Acid‐Base, Fluids, and Electrolytes


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Acid‐Base, Fluids, and Electrolytes


Joshua Dilday, DO1, Catherine Cameron, MD2, and Christopher Bell, MD3


1 Department of Acute Care Surgery, University of Southern California, LAC + USC Medical Center, Los Angeles, CA, USA


2 Landstuhl Regional Medical Center, Germany


3 William Beaumont Army Medical Center, El Paso, TX, USA



  1. A 73‐year‐old man with an extensive abdominal surgical history and multiple previous small bowel obstructions presents with several days of colicky abdominal pain. He cannot keep any food or water down and is vomiting multiple times over the past four days. Which is most likely this scenario?

    1. Metabolic acidosis, hypokalemia, hyperchloremia, acidic urine
    2. Respiratory alkalosis, hypokalemia, hyperchloremia, acidic urine
    3. Metabolic alkalosis, hypokalemia, hypochloremia, acidic urine
    4. Metabolic alkalosis, hypokalemia, hyperchloremia, alkaline urine
    5. Metabolic acidosis, hypokalemia, hypochloremia, acidic urine
    6. Respiratory acidosis, hypokalemia, hyperchloremia, alkaline urine

    The above patient is presenting with another small bowel obstruction. If the vomiting from a small bowel obstruction is significant, it can lead to acid/base and electrolyte abnormalities. The main problem is the severe dehydration. Proximal GI losses can cause a contraction metabolic alkalosis. Sodium is the key element as it tries to maintain intravascular volume through retention of water. During volume contraction, bicarbonate and sodium are reabsorbed to maintain volume and electrical balance as there is insufficient chloride. The body does all it can to hold on to the sodium and the distal convoluted tubules will compensate sodium reabsorption by excreting potassium, leading to hypokalemia. This continues until the potassium concentration is below tolerable levels at which point it starts to excrete hydrogen. The reabsorption of bicarbonate causes a loss of chloride and thus the hypochloremia. In this setting, the person is alkalotic but because they are urinating hydrogen ions despite being alkalotic, the kidneys are causing a “paradoxical aciduria”. The chloride is also lost from GI losses. The first and foremost important treatment is the replenishment of water with any crystalloid solution. Answer A is incorrect as it is not metabolic acidosis nor hyperchloremia. B is incorrect because it is not hyperchloremic. Answer D is incorrect because it is not hyperchloremia as explained. Answers E and F are wrong due to previous explanations.


    Answer: C


    Khanna, A., & Kurtzman, N. A. (2001). Metabolic alkalosis. Respiratory Care , 46 (4), 354–365.


    Galla, J. H. (2000). Metabolic alkalosis. Journal of the American Society of Nephrology , 11 (2), 369–375.


    Luke, R. G., & Galla, J. H. (2012). It is chloride depletion alkalosis, not contraction alkalosis. Journal of the American Society of Nephrology , 23 (2), 204–207.


  2. A 45‐year‐old woman is postoperative day 4 after a small bowel resection for adenocarcinoma. On examination, she has mild edema in her lower extremities. Her chest x‐ray shows mild pulmonary congestive and an enlarged cardiac silhouette. She has been receiving D5 1/2 Normal Saline at 150 mL/hr. Her serum sodium level is 131 mEq/L and her urine output is 50–60 ml/hr. She has jugular venous distention and edema. The patient states that she takes diuretics at home on occasion when she develops edema in her legs. How would you treat her serum sodium?

    1. Increase IV fluids to 200 ml/hr
    2. Decrease IV fluids to 75 ml/hr
    3. Treat with thiazide diuretics
    4. Change her IV fluids to 3% hypertonic saline at 150 cc per hour
    5. Treat with loop diuretics, ACE inhibitors, and beta‐blockers

    When addressing the disorder of hyponatremia, it is important to first classify the disorder based on volume status of hypovolemia, euvolemia, and hypervolemia. In general, hyponatremia is treated with fluid restriction in the setting of euvolemia, isotonic saline in the setting of hypovolemia, and diuresis in the setting of hypervolemia. Sometimes, a combination of these therapies may be needed based on the presentation. If the patient is hypovolemic from bowel obstruction, the therapy usually consists of isotonic or hypertonic saline. Euvolemic hyponatremia such as an SIADH is usually treated with fluid restriction and loop diuretics. Hypervolemic hyponatremia such as in patients with heart failure are treated with diuretics, angiotensin converting enzyme (ACE) inhibitors, and beta‐blockers. With the addition of ACE inhibitors, congestive heart failure is treated and vasopressin secretion is reduced. One liter of normal saline will increase plasma sodium concentration by 1 mEq/L. Thus, treating hyponatremia in edematous patients with saline will only exacerbate the problem. The patient in this case is overloaded with fluids and thus choice A is incorrect. Decreasing IV fluids to 75 cc to restrict fluids may be helpful but restricting fluids is a better option and that is why choice B is incorrect. Choice C is incorrect because thiazide diuretics are contraindicated since it blocks reabsorption of sodium and chloride in the distal tubules and thus prevents the generation of maximally dilute urine. 3% hypertonic saline would in this case worsen the salt load and the hypervolemia. In a hypovolemic state, this may be a treatment option. If the patient is in a state of hypovolemia and hyponatremia and is symptomatic, a bolus of 100–150 mL of 3% hypertonic saline may be useful.


