Mastering Perioperative Fluid and Acid-Base Management

Management of Perioperative Acid-Base Disturbances

• Acid-base equilibrium is described using the Henderson-Hasselbalch equation, where pH is the dependent variable, and bicarbonate concentration and PaCO2 are independent variables.

• Metabolic alkalosis, characterized by hyperbicarbonatemia (>27 mEq/L) and usually by an alkalemic pH (>7.45), occurs frequently in postoperative and critically ill patients.

• Metabolic alkalosis is associated with hypokalemia, ionized hypocalcemia, secondary ventricular arrhythmias, increased digoxin toxicity, and compensatory hypoventilation (hypercarbia).

• Metabolic acidosis, characterized by hypobicarbonatemia (<21 mEq/L) and usually by an acidemic pH (<7.35), can be innocuous or reflect a life-threatening emergency.

• Metabolic acidosis occurs as a consequence of buffering by bicarbonate of endogenous or exogenous acid loads or as a consequence of abnormal external loss of bicarbonate.

• Calculation of the anion gap [AG; [Na+] − ([Cl−] + [HCO3−])] distinguishes between two types of metabolic acidosis.

• The anesthetic risk associated with metabolic acidosis is proportional to the severity of the underlying process that produces the metabolic acidosis.

• If a patient with metabolic acidosis requires mechanical ventilation, every attempt should be made to maintain an appropriate level of ventilatory compensation until the primary process can be corrected.

• Disorders of the concentration of sodium, the principal extracellular cation, depend on the total body water (TBW) concentration and can lead to neurologic dysfunction.

• Disorders of potassium, the principal intracellular cation, are influenced primarily by insults that result in increased total body losses of potassium or changes in the distribution between extracellular and intracellular compartments.

• Calcium, phosphorus, and magnesium are all essential for maintenance and function of the cardiovascular system and neuromuscular transmission.

• Precise perioperative management of acid-base status, fluids, and electrolytes may limit perioperative morbidity and mortality.Acid-Base and Fluid Management Physiology in Anesthesia

• Respiratory alkalosis is characterized by hypocarbia and alkalemic pH, resulting from an increase in minute alveolar ventilation that is greater than that required to excrete metabolic CO2 production.

• Respiratory alkalosis may be a sign of pain, anxiety, hypoxemia, central nervous system disease, or systemic sepsis, and requires prompt evaluation.

• Respiratory alkalosis may produce hypokalemia, hypocalcemia, cardiac dysrhythmias, bronchoconstriction, and hypotension, and may potentiate the toxicity of digoxin.

• Acute hyperventilation may be useful in neurosurgical procedures to reduce brain bulk and to control intracranial pressure during emergent surgery for noncranial injuries associated with acute closed head trauma.

• Respiratory acidosis is characterized by hypercarbia and a low pH, occurring because of a decrease in VA, an increase in production of carbon dioxide, or both.

• Chronic hypercarbia due to intrinsic pulmonary disease requires careful preoperative evaluation, and the ventilatory restriction imposed by upper abdominal or thoracic surgery may aggravate ventilatory insufficiency after surgery.

• Acid-base assessment involves the integration of three sets of data: arterial blood gases, electrolytes, and history, and a systematic approach facilitates interpretation.

• Metabolic acidosis occurring during prolonged anesthesia and surgery could suggest lactic acidosis and prompt additional fluid therapy or other attempts to improve perfusion.

• The arterial blood gases and serum electrolytes are compatible with the clinical picture of hyperchloremic metabolic acidosis secondary to infusion of high-chloride fluid, which usually requires no treatment.

• The diagnosis of a primary metabolic alkalosis with compensatory hypoventilation is consistent with the history of recurrent vomiting.

• The sum of intracellular volume and ECV equals total body water, which approximates 60% of total body weight, and red cell volume is approximately 2 L, part of ICV.

