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ABG

Terms in this set (55)

Which is the stronger organ for acid/base balance?
Kidneys are stronger organ than lungs; lungs are more compensatory
What are the normal ranges for an ABG?
pH 7.35-7.45
PaCO2 35-45
HCO3 22-26
Base Excess -2 to +2
PaO2 80-100
SaO2 >95%
What does base excess tell us?
Tells the direction of the bicarb value
What is the first step of ABG interpretation?
Look at the PaO2 level and determine if the level shows hypoxemia. (oxygenation status)
What does PaCO2 tell us in ABG interpretation?
Ventilatory status
What does PaO2 reflect?
2% of oxygen normally dissolved in plasma of the blood.
What does O2 saturation reflect?
98% of oxygen normally bound to hemoglobin
Explain how PaO2 levels greater than 100 occur.
Hemoglobin picks up oxygen, once saturated the oxygen is in the plasma hence causing an increase in PaO2. If PaO2 levels over 100 for too long can lead to oxygen toxicity.
What are the three things that affect oxygenation?
Hemoglobin
Ventilation
Perfusion (specifically cardiac output)
How does age affect oxygenation status?
Take the low normal value of PaO2 and subtract 1 mm Hg for each year over 80 and that is the typical low normal for that age person. Should not drop below 60 mm Hg.
What is a shift to the left in the oxyhemoglobin curve?
Lower affinity for hgb to release oxygen from hemoglobin causes a dangerous shift to the left meaning that oxygen will not be released from Hgb at the tissue level. This leads to hypoxia or worsening oxygenation. Occurs in alkalosis, hypothermia, sepsis, massive transfusions, etc.
What does a shift to the right of the oxyhemoglobin curve indicate?
Hemoglobin releases oxygen more rapidly leading to poorer saturation of hemoglobin but more oxygen released to plasma. Occurs in acidosis, anemia, hyperthyroidism
Which direction is better to shift to in the oxyhemoglobin curve?
Want to shift to the right when attempting to oxygenate - hemoglobin will give more oxygen at the tissue level when shift to the right occurs
How do you calculate the expected PaO2 for patients on oxygen therapy?
FiO2 x 5; i.e. 50% FiO2 x 5 = 250 expected PaO2
What are the percentages of oxygen associated with NC?
1L = 24%
2L = 28%
3L = 32%
Add 4 points to % per L
What are the percentages of oxygen associated with FM?
5-6L = 40%
6-7L = 50%
7-8L = 60%
What are the percentages of NRB (reservior) oxygen therapy?
6L = 60%
7L = 70%
8L = 80%
What is the A-a oxygen gradient? What are normal values? What do the numbers mean?
Difference between PO2 in alveolar air and PO2 in the blood
PAO2 - PaO2 = A - a Oygen gradient
Normal = <20 (increases with age)
Helps to determine the degree of shunting, the greater the number the greater the shunt
How do you estimate the A-a gradient?
Subtract the water vapor pressure from barometric pressure (760 - 47 = 713)
Multiply the resulting pressure by the patient's FiO2
Subtract from 1.25 times the patient's arterial PCO2
Example:
Patient at sea level breathing room air with PO2 of 90 and PCO2 of 40: (760-47) x (.21) - (1.25 x 40) = 10
When would you expect abnormal A-a oxygen gradients?
Right to left Shunts
Diffusion abnormalities - fibrosis, thickening of alveolar walls, alveoli not open
Ventilation - perfusion mismatches - atelectasis, alveoli closed
Explain the a/A Ratio. What is the normal value?
measured arterial PO2 / calculated alveolar PO2
Normal is >0.75
Does not vary with changes in FiO2
Explain the PaO2/FiO2 Ratio. What are the ranges and what do they indicate?
Ratio that shows oxygenation status
Normal = 100/0.21 = 480
Ratio <300 = severe defect in gas exchange
Ratio <200 = ARDS (Defining characteristic or ARDS)
What is the second step in ABG interpretation?
Look at the pH level and determine if the level is on the acid or alkaline side of 7.4.
What is the third step in ABG interpretation?
Look at the PaCO2 level and determine if the PaCO2 level shows respiratory acidosis, alkalosis or normalcy. Below 35 = alkalosis; above 45 acidosis
What is the fourth step in AGB interpretation?
Look at the HCO3 level and determine if the level shows metabolic acidosis, alkalosis, or normalcy.
Explain the acronym ROME.
R = Respiratory
O = Opposite (direction of pH)
M = Metabolic
E = Equal (direction of pH)
As PCO2 increases pH decreases; as HCO3 increases pH increases
Explain the expected changes seen in metabolic acidosis.
