מאזןו חומצה בסיס - 4

Acid-Base Balance in the Human Body

Fundamentals of Acid-Base Balance

• Acid: A substance that releases hydrogen ions ($H^+$).

• Base: A substance that absorbs hydrogen ions.

• pH Scale: Used to measure acidity.

  • Low pH = Acidic.

  • High pH = Basic (Alkaline).

• Buffers: Substances that neutralize acids or bases.

  • Bicarbonate/Carbonic Acid Buffer: The primary buffer in the body.

• Lungs: Eliminate carbon dioxide ($CO_2$).

• Kidneys: Eliminate hydrogen ions and generate bicarbonate.

Importance of pH Balance

• Normal Range: A narrow range of 7.35-7.45 is crucial.

• Enzyme Activity: Essential for proper enzyme function.

• Protein Structure: Influences protein structure and function.

• Cellular Function: Necessary for cell function and metabolic processes.

• The body produces a large amount of acids from routine metabolic processes daily.

• The body's systems work together to maintain a proper acid-base balance and prevent excessive accumulation of acid or base in the blood.

Sources of Acids in the Body

• Carbon Dioxide ($CO_2$):

  • Produced during cellular respiration in all body cells.

  • Dissolves in the blood and forms carbonic acid.

• Lactic Acid:

  • Produced during anaerobic metabolism when there is a lack of oxygen or during strenuous physical activity.

• Fatty Acids and Amino Acids:

  • The breakdown of fatty acids (beta-oxidation) and amino acids produces metabolic acids.

• Acids are naturally created from different sources in the body as part of the daily metabolism.

• These acids must be neutralized or excreted to maintain a proper pH balance.

• Excessive accumulation of these acids will lead to disturbances in the acid-base balance.

Arterial Blood Gas (ABG) Test

• Purpose:

  • Performed in the hospital as part of patient assessment.

  • Aids in obtaining a tentative diagnosis and making treatment decisions to determine the acid-base balance.

• Parameters Examined:

  • Bicarbonate.

  • Carbon dioxide ($PaCO_2$).

  • pH in the blood.

  • Also measures oxygen levels, electrolytes, and metabolites.

• Clinical Uses:

  • Assessment of respiratory diseases, metabolic disorders, poisoning, kidney failure, diabetic complications, and other critical medical conditions.

• A venous blood test can also be performed, but the values obtained will be slightly different from those of an arterial blood test.

• ABG testing is an essential tool in assessing urgent clinical conditions and provides immediate information about the patient's internal balance.

Normal Values in Blood Gas Testing

• pH: 7.35-7.45

  • This narrow range is essential for the proper functioning of the body's systems.

• Bicarbonate: 22-26 mEq/L

  • Indicates kidney activity in regulating acidity.

• PaCO2: 35-45 mmHg

  • Concentration of carbon dioxide, indicating respiratory activity.

• Blood gas testing is an important diagnostic tool that provides an accurate picture of the acid-base balance in the body.

• Deviation from normal ranges may indicate metabolic or respiratory disorders.

• These values are the basis for determining the type of disorder and monitoring the effectiveness of treatment.

Possible Acid-Base Imbalances

• Acidemia/Acidosis: pH less than 7.35

  • Due to a decrease in bicarbonate (base loss).

  • Or due to an increase in $PaCO_2$ (acid retention).

  • Symptoms: Rapid and deep breathing, confusion, headache, drowsiness.

• Alkalemia/Alkalosis: pH greater than 7.45

  • Due to an increase in bicarbonate (base retention).

  • Or due to a decrease in $PaCO_2$ (acid loss).

  • Symptoms: Numbness in the fingertips, dizziness, muscle spasms, decreased level of consciousness.

• Acid-base imbalances can occur in a variety of medical conditions, some of which are life-threatening.

• Rapid identification of the type and severity of the disorder is essential for effective treatment and prevention of complications.