• The volume of distribution for colloid solutions is the ECV, and albumin, the most important oncotically active colloid in the ECV, is unequally distributed in PV and IFV.Fluid Therapy in Anesthesia and Critical Care: Physiology and Kinetics

• Static analysis of fluid therapy involves calculating the volume of a given fluid required to produce a certain plasma volume (PV) increment.

• The static analysis formula for restoring blood volume using 5% dextrose in water (D5W), lactated Ringer solution, or 5% or 25% human serum albumin is simplistic.

• A more comprehensive kinetic model proposed by Svensén and Hahn allows clinicians to predict more accurately the time course of volume changes produced by infusions of fluids of various compositions.

• Kinetic analysis permits estimation of peak volume expansion and rates of clearance of infused fluid and complements analysis of pharmacodynamic effects.

• Regulation of osmolarity and effective circulating volume is essential in maintaining fluid balance.

• Thirst is the primary mechanism of controlling water intake, triggered by an increase in body fluid tonicity or by a decrease in effective circulating volume.

• Renal reabsorption of filtered water and sodium is regulated by the renin/angiotensin/aldosterone system, antidiuretic hormone (ADH), and natriuretic peptides.

• Calculation of maintenance fluid requirements is of limited value in determining intraoperative fluid requirements.

• Intraoperatively, fluids containing sodium-free water are rarely used in adults because of the necessity for replacing isotonic losses and the risk of postoperative hyponatremia.

• Traditionally, glucose-containing intravenous fluids have been given in an effort to prevent hypoglycemia and limit protein catabolism.

• Iatrogenic hyperglycemia can limit the effectiveness of fluid resuscitation by inducing an osmotic diuresis and, in animals, may aggravate ischemic neurologic injury.

• Hyperglycemia may also constitute a hormonally mediated response to more severe injury.

• Targeted blood glucose management, at a target of 180 mg/dL or less, was associated with reduced mortality and morbidity in comparison with a tighter control target of 81 to 108 mg/dL.Perioperative Fluid Management: Physiology, Pharmacology, and Clinical Practice

• Perioperative fluid management requires consideration of pre-existing comorbidities and the type of surgery being performed.

• Replacement of gastrointestinal fluids requires accurate replacement of electrolytes, depending on the type of loss.

• Intraoperative fluid management has traditionally included replacement of estimated third space fluid, but recent clinical trials suggest that perioperative fluid management is linked to both minor and major morbidities.

• Large volume preoperative intravenous fluid infusion can reduce postoperative nausea and vomiting and pain, but fluid restriction may be less well tolerated in patients undergoing surgery of limited scope.

• Restrictive perioperative fluid management may be associated with fewer cardiopulmonary and tissue-healing complications, but may also lead to less prompt return of gastrointestinal function and longer hospital stays.

• Colloids preferentially expand plasma volume (PV) over interstitial fluid volume (IFV), but there is no conclusive evidence of the superiority of either colloid-containing or crystalloid-containing fluids for intraoperative or postoperative use.

• Albumin has been used in critically ill patients for decades, but the efficacy of albumin and other colloids remains uncertain.

• Hypertonic solutions cause water to shift from the intracellular to extracellular compartment, and may be useful in treating intracranial hypertension or hypotension.

• Crystalloids are the most commonly used intraoperative and postoperative fluids, and are available in isotonic, hypotonic, and hypertonic forms.

• Lactated Ringer's solution is similar in composition to plasma, but may be associated with hyperchloremic metabolic acidosis when used in large volumes.

• Normal saline is the most commonly used crystalloid, but may be associated with hyperchloremic metabolic acidosis and hypernatremia when used in large volumes.

• Goal-directed fluid therapy may be associated with fewer adverse outcomes than non-goal-directed, liberal fluid administration, but the data do not establish whether goal-directed fluid therapy is superior to non-goal-directed restrictive fluid therapy.

Management of Perioperative Acid-Base Disturbances

• Acid-base equilibrium is described using the Henderson-Hasselbalch equation, where pH is the dependent variable, and bicarbonate concentration and PaCO2 are independent variables.

