Acid-base disorders Flashcards
In the most general terms what causes metabolic acidosis?
Endogenous acid production or loss of bicarbonate.
What is the normal bicarbonate range?
22 - 26mEq/L
What is often seen on physical exam in patients with metabolic acidosis?
Compensatory tachypnea
What is one of the first laboratory signs of acidosis?
- low bicarbonate
- normal homeostasis acidosis is buffered by kidneys via H+ excretion and bicarbonate reabsorption and regeneration
- when this system is overwhelmed (uses up all bicarbonate) acidosis occurs
What other lab/ ABG value directly correlates with bicarbonate?
Base deficit
What blood gas abnormalities are seen in metabolic acidosis?
pH < 7.35
PCO2 < 35
HCO3 < 22/normal
BDE </= -3
What blood gas abnormalities are seen in metabolic alkalosis?
pH > 7.45
PCO2 > 40-45
HCO3 > 26
BDE >/= 3
What blood gas abnormalities are seen in respiratory acidosis?
pH < 7.35
PCO2 > 45
HCO3 > 26
BDE -
What blood gas abnormalities are seen in respiratory alkalosis?
pH > 7.45
PCO2 < 35
HCO3 < 22
BDE -
What are causes of anion gap acidosis?
Methanol
Uremia
DKA
Paraldehyde
Infection
Lactate
Ethanol
Salicylates
Is the ABG or the BMP/CMP a more accurate assessment of plasma bicarbonate?
BMP/CMP
-bicarbonate on the blood gas is a calculated value where it is actually measured in chemistry values
What is the base deficit/excess?
Calculated value of the amount of strong acid or base required to bring 1L blood in vitro at temp 38C and PCO2 40mmHg to pH 7.4
-approximates the severity of acidosis
-only useful in metabolic derangements
What is the anion gap?
The difference in concentration between routinely measured cations (Na+ and K+) and anions (Cl- and HCO3-)
What is the normal anion gap range?
8-12mEq/L
How is lactate produced?
In anaerobic glycolysis through the reduction of pyruvate by lactate dehydrogenase
How is lactate cleared?
Mostly hepatic some renal and other organs
What lactate isomers is tested on routine labs?
L- lactate
Where is D- lactate thought to be produced?
GI tract through metabolism via gut bacteria breakdown of carbohydrates
When should you consider D-lactate acidosis?
In anion gap acidosis in setting of intestinal disease and confusion especially if it happens after a carb load
Between lactate and base deficit which is a better predictor of mortality?
Lactate
-Elevated lactate correlates w/ higher mortality and longer length of stay
-BDE showed no correlation w/ mortality
What are some non-focal ischemia causes of lactic acidosis?
-lung injury/ARDS
-asthma d/t elevated oxygen demand and liver ischemia
-seizures
-pheochromocytoma
-burns
-neuroleptic malignant syndrome
-cardiopulmonary bypass d/t inadequate perfusion causing a shock like state
-use of epinephrine d/t potentiation of tissue malperfusion
-liver mets
-metformin (and other biguanides)
-malnutrition (d/t thiamine and biotin deficiency which are required for pyruvate metabolism, if it isn’t broken down it accumulates and leads to lactate)
What is type A lactic acidosis?
Due to hypoperfusion or hypoxemia
What is type B lactic acidosis?
Lactic acidosis in the absence of hypoperfusion and hypoxemia
From an acid/base status what occurs when GFR < 25mL/min?
Impaired renal acidification, reduced bicarbonate reabsorption, impaired renal homeostatis all leading to metabolic acidosis
What test can be ordered to help determine if ketoacidosis is the cause of anion gap acidosis?
beta-hydroxybuterate
What is the treatment for DKA?
-low-dose insulin infusion to correct hyperglycemia and electrolyte abnormalities (ie. total body potassium depletion)
-continue until gap has improved not just hyperglycemia
-adequate fluid resuscitation as hyperglycemia has an osmotic diuresis effect
What is the treatment for alcoholic or starvation ketoacidosis?
-treat electrolyte derrangements
-glucose in an isotonic solution
-avoid refeeding syndrome
Alcohols can cause an anion gap and what else?
osmolar gap
What are the common causes of non-gap acidosis?
-renal tubular acidosis
-GI losses (diarrhea and proximal fistula)
-iatrogenic (TPN, NS, medications)
What is the general cause of RTA types 1 and 4?
reduced ammonia production
What is the general cause of RTA type 2?
-impaired chloride resorption
-part of Fanconi syndrome
How do you treat RTA?
-treat the underlying cause
-prevent hypercalciuria
-in types 1 and 2: NaHCO3 or citrate and hyperkalemia management
-type 4: furosemide and treatment of adrenal insufficiency, alkalinization is less commonly needed
What is the treatment of non-gap acidosis d/t GI losses?
-these large fluid shifts result in a relative loss of Na compared w/ chloride (gut lumen has high levels of Na and HCO3)
-this discrepancy is exacerbated by NS resuscitation
-control the losses and resuscitate w/ LR to avoid hyperchloremia
What are the ventilation changes seen as a result of respiratory compensation to metabolic acidosis?
