Acid-base balance

Acid-base balance US Medical PG Question 1: A 27-year-old man with a past medical history of type I diabetes mellitus presents to the emergency department with altered mental status. The patient was noted as becoming more lethargic and confused over the past day, prompting his roommates to bring him in. His temperature is 99.0°F (37.2°C), blood pressure is 107/68 mmHg, pulse is 120/min, respirations are 17/min, and oxygen saturation is 98% on room air. Laboratory values are ordered as seen below. Serum: Na+: 144 mEq/L Cl-: 100 mEq/L K+: 6.3 mEq/L HCO3-: 16 mEq/L BUN: 20 mg/dL Glucose: 599 mg/dL Creatinine: 1.4 mg/dL Ca2+: 10.2 mg/dL Which of the following is the appropriate endpoint of treatment for this patient?

• A. Clinically asymptomatic
• B. Normal anion gap (Correct Answer)
• C. Normal glucose
• D. Vitals stable
• E. Normal potassium

Acid-base balance Explanation: ***Normal anion gap*** - A **normal anion gap** (approximately 8-12 mEq/L) indicates resolution of the **metabolic acidosis** characteristic of diabetic ketoacidosis (DKA). The current anion gap is high (Na - (Cl + HCO3) = 144 - (100 + 16) = 28 mEq/L). - The patient's presentation with **type 1 diabetes** and **altered mental status**, coupled with **hyperglycemia** (599 mg/dL) and **low bicarbonate** (16 mEq/L), is highly suggestive of DKA, for which anion gap normalization is a key treatment endpoint. *Clinically asymptomatic* - While a desirable outcome, resolution of symptoms occurs gradually and is not the primary biochemical endpoint for DKA treatment. - Patients may have residual symptoms even after metabolic derangements have significantly improved. *Normal glucose* - In DKA treatment, glucose is allowed to drop to a level (e.g., <200 mg/dL) but not necessarily to normal range, before initiating **dextrose-containing intravenous fluids** to prevent hypoglycemia while continuing insulin. - **Normal glucose** alone does not guarantee resolution of ketoacidosis, which is the main life-threatening aspect of DKA. *Vitals stable* - **Stable vital signs** indicate hemodynamic stability, which is crucial but does not confirm the resolution of the underlying DKA metabolic derangements. - Vitals can stabilize or worsen independently of acidosis resolution, especially if complications arise. *Normal potassium* - **Potassium levels** are critical to monitor and correct during DKA treatment, as insulin administration drives potassium into cells, potentially causing **hypokalemia**. - While important for patient safety, achieving a normal potassium level is part of supportive care and not the primary endpoint for resolving the ketoacidotic state itself.

Acid-base balance US Medical PG Question 2: A 32-year-old man is brought to the emergency department after he was found unresponsive on the street. Upon admission, he is lethargic and cyanotic with small, symmetrical pinpoint pupils. The following vital signs were registered: blood pressure of 100/60 mm Hg, heart rate of 70/min, respiratory rate of 8/min, and a body temperature of 36.0°C (96.8°F). While being assessed and resuscitated, a sample for arterial blood gas (ABG) analysis was taken, in addition to the following biochemistry tests: Laboratory test Serum Na+ 138 mEq/L Serum Cl- 101 mEq/L Serum K+ 4.0 mEq/L Serum creatinine (SCr) 0.58 mg/dL Which of the following values would you most likely expect to see in this patient’s ABG results?

• A. pH: decreased, HCO3- : increased, Pco2: increased (Correct Answer)
• B. pH: increased, HCO3- : decreased, Pco2: decreased
• C. pH: decreased, HCO3- : decreased, Pco2: decreased
• D. pH: increased, HCO3- : increased, Pco2: increased
• E. pH: normal, HCO3- : increased, Pco2: increased

Acid-base balance Explanation: ***pH: decreased, HCO3- : increased, Pco2: increased*** - The patient's **depressed respiratory rate** (8/min) indicates severe hypoventilation, leading to **CO2 retention** and subsequent **respiratory acidosis** (decreased pH, increased PaCO2). - In **acute respiratory acidosis**, the body initiates immediate buffering, causing a **mild increase in HCO3-** (approximately 1 mEq/L per 10 mmHg rise in PaCO2). Over 3-5 days, renal compensation leads to more significant HCO3- retention, but in this acute presentation, some HCO3- elevation is expected from acute buffering mechanisms. - The **decreased pH** indicates that compensation is incomplete, which is typical in the acute setting. *pH: increased, HCO3- : decreased, Pco2: decreased* - This pattern is characteristic of **respiratory alkalosis** (increased pH, decreased PaCO2) with metabolic compensation (decreased HCO3-), which would occur in hyperventilation, opposite to the patient's presentation. - The patient's **slow respiratory rate** of 8/min directly contradicts the finding of decreased PaCO2. *pH: decreased, HCO3- : decreased, Pco2: decreased* - This suggests a **metabolic acidosis** (decreased pH, decreased HCO3-) with respiratory compensation (decreased PaCO2), typically seen in conditions like DKA or lactic acidosis. - Although the pH is decreased, the patient's severe bradypnea (RR 8/min) indicates increased CO2 retention, not decreased CO2. *pH: increased, HCO3- : increased, Pco2: increased* - This combination of findings is indicative of **metabolic alkalosis** (increased pH, increased HCO3-) with respiratory compensation (increased PaCO2). - This is inconsistent with the patient's pinpoint pupils, cyanosis, and **severe bradypnea**, which are classic signs of opioid overdose causing respiratory depression and acidosis, not alkalosis. *pH: normal, HCO3- : increased, Pco2: increased* - A normal pH despite increased HCO3- and PaCO2 indicates **fully compensated respiratory acidosis**, which requires days of renal compensation to develop. - In this **acute, severe drug overdose** with profound respiratory depression, the body would not have sufficient time to achieve full compensation, thus the pH would remain low.