• Clinical symptoms may be similar in different disorders, so a blood gas test is required for an accurate diagnosis.

Types of Acidosis and Alkalosis

• Respiratory Acidosis: Decrease in pH and increase in $PaCO_2$

  • Caused by $CO_2$ retention in the body.

• Metabolic Acidosis: Decrease in pH and bicarbonate

  • Caused by acid accumulation or bicarbonate loss.

• Respiratory Alkalosis: Increase in pH and decrease in $PaCO_2$

  • Caused by increased $CO_2$ removal (hyperventilation).

• Metabolic Alkalosis: Increase in pH and bicarbonate

  • Caused by excess bicarbonate or acid loss.

• Classification of acid-base disorders is essential for determining appropriate treatment.

• Identification of the pathophysiological mechanism underlying the disorder (respiratory or metabolic) enables targeted treatment of the root cause of the problem.

• There is often a combination of several disorders simultaneously, which requires a more complex approach to treatment.

Respiratory Compensation for a Metabolic Problem

• Primary Metabolic Disorder: Change in bicarbonate levels in the blood.

• Identification of the Change: Chemoreceptors detect the change in pH.

• Respiratory Response: Change in the rate and depth of breathing.

• The respiratory compensation mechanism is relatively rapid and begins within about half an hour of the onset of the metabolic disorder.

• The level of $CO_2$ will change in the same direction as the change in bicarbonate:

  • In metabolic acidosis (decrease in bicarbonate), there will be an acceleration of the breathing rate to lower the level of $PaCO_2$. The body has more hydrogen ions so uses it to produce carbon dioxide and water, creating a decrease in the level of hydrogen ions and an increase in pH.

  • In metabolic alkalosis (increase in bicarbonate), there will be a slowing of the breathing rate to increase the level of $PaCO_2$. The body uses the carbon dioxide created with the water to produce bicarbonate causing the level of hydrogen to increase, and decreasing the pH.

• Respiratory compensation is rapid but also temporary, as it only manages to return bicarbonate values to the normal range for a short time.

• However, it is a first line of defense for maintaining acid-base balance stability when a metabolic disorder occurs.

• Then, metabolic compensation will also work to restore balance to normal values in the longer term.

Metabolic Compensation for a Respiratory Problem

• Primary Respiratory Disorder: Change in $PaCO_2$ levels in the blood due to a change in respiratory function.

• Immediate Buffer Compensation: The bicarbonate buffer responds immediately to changes in acidity; this compensation is performed all the time.

• Delayed Renal Compensation: If the respiratory problem persists, metabolic renal compensation begins to adjust the bicarbonate level.

  • In respiratory acidosis (increase in $PaCO_2$), the kidneys release bicarbonate into the blood to neutralize the increased acidity.

  • In respiratory alkalosis (decrease in $PaCO_2$), the kidneys excrete bicarbonate into the urine to lower its level in the blood.

• Renal compensation begins within a few hours but reaches its peak only after 3-5 days.

Regulation of Compensation Mechanisms

• Chemoreceptors: Sensors that detect pH changes in the blood and extracellular fluid.

  • Central - in the brainstem.

  • Peripheral - in the aortic arch and carotid bifurcation.

• Respiratory System: Change in the rate and depth of breathing in response to pH changes.

  • Hyperventilation in response to acidosis.

  • Hypoventilation in response to alkalosis.

• Renal System: Change in hydrogen ion excretion and bicarbonate production.

  • Increased acid excretion in acidosis.

  • Increased bicarbonate absorption in acidosis.

• The level of compensation should match the severity of the condition: the more significant the change in pH, the more we would expect to see the compensation mechanisms operating vigorously.

• However, compensation mechanisms have physiological limitations: it is not possible to stop breathing for a long time or breathe fast and deeply for a long time, and hydrogen excretion from the kidney requires adequate fluids.

• If the respiratory problem persists, metabolic-renal compensation begins.

  • Respiratory acidosis - release of bicarbonate into the blood.