For every mmol/L fall in HCO3 from 25, the PCO2 should fall by 1mm Hg from 40
Explain the expected changes seen in metabolic alkalosis.
For every mmol rise in HCO3 from 25, the PCO2 should rise by 0.7 from 40.
What is the equation for Acute Respiratory Acidosis expected changes?
Change in pH = 0.008 x (PCO2 - 40)
What is the equation for Chronic Respiratory Acidosis?
Change in pH = 0.003 x (PCO2 - 40)
What is the equation for Acute Resp Alkalosis?
Change in pH = 0.008 x (40 - PCO2)
What is the equation for chronic respiratory alkalosis?
Change in pH = 0.003 x (40 - PCO2)
Explain how PCO2 changes affect the pH.
10mm PCO2 = 0.08 pH
If you increase ventilation to decrease PCO2 by 20mm you increase pH by 0.16 (0.08 x2) giving a pH of 7.4.
Explain how HCO3 affects the pH.
10 mEq HCO3 = 0.15 pH
Body HCO3 deficit (mEq/1) x Wt (kg)/4 determines the dosage needed of bicarb to increase pH
What is the goal pH when treating acidosis?
Usually want to raise the pH to about 7.2
How do you calculate the anion gap?
Na - (Cl + HCO3) = UA - UC
Define Anion Gap.
The measurement of relative abundance of unmeasured anions (normal = 3-12 mEq/L)
What does an anion gap greater than 12 mEq/L indicate?
metabolic acidosis
What does an anion gap greater than 25 mEq/L always indicate?
metabolic acidosis regardless of whether the HCO3 level is reduced or not.
What occurs to the anion gap in DKA? What if greater than 8?
Anion gap increase roughly equals the HCO3 decrease (1:1 ratio)
If the anion gap differs in change in HCO3 by more than 8mEq/L in a patient with DKA, a mixed acid base disorder is suggested.
What are some causes of increases unmeasured anions?
Endogenous metabolic acidosis
Exogenous anion ingestion
Therapeutic agents
increased plasma proteins
What are tsome causes of decreased unmeasured cations?
hypokalemia
hypocalcemia
hypomagnesemia
Is a decreased anion gap usually of great clinical concern?
Not usually of concern
What occurs to the anion gap when albumin levels decrease?
1 gm drop in albumin leads to a 2.5 drop in normal anion gap.
What are some causes of anion gap metabolic acidosis?
Renal failure (uremic)
Ketoacidosis
Lactic acidosis (shock states)
Toxin ingestion (methanol, paraldehyde, ethylene glycol, salicylates)
Aspirin
How can a normal anion gap occur in metabolic acidosis?
If metabolic acidosis is caused by the loss of bicarb the bicarb loss is counterbalanced with an increase in chloride (1:1), thus a normal anion gap.
How is metabolic alkalemia treated?
Treatment can be guided through measurement of urinary chloride.
If <10 mEq/L = saline responsive = give volume
If >20 mEq/L = saline resistant (not volume contracted) - fluids will not help
What is step 6 in ABG interpretation?
Look at the pH level and determine if the pH shows a compensated or an uncompensated condition.
A MUDPIE
Aspirin
Methyl alcohol
Uremia
Diabetic Ketoacidosis
Paraldehyde
Idiopathic Lactic acidosis
Ethylene
All causes of Anion Gap Acidosis
Calculation for Metabolic acidosis expected changes.
∆ PCO2 = ∆ HCO3 x 1.2
Take change in bicarb answer and subtract from 40 (norm PCO2) to get answer.
Calculation for Metabolic Alkalosis expected changes.
∆ PCO2 = ∆ HCO3 x 0.9
Take change in bicarb answer and add to 40 (PCO2 norm) to get answer.
Calculation for Acute Resp Acidosis expected changes.
∆ HCO3 = ∆ PCO2 x 0.07
Take change in PCO2 answer and add to 24 (bicarb norm) to get answer.
Calculation for chronic respiratory acidosis expected changes.
∆ HCO3 = ∆ PCO2 x 0.4
Take change in PCO2 answer and add to 24 (bicarb norm) to get answer.
Calculation for acute respiratory alkalosis expected changes.
∆ HCO3 = ∆ PCO2 x 0.2
Take change in PCO2 answer and subtract from 24 (norm bicarb) to get answer.
Calculation for chronic respiratory alkalosis expected changes.
∆ HCO3 = ∆ PCO2 x 0.5
Take change in PCO2 answer and subtract from 24 (norm bicarb) to get answer.
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