L4 - Objectives

Obj 1: pH

The negative logarithm of hydrogen concentration

pH = -log(H+)

Obj 1: Acidemia

pH < 7.35

Obk 1: Alkelemia

pH > 7.45

Obj 1: PaCO₂

Partial pressure of carbon dioxide in arterial blood.

Obj 1: PaO₂

The measure of oxygen concentration in the blood

Obj 1: SaO₂

The oxygen saturation of hemoglobin (O₂ sat)

Obj 1: BE or base excess

Refers to an increase in base in blood gas (mEq/L)

Noted by a positive number

Obj 1: BD or base deficit

Refers to a decrease in base in blood gas (mEq/L)

Noted by a negative number

Obj 1: Acid

Donates H+ and decreases pH

Obj 1: Base

Accepts H+ and increases pH

Obj 1: Buffer

A system composed of a weak acid and its salt, or a weak base and its salt, that resists changes in pH.

Obj 2: What are the four primary acid-base disorders?

Respiratory acidosis

Respiratory alkalosis

Metabolic acidosis

Metabolic alkalosis

Obj 2: What characteristics of blood gas indicate respiratory acidosis?

An increase in PaCO₂ and a decrease in pH

Obj 2: What is the compensation method for respiratory acidosis?

An increase in HCO3 in an attempt to increase base and pH

Obj 2: What characteristics of blood gas indicate metabolic acidosis?

A decrease in HCO3 and a decrease in pH

Obj 2: What is the compensation method for metabolic acidosis?

A decrease in PaCO₂ in an attempt to decrease acid and increase pH

Obj 2: What characteristics of blood gas indicate respiratory alkalosis?

A decrease in PaCO₂ and an increase in pH

Obj 2: What is the compensation method for respiratory alkalosis?

A decrease in HCO3 in an attempt to decrease base and pH

Obj 2: What characteristics of blood gas indicate metabolic alkalosis?

An increase in HCO3 and an increase in pH

Obj 2: What is the compensation method for metabolic alkalosis?

An increase in PaCO₂ in an attempt to increase acid and decrease pH

Obj 3: What is the anion gap?

A calculation used to estimate the unmeasured ions in the blood, mainly to help evaluate metabolic acidosis

Obj 3: Give the anion gap calculation

[Na+] - [Cl-] - [CO2]

Do NOT use corrected sodium or HCO3!

Obj 3: Give the normal and elevated values for the anion gap

Normal AG:

Obj 3: High anion gap metabolic acidosis

Indicates excess acids in the blood. The H+ of the acid consumes bicarbonate and lowers bicarbonate's value, increasing the gap.

(Not due to chloride increasing)

Obj 3: Normal anion gap metabolic acidosis

Bicarbonate is lost because chloride rose, keeping AG normal. There is no extra acid, just more chloride to offset the loss of bicarbonate

Obj 4: What is the normal value of pH in the blood?

7.35-7.45

Obj 4: What is the normal value of PaCO₂ in the blood?

40 mmHg

Obj 4: What is the normal value of HCO3 in the blood?

24 mEq/L

Obj 5: How can you determine if PaCO₂ is compensating for metabolic acidosis/alkalosis?

Calculate the value of PaCO₂ using the correct formula. If measured PaCO₂ equals the measured value, it is compensating; if it is normal (40 mmHg), it is not compensating

Obj 5: What is the equation to determine if PaCO₂ is compensating in metabolic acidosis?

40 - [(1.3) x (24-Pt's bicarb on ABG)]

If PaCO₂ equals this value, it is compensating

If PaCO₂ is 40, it is not compensating

Obj 5: What is the equation to determine if PaCO₂ is compensating in metabolic alkalosis?

40 + [(0.6) x (Pt's bicarb on ABG - 24)]

If PaCO₂ equals this value, it is compensating

If PaCO₂ is 40, it is not compensating

Obj 6: What are some etiologies of high anion gap metabolic acidosis?