  • Respiratory alkalosis - excretion of bicarbonate into the urine.

  • This is a late compensation that starts working after a few hours and takes several days to compensate for the pH breach.

Treatment of Acid-Base Disorders

• Support ABCs: Stabilize the airway, breathing, and circulation as an initial and vital step in any medical emergency.

• Identify the Cause: Identify the underlying cause of the acid-base imbalance and treat it specifically.

• Support Compensation Mechanisms: Support natural compensation mechanisms through medication, fluids, or respiratory support.

• Monitor: Repeat blood gas tests and vital signs to assess response to treatment.

• When the natural compensation limit is reached, medical intervention is needed to aid compensation mechanisms.

• Treatment may include oxygen, medication, mechanical ventilation, fluids, or correction of electrolyte imbalances.

• The primary treatment should be directed at the underlying cause and not just at correcting pH values.

Mixed Disorders in Acid-Base Balance

• Diagnostic Challenge: Combined disorders present a greater diagnostic challenge than simple disorders and require in-depth analysis of the clinical and laboratory situation.

• Unexpected Values: Tests that do not conform to the normal rules of simple disorders, such as normal pH despite extreme values in $PaCO_2$ and bicarbonate.

• Importance of Clinical Correlation: Interpreting laboratory results must be done in the context of the overall clinical picture, including the patient's complaints and findings on physical examination.

• Combined disorders occur when two or more mechanisms are impaired related to acid-base balance.

• For example, a patient with obstructive lung disease (respiratory acidosis) who develops dehydration due to diarrhea (metabolic acidosis).

• Accurate identification of the contributing disorders is essential for effective treatment and prevention of patient deterioration.

Causes of Metabolic Acidosis

• Shock: Shock conditions lead to decreased tissue perfusion and anaerobic metabolism, producing lactic acid.

• Metabolic Fever: High fever increases the rate of metabolism and acid production in the body.

• Kidney Failure: Impaired kidney function leads to acid retention and decreased bicarbonate production.

• Diabetic Ketoacidosis: Accumulation of acidic ketone bodies as a result of insulin deficiency.

• Metabolic acidosis occurs when there is excess acid in the body or a lack of bases.

• Conditions such as dehydration, diarrhea, poisoning, kidney failure, uncontrolled diabetes and alcohol poisoning can cause metabolic acidosis.

• Early identification of the cause of acidosis allows for targeted treatment and prevention of complications.

Causes of Respiratory Acidosis

• Respiratory acidosis develops when carbon dioxide is retained in the blood due to a lack of effective pulmonary ventilation.

• Common conditions that cause this include:

  • Pneumonia.

  • Pulmonary edema.

  • Obstructive lung diseases such as asthma and COPD.

  • Choking on a foreign object.

  • Aspiration.

  • Use of central nervous system depressants.

• Pathological breathing patterns such as apnea (cessation of breathing), bradypnea (slow breathing), and hypopnea (low breathing volume) are also common causes of respiratory acidosis.

Causes of Respiratory and Metabolic Alkalosis

• Respiratory Alkalosis:

  • Hyperventilation due to anxiety or fear.

  • High fever.

  • Salicylate poisoning.

  • High altitudes (hypobaric hypoxia).

  • Overly aggressive mechanical ventilation.

  • Pregnancy (increased progesterone levels).

• Metabolic Alkalosis:

  • Excessive vomiting or aspiration of gastric contents.

  • Administration of antacid medications containing bicarbonate.

  • Administration of large amounts of blood (contains citrate).

  • Administration of loop diuretics (furosemide).

  • Primary hyperaldosteronism - Conn's syndrome - overproduction of aldosterone by the adrenal gland.

  • Electrolyte imbalances (hypokalemia, hypochloremia).

• Respiratory alkalosis is caused by excessive loss of $CO_2$ due to rapid and deep breathing (hyperventilation).