-get a respiratory alkalosis
-increased minute ventilation d/t increased tidal volume and tachypnea
-tachypnea can get to dangerous and unsustainable rates
Approximately when is maximal respiratory compensation to metabolic acidosis?
12-24hrs
How do you calculate the expected compensation of metabolic acidosis?
Winter’s formula
PCO2(expected) = (1.5 x HCO3) +8
-if measured PCO2 is higher than this there is a superimposed respiratory acidosis
-if measured PCO2 is lower than this there is a superimposed respiratory alkalosis
What is the theoretical limit of respiratory compensation for metabolic acidosis?
PCO2 15mmHg
What effects does acidemia have on the CV system?
-venodilation
-arterioconstriction
-conduction abnormalities
-decreased inotropy
-splanchnic vasoconstriction
What is the direct effect of acidemia on the pulmonary system?
respiratory depression
-however compensation of metabolic acidosis does lead to an increase in minute ventilation
What electrolyte changes are seen d/t acidemia?
-hyperkalemia
-hypercalcemia
-hyperuricemia
What are some of the causes of metabolic alkalosis?
-GI losses (gastric drainage and emesis)
-villous adenomas that are chloride secreting not bicarb secreting
-laxative abuse
-diuretics (furosemide, chlorothiazide)
-mineralocorticoid excess
-exogenous NaHCO3
-citrate excess from pRBC transfusions
-bone lytic conditions
What cluster of electrolyte abnormalities are seen in patients with large upper GI losses?
hypokalemic, hypochloremic, metabolic alkalosis
How do you treat metabolic alkalosis due to GI losses?
-NS or potassium chloride
-severe forms might need hydrochloric acid
-make sure you also watch and replete potassium, many of these mechanisms lead to potassium losses as well
How do you treat metabolic alkalosis due to mineralocortic excess?
spironolactone
What do blood transfusion w/ pRBC cause metabolic alkalosis?
the citrate is metabolized to bicarbonate
What is the timeframe needed for respiratory compensation of metabolic alkalosis?
it’s almost immediate
What is the equation to predict the expected PCO2 rise due to respiratory compensation of metabolic alkalosis?
PCO2 = 0.9 x (HCO3) + 9
-if PCO2 is lower there is a superimposed respiratory alkalosis
What are soem of the deleterious effects of metabolic alkalosis?
-increased hemoglobin-oxygen affinity
-vasoconstriction (especially cerebral)
-calcium wasting
-hypokalemia
-hypomagnesemia
What is the treatment for metabolic alkalosis?
-volume expansion
-if patient can’t tolerate volume expansion consider chloride loading w/ KCl
-acetazolamide, but careful
-mitigate acid production in upper GI losses w/ H2 blockers or PPIs
What do you have to be careful of if giving acetazolamide for metabolic alkalosis?
-first this is a carbonic anhydrase inhibitor
-causes bicarbonate diuresis
-but it leads to hypokalemia and hypercapnia
What type of base deficit/excess do you seen in respiratory acidosis?
neither
What is the hallmark diagnostic feature of respiratory acidosis?
PCO2 elevation to > 45
A bicarbonate level of what is suggestive of chronic respiratory acidosis?
> 26mEq/L
What is the most common cause of respiratory acidosis through hypoventilation seen in the SICU?
overdosing narcotics, benzodiazepines, sleep aids, axiolytics
What is the hallmark diagnostic feature of respiratory alkalosis?
PCO2 decrease < 22mEq/L
What are some causes of central hyperventilation?
-sepsis
-hepatic failure
-pregnancy
-salicylate poisoning
What is the acid/base disorder seen in ethylene glycol poisoning?
metabolic acidosis
What type of shift on the oxyhemoglobin curve is seen as blood is warmed (what what does this do to the PaO2)?
rightward shift
-increases PaO2
What type of shift on the oxyhemoglobin curve is seen in hypothermia?
leftward shift
-so a hypothermic pt who has their blood sample warmed for the ABG will have a higher PaO2 on the ABG than exists in vivo
How do you calculate the O2 content?
O2 content = 1.34(Hgb)(%sat) + (0.003)(PaO2)
-Hgb g/dL
-PaO2 mmHg
How does the CO2 concentration effect cerebral vessels?
-hypercapnia dilates cerebral vessels
-hypocapnia constricts cerebral vessels
What is the proposed mechanism in which acute increases in CO2 leads to decreased consciousness?
-leads to intraneuronal acidosis
-excessive cerebral blood flow
-rising intracranial pressure
What two types of renal tubular acidosis can cause metabolic acidosis d/t inability to secrete a normal dietary acid load?
-type 1 (distal) RTA
-type 4 RTA (hypoaldosteronism)
What type of renal tubular acidosis can cause metabolic acidosis d/t an increased H+ load or HCO3 loss?
type 2 (proximal) RTA
What are the benefits of permissive hypercapnia?
-reduces tissue metabolixm
-improves surfactant function
-prevents nitration of proteins
What are the intracranial effects of hypocapnia?