    Answer: E


    Adrogue, H. J., & Madias, N. E. (2000). Hyponatremia. New England Journal of Medicine , 342, 1581.


    Braun, M. M., Barstow, C. H., & Pyzocha, N. J. (2015). Diagnosis and management of sodium disorders: hyponatremia and hypernatremia. American Family Physician , 91 (5), 299–307.


    Konstam, M. A., Kiernan, M. S., Bernstein, D., et al. (2018). Evaluation and management of right‐sided heart failure: a scientific statement from the American Heart Association. Circulation , 137(20), e578–622.


  3. A 22‐year‐old man is admitted to the ICU after a motorcycle accident causing an ankle fracture, a mesenteric hematoma, and a splenic laceration. The patient received multiple saline boluses to maintain normal blood pressure. Despite multiple saline boluses, the patient remains hypotensive. On the way to the OR, massive transfusion protocol is initiated but the blood gas shows: pH 7.21, pCO 2 32, pO 2 70mm Hg, HCO 3 16 mEq/L. Which of the following best describes his acid‐base status?

    1. Primary metabolic alkalosis with respiratory compensation.
    2. Primary metabolic acidosis with combined respiratory alkalosis.
    3. Primary metabolic acidosis with respiratory compensation.
    4. Primary metabolic acidosis with combined respiratory acidosis.
    5. Primary respiratory alkalosis with metabolic compensation.

    The patient has a metabolic acidosis likely from lactic acidosis in addition to hyperchloremic acidosis due to multiple saline boluses in an attempt to control his hypotension. Excessive saline infusion can cause a hyperchloremic non‐gap metabolic acidosis. The diagnosis of primary metabolic acidosis is made by the low pH and low plasma HCO3. Once the determination of metabolic acidosis is made, it should be determined if a gap acidosis is present. However, since no electrolytes were given to determine a gap, the next determination should be appropriate respiratory compensation. This can be done utilizing Winters’ formula [pCO2 = (1.5 × HCO3) + 8 ± 2]. The formula predicts the expected pCO2 in a primary metabolic acidosis. If the measured pCO2 is greater than expected, a superimposed respiratory acidosis is present. If the measured pCO2 is less than expected, a combined respiratory alkalosis is present. In the case above, the expected pCO2 = (1.5 × 18) + 8 ± 2 = 35 ± 2. The measured pCO2 in the above patient (34) is an expected respiratory compensation for primary metabolic acidosis. As a general rule of thumb, a change in the PCO2 by 10 results in a change in the pH by 0.08. For example, if the PCO2 was 30, the pH would be expected to be at 7.48. If the PCO2 was to be 50, then the pH would be 7.32. Thus, in the patient above, the respiratory compensation was not enough to normalized pH. Although many are biased against hyperchloremic metabolic acidosis caused by saline infusions, the numerous randomized clinical studies using hypertonic saline has not shown any clinically relevant complications from the iatrogenically caused hyperchloremic metabolic acidosis. Answers A and E are wrong because the patient is not alkalotic. Answer D is incorrect because the patient does not have respiratory acidosis. Answer B is incorrect as the patient is not alkalotic.


    Answer: C


    Bulger, E. M., May, S., Kerby, J. D., et al. (2011). Out‐of‐hospital hypertonic resuscitation after traumatic hypovolemic shock: a randomized, placebo controlled trial. Annals of Surgery , 253(3), 431–441.


    Kellum, J. A. (2007). Disorders of acid‐base balance. Critical Care Medicine , 35 (11), 2630–2636


    Winter, S. D., Pearson, J. R., Gabow, P. A., et al. (1990). The fall of the serum anion gap. Archives of Internal Medicine , 150 (2), 311–313.


  4. A 75‐year‐old man with uncontrolled COPD from a 50 pack‐year history of smoking is admitted to the ICU after a traumatic hip fracture. An admission ABG reveals pH 7.33; PaCO 2 55 mmHg; PaO2 80 mmHg; HCO3 32 mEq/L.

    Which of the following is true regarding the acid‐base status?



    1. Primary respiratory acidosis with metabolic acidosis.
    2. Primary respiratory acidosis with metabolic compensation.
    3. Primary metabolic acidosis with respiratory acidosis.
    4. Primary respiratory acidosis without metabolic compensation.
    5. Primary metabolic acidosis with respiratory compensation.