• In contrast, metabolic alkalosis is mainly caused by excess bicarbonate or increased loss of acids from the body.

• Unlike acidosis, which is often the result of pathological conditions, respiratory alkalosis can be a physiological response, such as in anxiety or unadjusted mechanical ventilation.

Rules for Interpreting Disorders - pH

• Clinical Picture: Examine the patient's chief complaint, anamnesis, vital signs, and physical examination.

• Check the pH Value: Initial determination of whether it is acidosis (pH less than 7.35) or alkalosis (pH greater than 7.45).

• Identify the Primary Disorder: Analyze bicarbonate and $PaCO_2$ values to determine the source of the disorder - respiratory or metabolic.

• Interpreting acid-base disorders requires a systematic and structured approach.

• Always start with the patient's clinical picture and use laboratory results as a supportive tool.

• After determining the existence of an acid-base disorder, identify the primary disorder according to the following rules:

  • Acidosis with low bicarbonate indicates metabolic acidosis.

  • Acidosis with high $PaCO_2$ indicates respiratory acidosis.

  • Alkalosis with high bicarbonate indicates metabolic alkalosis.

  • Alkalosis with low $PaCO_2$ indicates respiratory alkalosis.

Rules for Interpreting Disorders - Compensation

• Assess Compensation: Check whether the second parameter (bicarbonate or $PaCO_2$) changes in the appropriate direction for compensation.

• Look for Anomalies: Examine whether the compensating response is proportionate to the primary disorder - an anomaly may indicate a combined disorder.

• Match the Clinical Condition: Verify that the identified disturbance and compensation mechanism match the patient's clinical condition.

• After identifying the primary disorder, examine the extent of compensation.

  • An increase in bicarbonate compensates for respiratory acidosis.

  • A decrease in bicarbonate compensates for respiratory alkalosis.

  • A decrease in $PaCO_2$ compensates for metabolic acidosis.

  • An increase in $PaCO_2$ compensates for metabolic alkalosis.

• Finally, name the disorder and compensation.

Steps for Analyzing Blood Gases

• What is compensation:

  • Partial: pH is not yet within normal range.

  • Full: pH is back to normal but the compensating element ($PaCO<em>2$ or $HCO</em>3^-$) is not in range

  • None: Nothing is affected to help compensate

Case Study 1

• A person with no known medical history complains of shortness of breath.

• Blood gas tests show:

  • pH: 7.32

  • $PaCO_2$: 70 mmHg

  • Bicarbonate: 33 mEq/L

• Analysis:

  • The patient has acidosis (pH less than 7.35) with high $PaCO_2$ levels, indicating respiratory acidosis.

  • There is metabolic compensation expressed by a high bicarbonate value.

• The disorder is consistent with the complaint of shortness of breath and may be caused by obstructive lung disease, pneumonia, or respiratory failure for other reasons.

Case Study 2

• Patient presents complaining of diarrhea.

• Blood gas results show:

  • pH = 7.24.

  • $PaCO_2$ = 24 mmHg.

  • $HCO_3^-$ = 10 mEq/L.

• Analysis:

  • The patient has Acidosis (pH<7.35), meaning they have low levels of blood bicarbonate. They likely have metabolic acidosis.

  • Signs of respiratory control are demonstrated by their low levels of $PaCO_2$, meaning they are hyperventilating or breathing rapidly

  • The metabolic acidosis likely stems from hypovolemic shock caused by dehydration resulting from diarrhea. This overall causes loss of many fluids and overall less bicarbonate.

Case Study 3

• A person with no known medical information started complaining about feeling a headache. A blood sample was taken and they started breathing really rapidly and heavily.

• A blood test was taken with the following results:

  • pH of 7.52,

  • $PaCO_2$ level of 15 mmHg.

  • Bicarbonate level of 16 mEq/L.

• Analysis:

  • The patient has alkalosis coming from their pH being more than 7.45. They also have low $PaCO_2$ meaning they have respiratory alkalosis.