• Metabolic alkalosis, characterized by hyperbicarbonatemia (>27 mEq/L) and usually by an alkalemic pH (>7.45), occurs frequently in postoperative and critically ill patients.

• Metabolic alkalosis is associated with hypokalemia, ionized hypocalcemia, secondary ventricular arrhythmias, increased digoxin toxicity, and compensatory hypoventilation (hypercarbia).

• Metabolic acidosis, characterized by hypobicarbonatemia (<21 mEq/L) and usually by an acidemic pH (<7.35), can be innocuous or reflect a life-threatening emergency.

• Metabolic acidosis occurs as a consequence of buffering by bicarbonate of endogenous or exogenous acid loads or as a consequence of abnormal external loss of bicarbonate.

• Calculation of the anion gap [AG; [Na+] − ([Cl−] + [HCO3−])] distinguishes between two types of metabolic acidosis.

• The anesthetic risk associated with metabolic acidosis is proportional to the severity of the underlying process that produces the metabolic acidosis.

• If a patient with metabolic acidosis requires mechanical ventilation, every attempt should be made to maintain an appropriate level of ventilatory compensation until the primary process can be corrected.

• Disorders of the concentration of sodium, the principal extracellular cation, depend on the total body water (TBW) concentration and can lead to neurologic dysfunction.

• Disorders of potassium, the principal intracellular cation, are influenced primarily by insults that result in increased total body losses of potassium or changes in the distribution between extracellular and intracellular compartments.

• Calcium, phosphorus, and magnesium are all essential for maintenance and function of the cardiovascular system and neuromuscular transmission.

• Precise perioperative management of acid-base status, fluids, and electrolytes may limit perioperative morbidity and mortality.Acid-Base and Fluid Management Physiology in Anesthesia

• Respiratory alkalosis is characterized by hypocarbia and alkalemic pH, resulting from an increase in minute alveolar ventilation that is greater than that required to excrete metabolic CO2 production.

• Respiratory alkalosis may be a sign of pain, anxiety, hypoxemia, central nervous system disease, or systemic sepsis, and requires prompt evaluation.

• Respiratory alkalosis may produce hypokalemia, hypocalcemia, cardiac dysrhythmias, bronchoconstriction, and hypotension, and may potentiate the toxicity of digoxin.

• Acute hyperventilation may be useful in neurosurgical procedures to reduce brain bulk and to control intracranial pressure during emergent surgery for noncranial injuries associated with acute closed head trauma.

• Respiratory acidosis is characterized by hypercarbia and a low pH, occurring because of a decrease in VA, an increase in production of carbon dioxide, or both.

• Chronic hypercarbia due to intrinsic pulmonary disease requires careful preoperative evaluation, and the ventilatory restriction imposed by upper abdominal or thoracic surgery may aggravate ventilatory insufficiency after surgery.

• Acid-base assessment involves the integration of three sets of data: arterial blood gases, electrolytes, and history, and a systematic approach facilitates interpretation.

• Metabolic acidosis occurring during prolonged anesthesia and surgery could suggest lactic acidosis and prompt additional fluid therapy or other attempts to improve perfusion.

• The arterial blood gases and serum electrolytes are compatible with the clinical picture of hyperchloremic metabolic acidosis secondary to infusion of high-chloride fluid, which usually requires no treatment.

• The diagnosis of a primary metabolic alkalosis with compensatory hypoventilation is consistent with the history of recurrent vomiting.

• The sum of intracellular volume and ECV equals total body water, which approximates 60% of total body weight, and red cell volume is approximately 2 L, part of ICV.

• The volume of distribution for colloid solutions is the ECV, and albumin, the most important oncotically active colloid in the ECV, is unequally distributed in PV and IFV.Fluid Therapy in Anesthesia and Critical Care: Physiology and Kinetics

• Static analysis of fluid therapy involves calculating the volume of a given fluid required to produce a certain plasma volume (PV) increment.