    The above patient has a respiratory acidosis as evidenced by his decrease in pH and increase in PaCO2. Given the information that he has suboptimal COPD control, it can be assumed that he likely has a chronic respiratory acidosis. The adequate metabolic compensation in a chronic respiratory acidosis can be determined by the following equation:


    HCO3 = 24+ [0.4 x (PaCO2 – 40)]


    HCO3 = 24+ [0.4 x (55 – 40)]


    HCO3 = 24+ [0.4 x (15)]


    HCO3 = 24+ [6]


    HCO3 = 30


    Thus, the expected bicarbonate level is within the correct range for compensation. If the patient were to have a combined metabolic acidosis, the bicarbonate would be lower than expected. As a general rule, a change in the PCO2 by 10 results a change in the pH by 0.08. For example, if the PCO2 was 30, the pH would be expected to be at 7.48. If the PCO2 was to be 50, then the pH would be 7.32. The patient above has a PCO2 of 55 and a pure respiratory acidosis would result in pH of lower than 7.27. Since the pH is higher than expected, there is some metabolic compensation. Answers A, C, and E are incorrect as the patient does not have metabolic acidosis. Answer D in incorrect as the patient does have metabolic compensation with a higher than normal bicarbonate level.


    Answer: B


    Kellum, J. A. (2007). Disorders of acid‐base balance. Critical Care Medicine , 35 (11), 2630–2636.


    Plant, P. K., Owen, J. L., & Elliott, M. W. (2000). One year period prevalence study of respiratory acidosis in acute exacerbations of COPD: implications for the provision of non‐invasive ventilation and oxygen administration. Thorax , 55 (7), 550–554.


    Winter, S. D., Pearson, J. R., Gabow, P. A., et al. (1990). The fall of the serum anion gap. Archives of Internal Medicine , 150 (2), 311–313.


  5. A 16‐year‐old woman is admitted to the ICU after ingesting an unknown substance. An admission ABG shows pH 7.25; Na 143 mEq/L, K 4.6 mEq/L, Cl 99 mEq/L, HCO 3 21, PaCO 2 39 mm Hg.

    Which of the following is true regarding the acid‐base status?



    1. Non‐anion gap metabolic acidosis
    2. Anion gap metabolic acidosis
    3. Combined metabolic acidosis and respiratory acidosis
    4. Respiratory acidosis
    5. Normal gap metabolic acidosis

    The above patient has an anion gap metabolic acidosis. The first step in management of acidemia is determining whether it is respiratory or metabolic in nature. Since the bicarbonate level is low and in the same direction as the pH, a metabolic acidosis is present. The next step is to determine if an anion gap (AG) is present. The AG can provide information as to whether the acidosis is due to increased acid accumulation or bicarbonate loss. This can be determined by the equation AG = Na – (Cl + HCO3). Normal AG ranges are 3–12 mEq/L. The above patient has an AG of 23 mEq/L (143‐(99+21), thus causing a high anion gap metabolic acidosis. Common causes of this can be remembered by the mnemonic – MUDPILES: (Methanol, Uremia, Diabetic ketoacidosis, Paraldehyde, Isoniazid. Iron, Lactic acidosis, Ethylene glycol, and Salicylates). This patient most likely ingested ethylene glycol which is antifreeze and found in the garage. Hyperchloremic metabolic acidosis is usually normal AG acidosis. Low AG is typically associated with hypoalbuminemia. Albumin constitutes 80% of the unmeasured anions. Answers A and E are incorrect because the patient has AG acidosis. Answers C and D are incorrect as the patient does not have respiratory acidosis as the PaCO2 is 39 mm Hg.


    The utilization of Winter’s formula, [expected pCO2 = (1.5 × HCO3) + 8 ± 2], determined that there is an appropriate respiratory response. Expected pCO2 = 31.5 +8 = 39.5. Thus, the respiratory response is appropriate.


    Answer: B


    Kellum, J. A. (2007). Disorders of acid‐base balance. Critical Care Medicine , 35(11), 2630–2636


    Kraut, J. A., & Xing, S. X. (2011) Approach to the evaluation of a patient with an increased serum osmolal gap and high‐anion‐gap metabolic acidosis. American Journal of Kidney Diseases , 58 (3), 480–484.


  6. A 27‐year old man presents with a gunshot wound to the right anterior thigh with significant blood loss. His heart rate is 110 per minute and his systolic blood pressure is 90 mmHg. Which of the following is true regarding blood transfusion?

    1. Worsening base deficit is associated with need for blood product transfusion and outcome.
    2. Transfusion should only be given if the hemoglobin falls below 7 g/dL.
    3. Blood products such as fresh frozen plasma should only be given if the patient has demonstrable coagulopathy.
    4. Lactate is not a useful marker of resuscitation.
    5. Base deficit is not a useful in the initial phase of treatment.

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Dec 15, 2022 | Posted by in CRITICAL CARE | Comments Off on Acid‐Base, Fluids, and Electrolytes

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