  • Metabolic compensation is shown with a lower than average bicarbonate value. This respiratory alkalosis comes from hyperventilation likely caused by pain, anxiety, other existing medical conditions, and possibly hypoxemia.

Case Study 4

• The patient is complaining about excessive vomiting.

• A blood sample was taken and analyzed with the results presented as follows:

  • pH of 7.54,

  • $PaCO_2$ level of 54 mmHg,

  • Bicarbonate count of 35 mEq/L.

• Analysis:

  • The patient has alkalosis from high pH that is over level 7.45. They also have high bicarbonate counts which lead us to believe the patient has metabolic alkalosis.

  • Because $PaCO_2$ level is already pretty, a response is seen in the patient to perform hypoventilation. To summarize, one can deduce that the vomiting is the root cause, which causes a loss of stomach acid, and in turn, gives a change in bicarbonate averages in count.

Combined pH Imbalances

• Combined respiratory disorders include respiratory acidosis and respiratory alkalosis (respiratory failure stemming from disease).

• Combined metabolic disorders involve alkalosis and metabolic acidosis (like kidney failure stemming from emesis).

• Combined respiratory-metabolic disorders include lung disease and/or kidney failure.

• Normal pH can exist with abnormal values of $PaCO_2$ and bicarbonate because they are in the opposite direction.

• They are categorized with a tough clinical diagnosis.

• Results don't directly align with the simple disorders, instead, clinical evaluation is used to determine the disorder. Additionally, extreme values can be expressed from compensatory results leading to that normal value.

Case studies of combined imbalances

• There is metabolic acidosis without appropriate or effective compensation.

  • pH = 7.12 being a low pH value that entails the existence of acidosis.

  • Bicarbonate, or $HCO_3^-$, is 12.

  • $PaCO_2$ level is 55.

    • Levels of pH are low equating to acidosis, but the level of $PaCO_2$ is high which equates to respiratory-themed acidosis.

• Now to identify if compensation should have occurred. The way of determining this relies on an understanding that a higher bicarbonate level would have revealed to us an existence of compensation.

• Because the result shows us lower bicarbonate, it is considered the patient is now encountering both metabolic and respiratory acidosis alike.

Case Studies of Combined Imbalances

• pH equals 7.39 and appears to be normal.

• $PaCO_2$ equals 25 and is determined to be too low which is classified as basic.

• $HCO_3^-$ equals 15 and we determine it is also below average or too low, overall meaning this is acidic-themed.

• Both values are too low, but they appear to be in their opposing direction.

  • Meaning because pH results are normal, and the $PaCO_2$ values are low, we see respiratory alkalosis.

  • However, because the bicarbonate is low, we see metabolic acidosis instead.

  • Therefore the patient has metabolic acidosis and respiratory alkalosis.

Clinical Signs of Acid-Base Disorders

• Acidosis: Kussmaul's breathing (deep and rapid breathing), confusion, fatigue, headache, heart rhythm disturbances, decreased blood pressure, and even loss of consciousness in severe cases.

• Alkalosis: Numbness in the fingertips and lips, muscle spasms, tremors, tetany (muscle contraction), restlessness, increased anxiety, and seizures in severe cases.

• Hypoxemia Cyanosis (blueness), tachycardia, dyspnea (shortness of breath), dizziness, confusion, restlessness, and changes in the level of consciousness. Often appears together with acid-base disorders.

Effects of Acid-Base Disorders on Body Systems

• Acid-base imbalances affect all body systems and can cause significant changes in their function.

• These effects vary depending on the type, severity, and duration of the disorder.

• Understanding the physiological effects of changes in pH on body systems is essential for proper assessment and treatment of patients with acid-base disorders.

Specific Treatment of Acid-Base Disorders

• Treatment of Respiratory Acidosis: Improve pulmonary ventilation through mechanical ventilation, bronchodilators, removal of pulmonary secretions, and treatment of pulmonary infection if present.

