Arterial Blood Gases Flashcards
What are Arterial Blood Gases used for?
Used to assess and manage a patient’s respiratory (ventilation) and metabolic (renal) acid-base and electrolyte homeostasis as well as assessing adequacy of oxygenation.
Used to monitor patients on ventilators, monitor critically ill patients, establish preoperative baseline parameters, and regulate electrolyte therapy.
Also used to monitor oxygen flow rates in the hospital and at home
What are the normal findings for the Arterial Blood Gases?
Normal findings: pH: 7.35-7.45 (venous 7.31-7.41) - remember 7.4 PCO2 : 35-45 mm Hg – remember 40 HCO3 : 21-28 mEq/L – remember 24 PO2 : 80-100 mm Hg – remember >= 80 O2 saturation: 95-100% - remember >= 95 Base excess: 0 +/- 2 mEq/L Alveolar to Arterial O2 difference: < 10 mmHg
What is Acidemia?
Low blood pH (<7.38)
What is Alkalemia?
high blood pH (>7.42)
What is acidosis? What are its subtypes?
Any process that, if left unchecked, will lead to acidemia. This can occur through one of two mechanisms:
Respiratory acidosis and
Metabolic acidosis
What is Respiratory acidosis?
PCO2 is high (>44)
What is Metabolic acidosis
HCO3 is low (<22)
What is alkalosis? What are its subtypes?
Any process that, if left unchecked, will lead to alkalemia. This can occur through one of two mechanisms:
Respiratory alkalosis and
Metabolic alkalosis
What is respiratory alkalosis?
PCO2 is low (<36)
What is metabolic alkalosis?
HCO3 is high (>26)
How can you make sure it is an arterial sample, not a venous sample?
Observe how the blood comes back into the syringe as the sample is being drawn.
Pulsatile flow is seen with an arterial sample but not venous.
Arterial samples fill the syringe much faster than venous.
If the patient was not very hypoxemic when the sample was drawn but the PO2 is very low, most likely it is a venous sample
How do you make sure there are no measurement errors?
The bicarbonate value from the blood gas (a calculated value) should closely approximate the bicarbonate from the chemistry panel (a measured value).
This only works if the samples were measured at roughly the same time
What is pH in regards to alkalinity and acidity? Critical values?
pH measures alkalinity (pH > 7.45) and acidity ( pH < 7.35)
In respiratory or metabolic alkalosis the pH is elevated while in respiratory or metabolic acidosis it is decreased
Critical values: < 7.25 or >7.55
What is PCO2? Critical values?
PCO2 measures the partial pressure of CO2 in the blood.
Measure of ventilation – the faster and more deeply a patient breathes, the more CO2 is blown off, and the level drops.
PCO2 is referred to as the respiratory component in acid base determination as this is mainly controlled by the lungs.
Critical values: < 20 or > 60
When is PCO2 elevated?
Elevated in primary respiratory acidosis.
When is PCO2 decreased?
Decreased in primary respiratory alkalosis
How do the lungs compensate for primary metabolic acid-base disturbances?
Lungs also compensate for primary metabolic acid-base disturbances, so PCO2 levels are affected by metabolic disturbances as well.
In metabolic acidosis the lungs compensate by blowing off CO2 to raise pH.
In metabolic alkalosis the lungs compensate by retaining CO2 to lower pH
What is PO2? Critical value?
PO2 is a measure of the tension (pressure) of O2 dissolved in the plasma so is an indirect measure of O2 content of arterial blood.
This pressure determines the force of O2 to diffuse across the pulmonary alveoli membrane.
Used to determine effectiveness of oxygen therapy.
Critical value: <40
When is PO2 decreased?
Patients unable to oxygenate the arterial blood because of O2 diffusion difficulties (pneumonia, shock lung, CHF).
Patients in whom venous blood mixes prematurely with arterial blood (congenital heart disease).
Patients who have underventilated and overperfused pulmonary alveoli (pickwickian syndrome – obese patients who cannot breathe well when in supine position or in patients with significant atelectasis)
What is HCO3?
Most of the CO2 content in the blood is HCO3.
The bicarbonate ion is a measure of the metabolic (renal) component of the acid-base equilibrium and is regulated by the kidney.
It can be measured directly by the bicarbonate value or indirectly by the CO2 content.
When does HCO3 increase and decrease? Critical values?
As HCO3 increases, the pH also increases so they are directly proportional.
HCO3 is increased in metabolic alkalosis and decreased in metabolic acidosis
Critical values: < 15 or > 40
Hoes does the kidney compensate for primary respiratory acid-base abnormalities?
Kidneys also compensate for primary respiratory acid-base abnormalities.
In respiratory acidosis, the kidneys compensate by reabsorbing increased amounts of HCO3 while in respiratory alkalosis the kidneys compensate by excreting HCO3 in increased amounts to lower pH
What is O2 Saturation? Critical Value?
Indication of the percentage of hemoglobin saturated with O2 .
At 92-100% of hemoglobin carrying oxygen, the tissues are adequately provided with oxygen, assuming normal oxygen dissociation.
Critical value: <75%
How do you measure O2 Saturation?
Pulse oximetry is a noninvasive way of determining O2 saturation.
It measures all forms of O2 saturated hemoglobin, including carboxyhemoglobin.
Therefore, in carbon monoxide poisoning, when carboxyhemoglobin is high, oximetry will inaccurately indicate a high O2 saturation.
A clip like sensor is applied to the tip of a finger or the earlobe – the oximeter transmits light from one side and records the amount of light on the other side, determining the O2 saturation.
What is Base excess/deficit? Critical value?
Calculated by the blood gas machine using pH, PCO2 , and hematocrit.
Represents the amount of buffering anions in the blood.
Base excess takes all anions into account when determining acid-base treatment based on the metabolic component.
Critical value: +/- 3 mEq/L
What does a negative Base excess/deficit mean?
indicates metabolic acidosis (ex. lactic acidosis).
What does a positive Base excess/deficit mean?
indicates metabolic alkalosis or compensation to prolonged respiratory acidosis
What is Alveolar (A) to Arterial (a) O2 difference (A-a gradient)? Normal value?
Calculated number that indicates the difference between alveolar O2 and arterial O2 .
Normal < 10 mm Hg
If the gradient is abnormally high, there is either a problem in diffusing oxygen across the alveolar membrane (thickened edematous alveoli) OR unoxygenated blood mixing with oxygenated blood
When do thickened alveolar membranes occur?
pulmonary edema, pulmonary fibrosis, and acute respiratory distress syndrome (ARDS)
When does mixing of unoxygenated blood occur?
congenital cardiac septal defects, arteriovenous shunts, or underventilated alveoli that are still perfused (mucus plug)
What is Respiratory Acidosis? Two types?
PCO2 > 40 mmHg and decreased pH due to inadequate elimination of carbon dioxide by the lungs.
Two types:
Acute respiratory acidosis
Chronic respiratory acidosis
What is Acute Respiratory acidosis?
acute carbon dioxide retention leading to an increased PCO2 , but a minimal change in HCO3
