South College AVL Lab Med: Acid/Base Balance - Lecture 7

pH, PaO2, PaCO2, HCO3 (bicarb), O2 saturation

What are some of the lab values we are looking for when it comes to acid/base balance?

Arterial Blood Gas (ABG)

a test performed on arterial blood to determine levels of oxygen, carbon dioxide, and more

Venous Blood Gas (VBG)

lab test that determines the partial pressure of gases, pH, and more within the venous blood

Gold standard

More difficult access

More painful

arterial puncture, increased risk (more room for error)

hypoxemic patients

patients in shock (hematoma and arterial thrombosis)

What are the key things to know about an ABG?

obtained as nurse obtains IV access (easier to obtain)

decreased risk - no arterial injury

can be used for pH, screening for hypercarbia and lactate trending

What are the key things to know about VBG?

pH, HCO3, and base deficit

which lab values have a correlation between ABG and VBG (in other words, you can obtain these values from VBG)?

PaCO2

which lab value has a correlation, but a larger confidence interval?

PaO2

which lab value has NO CORRELATION?

pH: 7.35-7.45

PaCO2: 35-45 mmHg (lungs)

PaO2: 80-100 mmHg

SaO2: 93-100%

HCO3-: 22-26 mEq/l (kidneys)

What are the normal ABG lab values? (must know these)

pH

when dealing with acid/base disorders, which lab value should you ALWAYS look at first?

<7.35 (<7.4 slightly acidotic), >7.45 (>7.4 slightly alkalotic)

what is considered an acidic pH/ basic pH?

respiratory problems! (acidosis/alkalosis)

which acid/base disorder has an inverse relationship between pH and PaCO2?

inadequate lung functioning, retaining CO2

decreased pH (acidic), increased PaCO2 (inverse relationship), increased HCO3 (hypoventilation - trying to compensate for low pH)

what happens in respiratory acidosis? what are the expected pH, PaCO2, and HCO3 levels?

increased arterial pressure, blowing off too much CO2

increased pH (basic), decreased PaCO2 (inverse relationship), decreased HCO3 (hyperventilation - trying to compensate for high pH)

what happens in respiratory alkalosis? what are the expected pH, PaCO2, and HCO3 levels?

metabolic problems! (acidosis/alkalosis)

which acid/base disorder has a similar relationship between pH and PaCO2?

gain of H+ or loss of HCO3

decrease pH (acidic), decrease PaCO2 (compensation, same relationship), decrease HCO3 (same relationship)

what happens in metabolic acidosis? what are the expected pH, PaCO2, and HCO3 levels?

loss of H+ or gain of HCO3

increase pH (basic), increase PaCO2 (compensation, same relationship), increase HCO3 (same relationship)

what happens in respiratory alkalosis? what are the expected pH, PaCO2, and HCO3 levels?

chemical buffers (seconds - happens all the time - manage minor acid/base balance)

respiratory compensation (minutes - retention/elimination of CO2 - manages mild-moderate acid/base balance)

renal compensation (hours/more permanent - regulate bicarb to combat hydrogen loss/gain - starts when other two mechanisms fail, requires 5 days to manage severe acid/base imbalance)

What are the three compensatory mechanisms we have as part of our physiology?

COPD, asthma, respiratory muscle weakness, CNS depression

what are some common causes of respiratory acidosis?

pain, anxiety, panic attack, pregnancy, high altitudes, drug tox, hyperventilation while on mechanical ventilation

what are some common causes of respiratory alkalosis?

AGMA (Anion Gap Metabolic Acidosis)

lactic acidosis, ketoacidosis, renal insufficiency

NAGMA (Normal/Non-Anion Gap Metabolic Acidosis)

renal tubular acidosis, GI loss of bicarb, GI fistulas

what are some common causes of metabolic acidosis?

Chloride-responsive (urine chloride normal)

vomiting/nasogastric suction, hypovolemia, loop/thiazide diuretic

Chloride-resistant (urine chloride elevated)

hyperaldosteronism, Cushing's syndrome

what are some common causes of metabolic alkalosis?

metabolic alkalosis

What is another way the body compensates for respiratory acidosis?

metabolic acidosis

What is another way the body compensates for respiratory alkalosis?

respiratory alkalosis

What is another way the body compensates for metabolic acidosis?

respiratory acidosis

What is another way the body compensates for metabolic alkalosis?

Anion Gap

Difference between the concentrations of serum cations and anions: determined by measuring the concentrations of sodium cations and chloride and bicarbonate anions (basically everything else on CMP/BMP besides Cl and CO2)

Anion gap = Na+ - (Cl- + HCO3-)

sodium (sometimes potassium)

what cation do we typically care about when obtaining anion gap value?

chloride and CO2/HCO3-

what anions do we typically care about when obtaining anion gap value?

albumin (also contains phosphate, sulfate, and other anions)

what is an anion gap primarily composed of?

YES! (high anion gap metabolic acidosis and anion gap metabolic acidosis mean the same thing)

Do HAGMA and AGMA mean the same thing?

hyperchloremic (Cl- going up)

What is important to note about NAGMA?

MUDPILES

(metanol, uraemia (AKI/CKD), diabetic ketoacidosis, propylene glycol, isoniazid/iron, lactic acidosis, ethanol/ethylene glycol, salicylates (ex: aspirin))

What is the pneumonic to remember the HAGMA/AGMA metabolic acidotic conditions?

HARD-UP

(Hyperalimentation, acetazolamide use, RTA (renal tubular acidosis), diarrhea, ureterosigmoid fistula, pancreatic fistula)

What is the pneumonic to remember the NAGMA metabolic acidotic conditions?