PaCO₂, and VCO₂

What is the PaCO₂ when the VCO₂ is 475 mL/min and V_A is 4.5 L/min?

Changes in PaCO₂ Affecting PaO₂

As PaCO₂ rises the P_AO₂ will fall and vice versa.

In the absence of shunt, theoretically P_AO₂ = PaO₂, and rising PaCO₂ with cause a reduction in PaO₂.

Rule of Thumb: as the PaCO₂ increases by 1 mmHg, the PaO₂ will decrease by 1.25 mmHg.

Changes in PaCO₂ Affecting PaO₂

Assuming the barometric pressure (P_b) is 760 mmHg, fractional inspired oxygen (F_IO₂) is 0.21, and P_H2O is 47 mmHg, what is the P_AO₂ if:

PaCO₂ is 20 mmHg

PaCO₂ is 40 mmHg

Changes in PaCO₂ Affecting PaO₂

This is defined as the alveolar – arterial oxygen difference or P(A-a)O₂.

Normal P(A-a)O₂ is 5 mmHg at age 20, breathing room air, and increases each decade after 20 by 4 mmHg.

Changes in pH, PaCO₂, and Bicarbonate

As the arterial partial pressure of CO₂ increases, the level of acid available in the blood also increases.

The relationship between pH, PaCO₂ and bicarbonate can be described with the Henderson-Hasselbalch equation:

Changes in pH, PaCO₂, and Bicarbonate

¨ The Henderson-Hasselbalch equation can be rearranged to express the concentration of [H⁺] or bicarbonate.

Relationship between pH and hydrogen ion (H⁺) concentration

Changes in pH, PaCO₂, and Bicarbonate

– What is the bicarbonate when pH is 7.40 and PaCO₂ is 60 mmHg?

¨ The ratio between the change in hydrogen ion concentration and the change in PaCO₂ can be used to determine acute, chronic, or acute-on-chronic acid-base disturbances.

– In acute hypercapnia, (D[H⁺]/DPaCO₂) will be about 0.7.

– pH = 7.25 PaCO₂ = 92 mmHg HCO₃^- = 38 mEq/L.

Changes in pH Caused by Changes in PaCO₂

¨ When starting at a PaCO₂ of 40 mmHg, for every 20 mmHg increase in PaCO₂, the pH decreases by 0.10 units.

Changes in Plasma Bicarbonate Caused by Changes in PaCO₂

¨ As CO₂ is added or removed from the blood it raises and lowers the bicarbonate level, respectively.

¨ For each 10 mmHg increase in PaCO₂, the bicarbonate increases 1 mEq/L.

¨ For each 10 mmHg decrease in PaCO₂, the bicarbonate decreases about 1.5 mEq/L.

Changes in Plasma Bicarbonate Caused by Changes in PaCO₂

Changes in Plasma Bicarbonate Caused by Changes in PaCO₂

¨ As hypoventilation produces a respiratory acidosis, the kidneys compensate by retaining about 2 mEq/L of HCO₃^- for every 10 mmHg increase in PaCO₂.

¨ During hyperventilation the plasma becomes more alkalotic and is compensated in the kidneys by the excretion of bicarbonate, causing a 3 mEq/L decrease for every 10 mmHg decrease in PaCO₂.