Arterial Blood Gases

what are the normal values for PaO2?

80-100 mmHg

what is the A-a gradient and its significance?

-PAO2-PaO2

-normal value increases with age and is about (age/4)+4

-a high value suggests insufficient O2 movement from alveolar space to arterial blood

what is the normal range for SaO2?

>95%

what are the normal values for CaO2?

18-22 mL O2/dL blood

what are the main influences on PaO2, SaO2, and CaO2?

-PaO2: PAO2 and lung architecture

-SaO2: PaO2 and Hgb dissociation curve

-CaO2: PaO2, SaO2, and Hgb

what is the main difference between hypoxemia and hypoxia?

-hypoxemia: low O2 in blood

-hypoxia: low O2 in tissue

what values of V/Q lead to shunt or dead space?

-V/Q=0 leads to shunt

-V/Q=∞ leads to dead space

what 2 things are used to clarify likely causes for hypoxemia?

-status of A-a gradient

-ability of supplemental O2 to correct the disorder

what are the 3 main disorders that are the result of a normal A-a gradient and a good response to O2?

-hypoventilation

-reduced inspired O2

-reduced venous O2

what are the 2 main disorders that are the result of a widened A-a gradient and a good response to O2?

-low V/Q

-diffusion limitation

what is the main disorder that is the result of a widened A-a gradient and a poor response to O2?

right to left shunt

what do PCO2 and HCO3- represent in the acid base balance?

-PCO2: respiratory component

-HCO3-: metabolic and chemical components

describe the compensatory response for metabolic and respiratory disorders

-metabolic: respiratory compensation has an onset of 30 min, takes 12-24 hrs for full effect

-respiratory: metabolic compensation has an onset of 6-12 hrs, takes 3-5 days for full effect

what are the normal values for pH, HCO3-, and PCO2?

-pH: 7.36-7.44

-HCO3-: 22-26 mEq/L

-PCO2: 36-44 mmHg

how do you determine whether a disorder is respiratory or metabolic?

-respiratory: PaCO2/HCO3- are altered in the opposite direction as pH

-metabolic: PaCO2/HCO3- are altered in the same direction as pH

what parameter is used to compensate for a primary metabolic disorder?

PaCO2

how do you determine if there is concurrent respiratory acidosis or alkalosis with a metabolic disorder?

-acidosis: actual PaCO2 > expected

-alkalosis: actual PaCO2 < expected

what parameter is used to compensate for a primary respiratory disorder?

HCO3-

how do you determine if there is concurrent metabolic alkalosis or acidosis with a respiratory disorder?

-alkalosis: actual HCO3- > expected

-acidosis: actual HCO3- < expected

what is the equation for the anion gap related to metabolic acidosis?

[Na+] - [Cl-] - [HCO3-]

what is the normal anion gap and what does it mean if values are out of range?

-normal is 8-12 mEq/L

-less than or equal to: non anion gap metabolic acidosis

-greater: anion gap metabolic acidosis

what value is used for high anion gap metabolic acidosis?

∆ anion gap / ∆ HCO3-, which has a normal value of 1

what does it mean if values are different than the normal ∆ anion gap / ∆ HCO3-?

-less than 1: concurrent non anion gap metabolic acidosis

-greater than 1: concurrent metabolic alkalosis

what are the main causes of respiratory acidosis?

inability to eliminate enough CO2 due to pulmonary, muscular, and/or neurological dysfunction

how are severe cases of metabolic acidosis or respiratory acidosis treated?

alkali therapy

what are 8 main causes of high anion gap metabolic acidosis?

-Methanol

-Uremia

-DKA

-Propylene glycol

-Iron/isoniazid

-Lactate

-Ethylene glycol

-Salicylates

what are 3 main causes of non anion gap metabolic acidosis?

-diarrhea

-normal saline

-renal insufficiency

what are 5 compounds used in alkali therapy?

-sodium bicarbonate

-sodium acetate

-sodium citrate

-sodium lactate

-THAM

what is the main cause of respiratory alkalosis and how is it treated?

stimulation of hyperventilation; consider rebreathing in symptomatic cases

what are the 3 general causes of metabolic alkalosis?

-loss of Cl-

-loss of K+

-mineralocorticoid excess

what are the 2 main methods of treating metabolic alkalosis?

-replace Cl- and K+

-consider hydrochloric acid in severe cases