-reduces total cerebral blood flow
-raises neuronal pH
-reduces ionized Ca causing disturbances in cortical and peripheral nerve function
To maintain a pH 7.4 what mut the ratio of HCO3 to (0.03 x PaCO2) be?
20:1
What equation can given the predicted PaCO2 of a given HCO3 in metabolic acidosis?
PaCO2 = (1.5 x HCO3) + (8 +/- 2)
-if PaCO2 is more than this value pt has both met acidosis and resp acidosis
-if PaCO2 is less than this value pt has met acidosis and resp alkalosis
What equation can given the predicted PaCO2 of a given HCO3 in metabolic alkalosis?
PaCO2 = (0.7 x HCO3) + (20 +/- 1.5)
-if PaCO2 is more than this value pt has both met alkalosis and resp acidosis
-if PaCO2 is less than this value pt has met alkalosis and resp alkalosis
What are the 4 basic mechanisms that can lead to metabolic acidosis?
-bicarbonate consumption from decreased H+ excretion
-bicarbonate consumption from increased H+ production
-bicarbonate loss
-bicarbonate dilution
What causes an anion-gap acidosis versus a non-anion gap acidosis
-AG is the addition of fixed acids
-nonAG is the loss of bicarb
What is the principal early manifestation of metabolic acidosis?
increased minute ventilation from increased tidal volume
Why is the a loss of vasomotor tone and reduction of myocardial contractility for pH < 7.2?
catecholamine resistance develops
Above what pH can you see ventricular irritability in metabolic alkalosis?
> 7.55
How much should the PaCO2 increase for every 1mMol/L increase in HCO3 in metabolic alkalosis?
PaCO2 should increase by 0.7mmHg
What should be the increase in HCO3 for every 1mmHg PaCO2 increase in acute respiratory acidosis? In chronic?
-acute = 3-4mEq/L
-chronic = 0.3mEq/L
Which respiratory compensation is more common in hypercapnea and metabolic aciosis, increased tidal volume or increased respiratory rate?
increased tidal volume
-Kassmual respiration
-if acidemia is severe can reach 30L/min
-starts in 1-2 hours after onset of acidemia and reaches peak at 12-24hrs
What is the equation to calcuate the expected PaCO2 d/t respiratory compensation in response to metabolic acidosis?
PaCO2 = 1.5 x measured HCO3 +8 (+/-2)
What are potential indications for direct treatment of metabolic acidosis (and not just treating the cause)?
-pH < 7.1
-overt physiologic compromise attributable to acidosis
-excessive work of breathing required to maintain pH > 7.2
How do you approximate the HCO3 deficit for HCO3 dosing?
HCO3 deficit = (0.5 x total body water) x (24 - HCO3)
Reduced amounts of which electrolytes can inhibit the excretion of excess HCO3?
Hypokalemia hypomagnesemia hypochloremia
What is the effect on the adrenal glands due to volume depletion?
Hyperaldosteronism (HCO3 retention K loss)
-aldosterone also promotes Na reabsorption leading to higher rates of Na+ for H+ exchange which worsens alkalosis
What are the chloride responsive causes of metabolic alkalosis?
- volume depletion
- vomiting/diarrhea
- NGT suctioning
- diuretics (loop and thiazide type)
- post-hypercapnia
- medications (PCN)
What are the chloride resistant causes of metabolic alkalosis?
-hyperaldosteronism
-exogenous steroids
-Cushing syndrome
-alkali ingestion
What diuretic is preferred in pts in an edematous state (HF, cirrhosis, nephritis syndrome) to prevent metabolic alkalosis?
Acetazolamide
-dose 125 - 500mg
-carbonic anhydrase inhibitor that increases renal excretion of NaHCO3
What are the causes of respiratory acidosis due to inhibition of the medullary respiratory center?
Acute
- meds (opiates, anesthetics, sedatives)
- oxygen in someone who has chronic hyperopia
- cardiac arrest
- OSA
Chronic
- obesity (Pickwickian syndrome)
- CNS disease
- metabolic alkalosis
What are the causes of respiratory acidosis due to disorders of the chest wall?
Acute
- muscle weakness (MG, Guilain-Barre, hypokalemia, hypophosphatemia)
Chronic
- muscle weakness (SCI, ALS, MS, polio, myxedema)
- kyphoscoliosis
- extreme obesity
What are the causes of respiratory acidosis due to gas exchange disorders?
Acute
- exacerbation of lung disease
- ARDS
- cardiogenic pulmonary edema
-PTX
Chronic
- COPD
- extreme obesity
What are the causes of respiratory acidosis due to upper airway obstruction?
- aspiration
- OSA
- laryngospasm
What are symptoms of acute respiratory acidosis?
- HA
- blurred vision
- restlessness
- anxiety
- tremors
- asterixis
- delirium
- somnolence
- CSF pressure can be elevated causing papilledema
- hypotension if pH<7.1
What are causes of respiratory alkalosis?
- CNS disorders
- agitation
- pain
- inappropriate mechanical ventilation
- hypoxemia
- restrictive diseases