- Methanol ingestion

- Urea

- Diabetic ketoacidosis

- Paraldehyde

- Isoniazid/ischemia

- Lactic acid

- Ethylene glycol ingestion

- Salicylates/starvation

EXCESS ACIDS in the blood bind to bicarbonate

Obj 6: What medications can cause lactic acidosis in HAGMA?

Metformin, linezolid, propofol, pentobarbital, lorazepam, phenobarbital, etc.

Obj 6: What are some etiologies of normal anion gap metabolic acidosis?

GI: Diarrhea (loses bicarb)

Medications: Ca2+ chloride or rapid correction with 0.9% NaCl (increase in chloride decreased bicarb)

Renal tubular acidosis

Obj 6: What is saline-responsive alkalosis?

A metabolic alkalosis caused by chloride depletion, which improves with IV saline (NaCl) replacement.

Obj 6: What are common causes of saline-responsive metabolic alkalosis?

- Vomiting (loss of chloride and H+)

- Loop/thiazide diuretics

- Mild to moderate K+ deficiency

- Excessive sodium bicarbonate

- Correction of chronic hypercapnia

Obj 6: What is saline-resistant metabolic alkalosis?

A metabolic alkalosis that does not correct with IV saline, usually due to renal chloride wasting or excess mineralocorticoid activity.

Obj 6: What are common causes of saline-resistant metabolic alkalosis?

Mineralocorticoid excess: primary hyperaldosteronism, Cushing's syndrome (K+ loss)

Severe Mg2+ deficiency (K+ loss)

Milk-alkali syndrome

(Less K+ can lead to K+ shifting out of cells and H+ shifting in)

Obj 6: What general mechanisms lead to respiratory acidosis?

Inhibition of the respiratory center, impaired perfusion or gas exchange, or intrinsic lung/muscle/nerve disease.

Obj 6: What are the acute central causes of respiratory acidosis?

Medications (opioids, sedatives) - decrease breathing

Stroke or head injury - medullary chemoreceptors can't respond to PaCO2 concentrations/pH

Obj 6: What are acute airway/pulmonary causes of respiratory acidosis?

Asthma

COPD

Obj 6: What are the acute neuromuscular causes of respiratory acidosis?

Brainstem or spinal cord injury

Paralyzed diaphragm

Obj 6: What are other acute causes of respiratory acidosis?

Total parenteral nutrition (TPN) can contribute in certain contexts.

Obj 6: What are chronic neuromuscular causes of respiratory acidosis?

Tumors affecting motor pathways

Multiple sclerosis

Chronic diaphragm paralysis

Obj 6: What are chronic pulmonary causes of respiratory acidosis?

COPD

Interstitial lung disease

Obj 6: What are the general mechanisms leading to respiratory alkalosis?

Hyperventilation due to central stimulation, hypoxemia, peripheral chemoreceptor stimulation, or pulmonary abnormalities.

Obj 6: What are acute central causes of respiratory alkalosis?

Anxiety and pain.

Obj 6: What are acute causes of respiratory alkalosis due to hypoxemia?

High altitude, severe anemia.

Obj 6: What are causes of respiratory alkalosis due to peripheral stimulation of respiration?

Pulmonary embolism, asthma (rarely).

Obj 6: What chronic pulmonary abnormalities can cause respiratory alkalosis?

Asthma, pulmonary fibrosis, and congestive heart failure.

Obj 7: When should you not use acute treatment of metabolic acidosis?

If HCO3 is 12-20 mEq/L and pH > 7.2

Obj 7: What are some treatments used for mild-moderate metabolic acidosis?

Remove offending substance/underlying disorder

If they have renal failure, they may need solutions with bicarbonate when they can't make it themselves (Bicitra, Polycitra, etc).

Obj 7: What are some treatments used for severe metabolic acidosis (< 8 mEq/L HCO3 and pH < 7.2)?