Acid-base balance US Medical PG Question 3: A 24-year-old woman presents to the emergency department after she was found agitated and screaming for help in the middle of the street. She says she also has dizziness and tingling in the lips and hands. Her past medical history is relevant for general anxiety disorder, managed medically with paroxetine. At admission, her pulse is 125/min, respiratory rate is 25/min, and body temperature is 36.5°C (97.7°F). Physical examination is unremarkable. An arterial blood gas sample is taken. Which of the following results would you most likely expect to see in this patient?

• A. pH: increased, HCO3-: increased, Pco2: increased
• B. pH: decreased, HCO3-: decreased, Pco2: decreased
• C. pH: decreased, HCO3-: increased, Pco2: increased
• D. pH: increased, HCO3-: decreased, Pco2: decreased (Correct Answer)
• E. pH: normal, HCO3-: increased, Pco2: increased

Acid-base balance Explanation: ***pH: increased, HCO3-: decreased, Pco2: decreased*** - The patient's presentation with **agitation**, **dizziness**, **paresthesias** (tingling in lips and hands), and **tachypnea** (respiratory rate 25/min) is highly suggestive of **hyperventilation** due to an anxiety attack. - **Hyperventilation** leads to excessive **CO2 expulsion**, causing a decrease in Pco2, which results in respiratory alkalosis (increased pH) and a compensatory decrease in HCO3-. *pH: increased, HCO3-: increased, Pco2: increased* - An **increased pH** coupled with **increased HCO3-** and **increased Pco2** would suggest a **metabolic alkalosis with respiratory compensation**, which is not consistent with the patient's acute hyperventilation. - While pH is increased, the other values contradict the primary respiratory cause suggested by the symptoms. *pH: decreased, HCO3-: decreased, Pco2: decreased* - This profile describes **metabolic acidosis with respiratory compensation**, which would typically present with **Kussmaul breathing** and other signs of acidosis, not acute hyperventilation and agitation. - Symptoms such as dizziness and tingling are associated with alkalosis, not acidosis. *pH: decreased, HCO3-: increased, Pco2: increased* - This pattern is characteristic of **respiratory acidosis with metabolic compensation**, often seen in conditions like **COPD exacerbation** or **opioid overdose** with hypoventilation. - The patient's rapid breathing and clinical picture are not consistent with respiratory acidosis. *pH: normal, HCO3-: increased, Pco2: increased* - A **normal pH** with **increased HCO3-** and **increased Pco2** would indicate a **compensated metabolic alkalosis**. - Her acute symptoms point to an uncompensated or acutely compensated respiratory disorder, not a compensated metabolic issue.

Acid-base balance US Medical PG Question 4: A 66-year-old man is brought to the emergency department by his daughter because of 3 days of fever, chills, cough, and shortness of breath. The cough is productive of yellow sputum. His symptoms have not improved with rest and guaifenesin. His past medical history is significant for hypertension, for which he takes hydrochlorothiazide. He has a 30-pack-year history of smoking. His temperature is 38.9 C (102.0 F), blood pressure 88/56 mm Hg, and heart rate 105/min. Following resuscitation with normal saline, his blood pressure improves to 110/70 mm Hg. His arterial blood gas is as follows: Blood pH 7.52, PaO2 74 mm Hg, PaCO2 28 mm Hg, and HCO3- 21 mEq/L. Which of the following acid-base disturbances best characterizes this patient's condition?