• Treatment of Metabolic Acidosis: Treat the root cause (shock, infection, kidney failure), provide fluids, and in severe cases administer bicarbonate as a drug.

• Treatment of Respiratory Alkalosis: Slow the breathing rate (using relaxation techniques), treat anxiety if that is the cause, and adjust parameters in the ventilator if the patient is ventilated.

• Treatment of Metabolic Alkalosis: Replace fluids, balance electrolytes (especially potassium), and in some cases administer solutions containing chloride such as ammonium chloride.

• Treatment of acid-base disorders focuses on treating the cause and supporting natural compensation mechanisms.

• In all types of disorders, rapid identification and treatment of the root cause is key to success.

• The use of drugs such as bicarbonate or ammonium chloride is limited to severe cases only, when natural compensation mechanisms are insufficient.

Challenges in Treating Acid-Base Disorders

• Accurate Diagnosis: Identify the disorder and distinguish between simple and combined disorders.

• Treat the Root Cause: Identify and treat the underlying cause of the disorder, not just the symptoms.

• Maintaining Subtle Balance: Prevent overcorrection that can lead to a disturbance in the opposite direction.

• Controlled Correction Rate: Slow and gradual correction to prevent complications such as cerebral edema.

• Complex Clinical Environment: Dealing with multiple factors affecting acid-base balance.

• Treatment of acid-base disorders presents many challenges for the medical staff.

• Too rapid correction can lead to serious complications such as cerebral edema (in the case of metabolic acidosis) or tetany and muscle cramps (in the case of alkalosis).

• Also, in patients with chronic diseases, certain disorders may represent necessary physiological compensation, and correcting them may do more harm than good.

Monitoring in Acid-Base Disorders

• Repeated Laboratory Tests:

  • Arterial Blood Gas (ABG) tests at varying frequencies depending on clinical status.

  • Electrolyte and kidney function tests.

  • Lactate test when lactic acidosis is suspected.

  • Glucose and ketone blood test testing for diabetic ketoacidosis

• Clinical Monitoring:

  • Continuous measurement of vital signs: pulse, blood pressure, breathing rate, and saturation.

  • Monitoring level of consciousness and neurological signs.

  • Monitoring urine output and fluid balance.

  • Monitoring breathing characteristics and quality.

• Advanced Monitoring Tools:

  • Capnography monitoring of exhaled $CO_2$.

  • Continuous blood gas testing using dedicated catheters.

  • Continuous electrolyte monitoring using new technologies.

  • Advanced hemodynamic monitoring in complex cases.

• Careful monitoring of patients with acid-base disorders is essential for assessing the effectiveness of treatment and early detection of deterioration or complications.

• The frequency of testing and depth of monitoring are determined by the severity of the disorder, patient stability, and response to treatment.

• In addition to laboratory tests, continuous clinical assessment is an integral part of optimal follow-up.

A-a Gradient

• Arterial-alveolar oxygen difference is an important measure in the evaluation of gas exchange within the lungs. Measures abnormalities such as ventilation or oxygenation.

Definition

• Difference between the calculated alveolar concentration of oxygen and the measured arterial concentration of oxygen

Calculation:

• $A-a gradient = PAO₂ - PaO₂$

• Normal values are 5-25 mmHg

• The gradient goes up based on the age of the patient

Pathophysiology of A-a gradient

• the A-a gradient is critical in measuring the effectiveness of gas exchange in the lungs, specifically measuring the lungs' ability to use inspired air to get to the bloodstream.

Physiological Factors:

There are three main factors to the A-a gradient: V/Q Mismatch, Diffusion abnormalities, and shant.

Abnormal conditions

Conditions that would demonstrate abnormal a-a gradients include, lung disease, pulmonary embolism, ARDS. Measuring the gradient is critical to being able to diagnose the correct disease mechanism. This is especially true because it allows people to see if the disease is the problem, and to determine whether hypoxemia is the underlying problem.