• The static analysis formula for restoring blood volume using 5% dextrose in water (D5W), lactated Ringer solution, or 5% or 25% human serum albumin is simplistic.

• A more comprehensive kinetic model proposed by Svensén and Hahn allows clinicians to predict more accurately the time course of volume changes produced by infusions of fluids of various compositions.

• Kinetic analysis permits estimation of peak volume expansion and rates of clearance of infused fluid and complements analysis of pharmacodynamic effects.

• Regulation of osmolarity and effective circulating volume is essential in maintaining fluid balance.

• Thirst is the primary mechanism of controlling water intake, triggered by an increase in body fluid tonicity or by a decrease in effective circulating volume.

• Renal reabsorption of filtered water and sodium is regulated by the renin/angiotensin/aldosterone system, antidiuretic hormone (ADH), and natriuretic peptides.

• Calculation of maintenance fluid requirements is of limited value in determining intraoperative fluid requirements.

• Intraoperatively, fluids containing sodium-free water are rarely used in adults because of the necessity for replacing isotonic losses and the risk of postoperative hyponatremia.

• Traditionally, glucose-containing intravenous fluids have been given in an effort to prevent hypoglycemia and limit protein catabolism.

• Iatrogenic hyperglycemia can limit the effectiveness of fluid resuscitation by inducing an osmotic diuresis and, in animals, may aggravate ischemic neurologic injury.

• Hyperglycemia may also constitute a hormonally mediated response to more severe injury.

• Targeted blood glucose management, at a target of 180 mg/dL or less, was associated with reduced mortality and morbidity in comparison with a tighter control target of 81 to 108 mg/dL.Perioperative Fluid Management: Physiology, Pharmacology, and Clinical Practice

• Perioperative fluid management requires consideration of pre-existing comorbidities and the type of surgery being performed.

• Replacement of gastrointestinal fluids requires accurate replacement of electrolytes, depending on the type of loss.

• Intraoperative fluid management has traditionally included replacement of estimated third space fluid, but recent clinical trials suggest that perioperative fluid management is linked to both minor and major morbidities.

• Large volume preoperative intravenous fluid infusion can reduce postoperative nausea and vomiting and pain, but fluid restriction may be less well tolerated in patients undergoing surgery of limited scope.

• Restrictive perioperative fluid management may be associated with fewer cardiopulmonary and tissue-healing complications, but may also lead to less prompt return of gastrointestinal function and longer hospital stays.

• Colloids preferentially expand plasma volume (PV) over interstitial fluid volume (IFV), but there is no conclusive evidence of the superiority of either colloid-containing or crystalloid-containing fluids for intraoperative or postoperative use.

• Albumin has been used in critically ill patients for decades, but the efficacy of albumin and other colloids remains uncertain.

• Hypertonic solutions cause water to shift from the intracellular to extracellular compartment, and may be useful in treating intracranial hypertension or hypotension.

• Crystalloids are the most commonly used intraoperative and postoperative fluids, and are available in isotonic, hypotonic, and hypertonic forms.

• Lactated Ringer's solution is similar in composition to plasma, but may be associated with hyperchloremic metabolic acidosis when used in large volumes.

• Normal saline is the most commonly used crystalloid, but may be associated with hyperchloremic metabolic acidosis and hypernatremia when used in large volumes.

• Goal-directed fluid therapy may be associated with fewer adverse outcomes than non-goal-directed, liberal fluid administration, but the data do not establish whether goal-directed fluid therapy is superior to non-goal-directed restrictive fluid therapy.

Test your knowledge on the management of perioperative acid-base disturbances, fluid therapy, and clinical practice with this informative quiz. Learn about the Henderson-Hasselbalch equation, metabolic alkalosis, metabolic acidosis, and the anion gap. Discover the physiology and kinetics of fluid therapy, including the regulation of osmolarity and effective circulating volume, thirst, and renal reabsorption. Explore perioperative fluid management, replacement of gastrointestinal fluids, and the use of colloids and crystalloids. Improve