• A. Metabolic acidosis
• B. Respiratory acidosis
• C. Respiratory alkalosis (Correct Answer)
• D. Normal acid-base status
• E. Metabolic alkalosis

Acid-base balance Explanation: ***Respiratory alkalosis*** - The patient's pH of **7.52** indicates alkalemia. A **PaCO2 of 28 mm Hg** (normal range 35-45 mm Hg) is low, indicating a respiratory component. - The **primary disturbance is respiratory alkalosis** due to hyperventilation from pneumonia/sepsis causing tachypnea and increased CO2 elimination. - The HCO3- of 21 mEq/L (normal range 22-26 mEq/L) is at the lower limit of normal. In acute respiratory alkalosis, bicarbonate remains near normal since **metabolic compensation takes 2-3 days** to develop significantly. - With a 3-day history, minimal renal compensation is expected, consistent with the near-normal bicarbonate. *Metabolic acidosis* - Metabolic acidosis would present with a **low pH** and a **low HCO3-**, which is not seen here. - The patient's pH is **alkaline (7.52)**, not acidic, ruling out this diagnosis. *Respiratory acidosis* - Respiratory acidosis would be characterized by a **low pH** and a **high PaCO2**, indicating hypoventilation. - The patient's **PaCO2 is low (28 mm Hg)** and the **pH is high**, directly contradicting respiratory acidosis. *Normal acid-base status* - A normal acid-base status would have a **pH between 7.35 and 7.45** and PaCO2 between 35-45 mm Hg. - The patient's **pH of 7.52** and **PaCO2 of 28 mm Hg** are both abnormal, specifically indicating alkalemia and hypocapnia. *Metabolic alkalosis* - Metabolic alkalosis would feature a **high pH** and a **high HCO3-** (typically >26 mEq/L), often resulting from conditions like vomiting or diuretic use. - While the patient is on hydrochlorothiazide, his **HCO3- is 21 mEq/L** (low-normal, not elevated), indicating this is not a primary metabolic alkalosis.

Acid-base balance US Medical PG Question 5: A 50-year-old woman presents to the ED 6 hours after ingesting three bottles of baby aspirin. She complains of nausea, vomiting, dizziness, and tinnitus. Her blood pressure is 135/80 mmHg, pulse is 110/min, respirations are 32/min, temperature is 100.1 deg F (37.8 deg C), and oxygen saturation is 99% on room air. Arterial blood gas at room air shows, PCO2 11 mmHg, and PO2 129 mmHg. Blood salicylate level is 55 mg/dL. Management should involve which of the following acid-base principles?

• A. Serum neutralization, urine alkalization
• B. Serum alkalization, urine alkalization (Correct Answer)
• C. Serum neutralization, urine acidification
• D. Serum acidification, urine acidification
• E. Serum acidification, urine alkalization

Acid-base balance Explanation: ***Serum alkalization, urine alkalization*** - Managing **aspirin overdose** involves **aggressive serum alkalization** to promote the shift of salicylic acid from the cells into the bloodstream, where it remains ionized and cannot freely diffuse into the CNS. This also reduces its toxicity by increasing the proportion of the ionized form. - Subsequently, **urine alkalization** with **sodium bicarbonate** is used to trap the ionized salicylate in the renal tubules, preventing reabsorption and enhancing its excretion. *Serum neutralization, urine alkalization* - This option is flawed because the goal is not to "neutralize" the serum pH to a neutral 7.0 but rather to raise it above normal towards an alkaline state (typically pH 7.45-7.55) to enhance salicylate elimination. - While urine alkalization is correct, the idea of serum neutralization is incorrect and could lead to inadequate treatment. *Serum neutralization, urine acidification* - This approach is entirely incorrect for **salicylate toxicity** as **acidifying the urine** would promote the reabsorption of salicylic acid from the renal tubules, worsening toxicity. - Serum neutralization, as mentioned, is not the correct term or goal for managing **aspirin overdose**. *Serum acidification, urine acidification* - This strategy would be **dangerous** in the context of **salicylate overdose** as it would significantly increase the proportion of **non-ionized salicylic acid**, allowing it to more readily cross cell membranes, including the blood-brain barrier, thereby increasing systemic and central nervous system toxicity. - It would also drastically reduce elimination. *Serum acidification, urine alkalization* - **Serum acidification** is contraindicated in **salicylate poisoning** as it drives salicylate into the tissues, exacerbating its toxicity, particularly in the central nervous system. - While urine alkalization is correct for enhancing elimination, combining it with serum acidification would counteract its benefits and worsen patient outcomes.

Acid-base balance Flashcard 1 of 5

Question: Respiratory compensation in response to metabolic acidosis or alkalosis is _____ (timing)

Answer: immediate

Acid-base balance Flashcard 2 of 5

Question: Hyperventilation as a respiratory compensation for metabolic acidosis is known as _____ breathing

Answer: Kussmaul

Acid-base balance Flashcard 3 of 5

Question: Winters formula is used to calculate the predicted respiratory compensation for a simple _____

Answer: metabolic acidosis

Acid-base balance Flashcard 4 of 5

Question: Renal compensation of respiratory alkalosis includes _____ excretion of fixed H+ as titratable acid and NH4+

Answer: decreased

Acid-base balance Flashcard 5 of 5

Question: Patients with diabetic ketoacidosis have _____ bicarbonate reuptake

Answer: increased