3 ABGs Flashcards
Arterial blood gases are frequently used to detect and monitor…
Oxygenation
Ventilation
Acid/base balance
Also quantify levels of carboxyhemoglobin and methemoglobin
For ABGs, blood is drawn from …
Artery, usually the radial artery
Sometimes from brachial or femoral artery
ABG blood is collected with __________, placed on ice, and taken quick to lab
Anticoagulant (heparin)
How quickly can you get results from an ABG?
5-15 minutes
O2 saturation on ABG correlates to…
Pulse Ox
What are the five values you get back with an ABG test?
pH pO2 O2 saturation pCO2 HCO3
The pO2 is used to determine…
How well patient is oxygenating
NOT used in determination of acid-base conditions
The pO2 from an ABG is much more reliable way to check…
Oxygenation
Better than the O2 saturation reading from pulse ox
______ are used to keep blood pH in a narrow range
Buffer systems
Body maintains precise control of hydrogen ions to maintain homeostasis
Acidemia is defined as pH < _____
< 7.35
Alkalemia is defined as pH > _____
> 7.45
Acidosis/alkalosis ≠ _____________.
Acidemia/Alkalemia
The former are acid/base disorders, the laters refers to pH of blood
Acid-base disturbances indicate …
An underlying disease process
Treatment is directed at the management of the underlying disease
You can have up to _____ different acid-base disorders at the same time
3
The body tries to compensate for an acid-base disorder by …
Using respiratory or metabolic processes that attempt to return a patient’s pH to normal (7.4)
__________ compensation for a __________ process is usually rapid
Respiratory compensation for metabolic process
Metabolic compensation can take _________.
Days or even a week
A septic patient will start breaking more rapidly because…
Their body is trying to compensate for metabolic acidosis by breathing off a bunch of CO2
ALWAYS pay attention if a patient suddenly becomes tachypnic
A primary respiratory problem involves ______ whereas a primary metabolic problem involves _______
Respiratory = pCO2 Metabolic = HCO3
pCO2 >45
Acidosis
pCO2 <35
Alkalosis
HCO3 > 26
Alkalosis
HCO3 < 22
Acidosis
What disorder:
pH >7.45
Primary disturbance = decreased pCO2
Compensatory response = decreased HCO3
Respiratory Alkalosis
What disorder:
pH < 7.35
Primary disturbance = Increased pCO2
Compensatory response = Increased HCO3
Respiratory acidosis
What disorder:
pH > 7.45
Primary disturbance = Increased HCO3
Compensatory response = Increased pCO2
Metabolic alkalosis
What disorder:
pH < 7.35
Primary disturbance = Decreased HCO3
Compensatory response = Decreased pCO2
Metabolic acidosis
How to evaluate an acid-base disorder
1) Evaluate pH (decreased, increased, or normal)
2) If decreased pH…
Normal HCO3 —> increased pCO2 = Respiratory Acidosis
Low HCO3 —> Metabolic acidosis —> look at anion gap/chloride
3) If increased pH…
High HCO3 —> metabolic alkalosis
Normal HCO3 —> decreased pCO2 —> respiratory alkalosis
If both pCO2 and HCO3 are HIGH —>
Respiratory acidosis OR metabolic alkalosis
If both pCO2 and HCO3 are LOW —>
Respiratory alkalosis OR metabolic acidosis
If pCO2 and HCO3 are moving in opposite directions…
MIXED DISORDER
What is MUDPILES?
Conditions that cause high anion gap metabolic acidosis
Methanol Uremia DKA Propylene glycol Iron/isoniazid Lactate (lactic acidosis) Ethanol/ethylene glycol Salicylate/starvation
Metabolic acidosis is
low pH and HCO3 <22
Conditions that cause non-anion gap metabolic acidosis
GI bicarb loss
—> Diarrhea, GI fistula, ureterosigmoidostomy
Renal bicarb loss • Early renal failure • Renal tubular acidosis • Carbonic anhydrase inhibitors • Aldosterone inhibitors (spironolactone)
Hyperchloremia due to saline resuscitation
How to treat metabolic acidosis
TREAT UNDERLYING CAUSE
Consider giving sodium bicarb to help in the immediate period until you’ve treated the underlying cause
Allow for normal respiratory compensation - DON’T GIVE MORPHINE
What is metabolic alkalosis?
High pH and HCO3>26
What to check first if you see Metabolic Alkalosis on ABG
Urine Chloride - helps to distinguish the etiology
If metabolic alkalosis and urine chloride < 25…
GI losses like vomiting, NG suction
Diuretics (“contraction alkalosis”)
CF
GIVE FLUIDS TO TREAT
If metabolic alkalosis and urine chloride >25…
Barter’s syndrome Cushing’s Hyperaldosteronism Potassium depletion Citrate toxicity Chronic diuretics Renin secreting tumor
Treat underlying cause, may need potassium
What is Respiratory Acidosis?
Low pH and pCO2 >45
Essentially you aren’t able to ventilate well enough, leading to an accumulation of CO2
Causes of respiratory acidosis
Acute airway obstruction Lung disease (esp COPD) CNS depression (ie too much morphine) Neuromuscular disorder (ie Guillan-Barré) Impaired lung motion Inappropriate mechanical ventilation settings
Treating respiratory acidosis
Treat underlying cause Respiratory support (BiPAP can be helpful)
Important to differentiate acute from chronic - acute may need BiPAP but chronic is typically more stable (ie chronic COPD)
How to tell acute from chronic respiratory acidosis
Look at pH
Chronic will have a normal pH but they are compensatory
What is respiratory alkalosis?
High pH but low pCO2 (<35)
Excessive elimination of CO2 from the lungs (“blowing off too much CO2”)
SSx of respiratory alkalosis
Lightheadedness, palpitations, tachypnea, +/- paresthesias
Possible causes of respiratory alkalosis
HYPERVENTILATION**** (ie anxiety) Compensatory mechanism in sepsis Pain CNS Salicylate overdose Pregnancy High altitude Hypoxemia Hepatic encephalopathy
Steps in determining acid/base disturbance…
Step 1 - look at pH to determine primary abnormality
Step 2 - Look at pCO2 and HCO3. Whichever change supports the direction of the pH change tells you the primary process
pCO2 reflects respiratory, HCO3 reflects metabolic
If pCO2 and HCO3 go in opposite directions, you likely have multiple disorders - always focus on HCO3 first in that case
Step 3 - determine compensation (if pH close to normal, they are compensating well)
Step 4 - calculate anion gap (very high anion gap —> Primary metabolic acidosis regardless of pH or HCO3)
How to calculate Anion gap
[Na] - [Cl+HCO3]
Normal range = 8-12
Elevated > 12
Anytime you have a very high anion gap (AG >20)…
There automatically has to be a primary metabolic acidosis, regardless of the pH or serum bicarbonate concentration
The body does not generate a large anion gap to compensate for a primary disorder
You can have both _______ and ______ at the same time but you can only have one __________.
Metabolic alkalosis and metabolic acidosis but only one respiratory disorder at a time
B/c you can’t breathe fast and slow at the same time lol
A normal pH does not mean…
There is not an acid/base disorder!
Low ______ is usually pathologic
HCO3
Make sure to investigate!
