113. Acid Base Flashcards
How is a normal pH in the body maintained?
kidneys regulation of plasma bicarb (hco3) and lungs regulation of pp of arterial co
What is the Henderson Hasselbach equation?
pH = pK +log10 [hco3]/[0.03xpaco2]
What does the HH equation describe? Relationship…
between hco3 and paco2 to ph
Acidemia and alkalemia describe what? vs acidosis and alkalosis
summary acid-base state or pH of flood (ie blood pH <7.35 vs pH >7.45)
acidosis and alkalosis are discrete conditions (ie acidosis is an A-B disturbance that incr H and lowers pH)
What does the “metabolic system” in broad strokes include?
cellular production
renal excretion of acid and bases
What (in general) causes metabolic acid-base disorders? Dysregulation in which 3 components…
abnormal cellular function
altered renal excretion of acid and bases
exogenous gain or loss of acids and gases through GI
How long does a paco2 take to compensate in a metabolic process?
12-24h for metabolic acidosis vs 24-36 in metabolic alkalosis
How long does a hco3 take to compensate in a respiratory process?
compensatory incr hco3 for resp acidosis 2-5d
compensatory decr hco3 after 2-5d
Basic 5 step approach for A-B problems with a VBG/ABG:
1.
Check the pH (N 7.35-7.45)
and hco3 (24) and pco2 (40)
Basic 5 step approach for A-B problems with a VBG/ABG:
2.
Check AG
Na - Cl + Hco3
Wide AG >12 ish
What is an elevated anion gap?
accumulation of unmeasured anions
Basic 5 step approach for A-B problems with a VBG/ABG:
If a metabolic acidosis is present:
3.
Check to see if concomittent resp-acid base disorder or if compensated:
Expected PCO2 = (1.5 x serum HCO3) + 8 ± 2
If PCO2 is lower than predicted by Winter’s formula, what is present?
resp alkalosis ie they are blowing off more acid than is required to combat the acidosis from the metabolic cause and therefore, are losing more acid than would be expected from this process alone
If PCO2 is higher than expected in Winter’s formula for a compensatory resp in metabolic acidosis, what does this mean?
concomittent resp acidosis
ie they are not blowing off co2 as expected to compensate for rise in acid from metabolic acidosis and therefore have more acid in body than expected
Basic 5 step approach for A-B problems with a VBG/ABG:
If a wide AG metabolic acidosis is present….
4.
check the delta gap
to see if there is additional metabolic acid base disturbances
How to check the delta gap
- know that a hco3 is 24
- note the current bicarb
- calculate the change from normal to current
- for every 1mmol increase in the AG above 15, expected drop hco3 should be one below 24
If the measured bicarb concentration is higher than predicted by the delta gap, this indicates a concomittent metabolic ___
alkalosis (ie bicarb should have dropped more so there is still more base in solution than should be)
If the measured bicarb concentration is lower than predicted by the delta gap, this indicates a concomittent metabolic ___
NAGMA
Basic 5 step approach for A-B problems with a VBG/ABG:
5. if the wide anion gap metabolic acidosis is present, but the cause isn’t clear, what equation should you check?
osmolar gap
How to calculate the osmolar gap?
- find the serum one in the labs
- calculate: 2xNa + glucose + bun
**+ (Ethanol (mmol/L) x 1.25) if present
measured - serum = osmolar
Normal is 10 or less
For every increase in pco2 (resp acidosis): expected hco3 in ACUTE
incr 1
For every increase in pco2 (resp acidosis) *10mmhg above 40: expected hco3 in CHRONIC
incr 4
For every decr in pco2 (resp alkalosis: expected hco3 in ACUTE
decr 2
For every decr in pco2 (resp alkalosis: expected hco3 in CHRONIC
decr 5
When applying bicarb compensation rule to a respiratory/pco2, what is the general rule in terms of how much it needsto be above 40/normal?
for every increase in 10 above or below 40 you get the change of 1/4/2/5 respectively depending on resp acido ac/chronic vs resp alkalosis ac chronic
DDX of metabolic acidosis - AGMA
Methanol
Uremia
Diabetic ketoacidosis/alcoholic ketoacidosis Paraldehyde/Polyethylene glycol/Paracetamol (acetaminophen) Iron
Lactic acidosis
Ethylene glycol
Salicylates
DDX of NAGMA
Hyperalimentation/Hospital-acquired administration of saline
Acid infusion/Addison disease / Carbonic Anhydrase Inhibitors Renal tubular acidosis (RTA)
Diarrhea
Ureterosigmoidostomy (and ileal diversion) Pancreatic drainage/fistula
Metabolic alkalosis- what is important to determine in terms of ___ responsive vs not
chloride!
