CHANGES IN ARTERIAL BLOOD GAS VALUES

MILD TO MODERATE CHRONIC BRONCHITIS

CONDITION: Acute Alveolar Hyperventilation

Acute Alveolar Hyperventilation is ventilation in excess of needs and the blood gas values would show the following:

pH > 7.45

PaCO2 < 35

HCO3 slightly low

 

DISORDER

CAUSE

SIGNS 

TREATMENT 

ACUTE RESPIRATORY ALKALOSIS

 

ANXIETY

SHOCK

SEPSIS

HYPOXEMIA

 

PARESTHESIA

HYPERACTIVE REFLEXES

LIGHT HEADEDNESS

DIZZINESS

 

TREAT CAUSE

REBREATHE CO2

OXYGEN

 

 

We can use the formulas given on the previous page to determine if the following blood gas changes are appropriate for acute alveolar hyperventilation / respiratory alkalosis

Blood Gas Example:

pH    7.53

PaCO 28 mm Hg

HCO3   20 mEq/L

RULE: Each 1 mm Hg ↓ in PaCO2 should give 0.01 ↑ in pH

When the PaCO2 < 40 mmHg the expected pH = 7.40 + (40 mm Hg – measured PaCO2)0.01

Expected pH = 7.40 + (40 – 28)0.01

      = 7.40 + .12

         = 7.52

Expected change matches actual. This indicates that the changes in the blood gas would be primarily due to PaCO2 and therefore would be an acute respiratory or ventilatory disturbance.

RULE: Each 5 mm Hg ↓ in PaCO2 should ↓ HCO3 by 1 mEq

Expected HCO3 = 22 – (12/5)

                           = 22 – 2.4

    = 19.6

Verifying that the changes in bicarb are tied to the changes in PaCO2 and not due to renal compensation by elimination of bicarb. 

SEVERE CHRONIC BRONCHITIS

CONDITION: Chronic Ventilatory Failure

pH  IN NORMAL RANGE BUT ON ACID SIDE OF 7.40 (7.35 - 7.39)

PaCO2 > 45

HCO3 significantly increased

Would be identified as a fully compensated respiratory acidosis

DISORDER

CAUSE

SIGNS 

TREATMENT

CHRONIC RESPIRATORY ACIDOSIS

COPD

PROLONGED EXPIRATION

ACCESSORY MUSCLE USE

NONE

 

We can evaluate the following for compensation by looking at the expected pH in relation to the measured PaCO2

Blood Gas Example

pH    7.38

PaCO2  66 mm Hg

HCO3   35 mEq/L

RULE: Each 1 mm Hg in ↑PaCO2 should give 0.006 ↓ in pH

When the PaCO2 is > 40, the expected pH = 7.40 - (measured PaCO2– 40 mm Hg)0.006

Expected pH = 7.40 - (66 – 40)0.006

         = 7.40 - .156

         = 7.24

This indicates that there is compensation as the pH of 7.38 is not as low as expected.

RULE: Each 10 mm Hg ↑ in PaCO2 should ↑ HCO3 by 1 mEq

Expected HCO3 = 24 + (26/10)

                           = 24 + 2.6

  = 26.6

Indicates that the elevated HCO3 is higher than expected and is the result of renal retention.

ACUTE CONDITIONS SUPERIMPOSED ON CHRONIC VENTILATORY FAILURE

CONDITION:Acute Alveolar Hyperventilation on Chronic Ventilatory Failure

pH > 7.45

PaCO2 > 45

HCO3 significantly increased

Can be confused with partially compensated metabolic alkalosis where the elevated PaCO2 would be related due to hypoventilation, not hyperventilation that has actually caused a much higher initial PaCO2 to be reduced.

Assess oxygenated status – usually have significant hypoxemia (not present with metabolic disturbances)

Blood Gas Example:

pH    7.55

PaCO2  62 mm Hg

HCO3   38 mEq/L

PaO2    51 mm Hg

RULE: Each 1 mm Hg in ↑PaCO2 should give 0.006 ↓ in pH

Expected pH = 7.40 - (measured PaCO2– 40 mm Hg)0.006

Expected pH = 7.40 - (62 – 40)0.006

         = 7.40 - 0.132

         = 7.268

Indicates that measured pH not as low as would be expected based on PaCO2

RULE: Each 10 mm Hg ↑ in PaCO2 should ↑ HCO3 by 1 mEq

Expected HCO3 = 24 + (22/10)

                           = 24 + 2.2

    = 26.2

Indicates additional bicarb retention by the kidneys.

Moderate hypoxemia of 51 mm Hg indicates this is a ventilatory problem. Patient's with chronic ventilatory failure will increase respiratory rate in response to decrease in PaO2. The patient has a chronically elevated CO2 that is higher than the 62 mmHg on the blood gas - but has blown off some of it in an effort to improve oxygenation. The renal system being a slow response system is not able to eliminate the HCO3 that had been retained as compensation, causing the patient's blood pH to swing from less than 7.4 to 7.55.

Another way to differentiate between a ventilatory problem and a partially compensated metabolic alkalosis to to calculate the P(A-a)O2. A wide gradient that would be evident in a patient with acute alveolar hyperventilation superimposed on chronic ventilatory failure, is not normally present in hypoventilation associated with a partially compensated metabolic alkalosis.

Also, look at the clinical presentation of the patient: What is their respiratory rate? What is their work of breathing? In acute alveolar hyperventilation superimposed on chronic failure the respiratory rate should be increased and the patient should be working hard to breathe. In partially compensated metabolic alkalosis the respiratory rate should be decreased.

CONDITION: Acute Ventilatory Failure on Chronic Ventilatory Failure

pH < 7.35 but not as low as would be expected

PaCO2 significantly > 45

HCO3 increased

Blood Gas Example

pH    7.23

PaCO2  107 mm Hg

HCO3   41 mEq/L

PaO  38 mm Hg

RULE: Each 1 mm Hg in ↑PaCO2 should give 0.006 ↓ in pH

Expected pH = 7.40 - (measured PaCO2– 40 mm Hg)0.006

Expected pH = 7.40 - (107 – 40)0.006

         = 7.40 - .402

         = 6.998

 Shows that pH change not as severe as it would be if there was no compensation

RULE: Each 10 mm Hg ↑ in PaCO2 should ↑ HCO3 by 1 mEq

Expected HCO3 = 24 + (67/10)

                           = 24 + 6.7

  = 30.7