Sodium bicarbonate or sodium acetate

(Acetate in bicarb shortage as it converts to bicarb in the liver)

Obj 7: What is the goal of treatment in severe metabolic acidosis?

HCO3 of 10 mEq/L

Obj 7: What equation is used to determine the amount of sodium bicarbonate is needed in severe metabolic acidosis?

Dose in mEq = (0.5L/kg) x (wt in kg) x (10 mEq/L - Pts HCO3)

Obj 7: What is the first-line treatment for saline-responsive metabolic alkalosis?

Administer IV isotonic fluid bolus

Obj 7: What is the second-line agent for saline-responsive metabolic alkalosis?

Acetazolamide (Diamox).

Obj 7: What is a third-line agent for saline-responsive metabolic alkalosis?

Arginine hydrochloride.

Obj 7: What is the first step in managing saline-resistant metabolic alkalosis in steroid users?

Switch to a steroid with lower mineralocorticoid activity

Obj 7: What medication can be used to treat saline-resistant metabolic alkalosis due to mineralocorticoid excess?

Spironolactone, an aldosterone receptor antagonist.

Blocks aldosterone → retains potassium, helping correct alkalosis.

Obj 7: How should potassium be managed in saline-resistant metabolic alkalosis?

Administer IV potassium bolus and/or add potassium to IV fluids to support alkalosis resolution. (H+ shifts out of cells into the blood when excess potassium)

Obj 8: What are four common IV fluids that can be administered to a patient?

Plasmalyte A, 0.9% NaCl, LR, and D5W

Obj 8: What is the sodium concentration and pH of 0.9% NaCl?

Sodium: 154 mEq/L

pH: 5.5

Obj 8: Why can 0.9% NaCl (normal saline) cause metabolic acidosis?

It has equal Na⁺ and Cl⁻ (154 mEq/L each). The excess chloride lowers plasma bicarbonate, causing hyperchloremic metabolic acidosis with a normal anion gap.

Obj 8: Does 0.9% NaCl have any buffering components?

Nope, it contains only sodium and chloride.

Obj 8: What is the pH of 0.9% NaCl in the bag? Does this directly cause acidosis?

5.5; no, the bag pH doesn't cause acidosis — the problem is chloride load and no buffering components to offset its addition.

Obj 8: What is the sodium concentration and pH of LR?

Sodium: 130 mEq/L

pH: 6.5

Obj 8: Why is Lactated Ringer's (LR) less likely to cause metabolic acidosis than saline?

LR has less chloride (109 mEq/L) and contains lactate (28 mEq/L), which is metabolized to bicarbonate, offsetting chloride's acidifying effect.

Obj 8: What is the pH of LR in the bag?

6.5 (slightly acidic), but once metabolized, it has an alkalinizing effect.

Obj 8: What is the sodium concentration and pH of Plasmalyte A?

Sodium: 140 mEq/L

pH: 7.4

Obj 8: Why is Plasma-Lyte A considered "balanced"?

Its chloride (98 mEq/L) matches plasma, and it contains acetate (27) and gluconate (23), which are metabolized to bicarbonate → maintain physiologic acid-base balance

Obj 8: What is the unifying reason LR and Plasma-Lyte don't cause acidosis like saline?

They both provide buffering anions (lactate, acetate, gluconate) that are converted to bicarbonate, offsetting chloride's acidifying effect.

Obj 8: What is the sodium concentration and pH of D5W?

Sodium: 0 mEq/L

pH: 5

Obj 8: What is D5W (5% dextrose in water) made of?

Glucose (50 g/L) dissolved in sterile water; no sodium, chloride, or buffering ions.

Obj 8: What is the pH of D5W in the bag?

5 (acidic), after metabolism, it can indirectly cause dilutional metabolic acidosis by diluting plasma bicarbonate. It lacks chloride load but also lacks buffers.