What is meant by a chloride responsive metabolic alkalosis
ciruclating vol is decreases
RAAS activated
kidneys reabsorb filtered na, hco3 and cl, leading to a decr [] of urine chloride
thus if you gave chloride via solution, would get a correction in metabolic alkalosis
(ur cl <40)
What is meant by a chloride UN-responsive metabolic alkalosis
Not correctable by fluids
typically by mechanism of hyperaldosteronism causing inapproppriate renal excretion of H and Cl
(spot urine cl >40)
Causes of chloride responsive metabolic alkalosis
Nasogastric suction
Vomiting
Chloride-wasting diarrhea
Villous adenoma
Persistent diuretic use
Chlroide unresponsive metabolic alkalosis causes
Primary hyperaldosteronism
Secondary hyperaldosteronism (Bartter syndrome, Gitelman syndrome, con-
gestive heart failure, liver failure, chronic renal failure) Steroids
Cushing disease
Severe hypercalcemia
Severe hypomagnesemia Bicarbonate ingestion
Licorice overdose (glycyrrhizic acid)
What are four categories of respiratory acidosis?
resp disease from lungs and airways
chest wall disease
resp m weakness
decreased resp drive
Causes of resp disease/lung and airways causing resp acidosis
Airway obstruction
Obstructive pulmonary diseases (e.g., chronic obstructive pulmonary disease) Pneumothorax
Pulmonary effusion
Pulmonary edema
Pneumonia
Mechanical ventilation (iatrogenic hypoventilation)
Causes of chest wall disease causing resp acidosis
Chest wall trauma (e.g., flail chest) Obesity hypoventilation syndrome
Causes of resp m weakness disease causing resp acidosis
Myopathies (e.g., muscular dystrophy)
Neuropathies (e.g., Guillain-Barré)
Electrolyte abnormalities (e.g., hypokalemia, hypophosphatemia)
Causes of decreased resp drive disease causing resp acidosis
Brain space-occupying lesion (e.g., intracranial mass, intracranial hemorrhage) Drugs/toxins (e.g., sedative-hypnotics, narcotics)
Hypoventilation causes resp ___
acidosis
Hyperventilation caues resp ___
alkalosis
DDX cateogries of resp alkalosis based on pathophysioologic mechanism: 7
resp
GI
GU
psych
toxic metabolic
fever
sepsis
pain
Name 8 causes of resp alkalosis
Respiratory:
Conditions that cause hypoxemia (e.g., pulmonary embolus) Mechanical ventilation (iatrogenic hyperventilation)
Gastrointestinal:
Hepatic encephalopathy
Neurologic
Brain lesion
Genitourinary:
Pregnancy
Psychiatric:
Anxiety
Toxic-Metabolic:
Drugs (e.g., salicylates, catecholamines, progesterone)
Hyperthyroidism
Infectious:
Fever
Sepsis
Miscellaneous:
Pain
How much sodium is in 1L of saline?
9g/1000ml
what are the mmol/L concentrations of na and cl for NaCl
154
154
What are the mmol/L [] of na, k, cl, ca lactate in LR?
130
4
109
2.7
28
What can you get from too much NCl?
nagma hyperchloremic metabolic acidosis
A patient presents to the emergency department with severe meta- bolic acidosis. In addition to initial stabilization, what should be a priority for the clinician?
a. Infuse sodium bicarbonate to normalize pH to approximately
7.40.
b. Induce a respiratory alkalosis to normalize pH to approximately
7.40.
c. Identify the cause of metabolic acidosis and target therapy at that
cause.
d. Infuse saline with the goal of decreasing the strong ion differ-
ence.
c
- What causes an increase in the anion gap?
a. A decrease in plasma bicarbonate concentration accompanied by an increase in plasma chloride concentration of the same magnitude
b. An increase in the concentration of unmeasured anions in the plasma
c. Saline infusion
d. Hypoventilation
b
A patient in the emergency department has the following labo-
ratory values. Select the term that best characterizes this patient’s acid-base status. pH = 7.36; Paco2 = 36 mm Hg; Pao2 = 135 mm Hg; Na = 142 mmol/L; Cl = 100 mmol/L; HCO3 = 21 mmol/L.
a. Acidemia
b. alkalemia
c. Respiratory alkalosis with metabolic compensation
d. Metabolic acidosis with respiratory compensation
d
. How can calculation of an osmolar gap be used clinically?
a. To rule out toxic alcohol ingestion
b. To assist with understanding potential causes of a wide anion
gap metabolic acidosis
c. To differentiate primary lung pathology from respiratory muscle
weakness in a patient with respiratory acidosis
d. To understand if metabolic compensation has occurred after
respiratory alkalosis
b
A patient presents to the emergency department with short- ness of breath. Plasma bicarbonate concentration is 44 mmol/L. What is a potential explanation for this laboratory value?
a. Administration of a large volume of saline
b. Acute respiratory failure from a spontaneous pneumothorax sustained 2 hours prior to presentation
c. Chronic respiratory failure from chronic obstructive pulmonary disease
d. Diabetic ketoacidosis
c
