Handbook of Blood Gas Acid-Base Interpretation - Ashfaq Hasan

Год выпуска: 2013

Автор: Ashfaq Hasan

Жанр: Интенсивная терапия

Формат: PDF

Качество: OCR

Описание: For the uninitiated, the analysis of blood gas can be a daunting task. Hapless medical students, badly constrained for time, have struggled ineffectively with Hasselbach's modification of the Henderson equation; been torn between the Copenhagen and the Boston schools of thought; and lately, been confronted with the radically different strong-ion approach of Peter Stewart.
I n the modern medical practice, the multi-tasking health provider's time has become precious — and his attention span short. It is therefore important to retain focus on those aspects of clinical medicine that truly matter. In the handling of those subjects rooted in clinical physiology (and therefore predictably difficult to understand), it makes perfect sense, in my opinion, to adopt an 'algorithmic' approach. A picture can say a thousand words; a well-constructed algorithm can save at least a hundred — not to say, much precious time—and make for clarity of thinking. I have personally found this method relatively painless — and easy to assimilate. The book is set out in the form of flow charts in logical sequence, introducing and gradually building upon the underlying concepts.
The goal of this book is to enable medical students, residents, nurses and respiratory care practitioners to quickly grasp the principles underlying respiratory and acid-base physiology, and to apply the concepts effectively in clinical decision making. Each of these sections, barring a few exceptions, has been designed to fit into a single powerpoint slide: this should facilitate teaching.
Over the years, many excellent books and articles have appeared on the subject. I have found the manuals by Lawrence Martin and Kerry Brandis thoroughly enjoyable as also the online tutorials of Alan Grogono and Bhavani Shankar Kodali : I have tried to incorporate into my own book, some of their energy and content.
No matter how small, a project such as this can never be accomplished without the support of well wishers and friends. I would like to acknowledge the unwavering support of my colleagues Dr. TLN Swamy and Dr. Syed Mahmood Ahmed; my assistants A. Shoba and P. Sudheer; and above all, my family who had to endure the painstaking writing of yet another manuscript.

Contents

«Handbook of Blood Gas Acid-Base Interpretation»

Gas Exchange

The Respiratory Centre
Rhythmicity of the Respiratory Centre
The Thoracic Neural Receptors
Chemoreceptors
The Central Chemoreceptors and the Alpha-Stat Hypothesis
Peripheral Chemoreceptors
Chemoreceptors in Hypoxia
Response of the Respiratory Centre to Hypoxemia
Respiration
Partial Pressure of a Mixture of Gases
- Atmospheric Pressure
- Gas Pressure
Partial Pressure of a Gas
The Fractional Concentration of a Gas (F_gas)
Diffusion of Gases
Henry's Law and the Solubility of a Gas in Liquid
Inhaled Air
The O₂ Cascade
PaO₂
The Modified Alveolar Gas Equation
The Determinants of the Alveolar Gas Equation
The Respiratory Quotient (RQ) in the Alveolar Air Equation
FIO₂, PAO₂, PaO₂ and CaO₂
DO₂, CaO₂, SpO₂, PaO₂ and FIO₂
O₂ Content: An Illustrative Example
Mechanisms of Hypoxemia
Processes Dependent Upon Ventilation
Defining Hypercapnia (Elevated CO₂)
Factors That Determine PaCO₂ Levels
Relationship Between CO₂ Production and Elimination
Exercise, CO₂ Production and PaCO₂
Dead Space
Minute Ventilation and Alveolar Ventilation
The Determinants of the PaCO₂
Alveolar Ventilation in Health and Disease
Hypoventilation and PaCO₂
The Causes of Hypoventilation
Blood Gases in Hypoventilation
Decreased CO₂Production
- Summary: Conditions That Can Result in Hypercapnia
V/Q Mismatch: A Hypothetical Model
V/Q Mismatch and Shunt
Quantifying Hypoxemia
Compensation for Regional V/Q Inequalities
Alveolo-Arterial Diffusion of Oxygen (A-aDO₂)
A-aDO₂ is Difficult to Predict on Intermediate Levels of FIO₂
Defects of Dif2fusion
Determinants of Diffusion: DLCO
Timing the ABG
A-aDO₂ Helps in Differentiating Between the Different Mechanisms of Hypoxemia

The Non-Invasive Monitoring of Blood Oxygen and Carbon Dioxide Levels

The Structure and Function of Haemoglobin
Co-operativity
The Bohr Effect and the Haldane Effect
Oxygenated and Non-oxygenated Hemoglobin
PaO₂ and the Oxy-hemoglobin Dissociation Curve
Monitoring of Blood Gases
- Invasive O₂ Monitoring
- The Non-invasive Monitoring of Blood Gases
Principles of Pulse Oximetry
Spectrophotometry
Optical Plethysmography
Types of Pulse Oximeters
Pulse Oximetry and PaO₂
P₅₀
Shifts in the Oxy-hemoglobin Dissociation Curve
Oxygen Saturation (SpO₂) in Anemia and Skin Pigmentation
Oxygen Saturation (SpO₂) in Abnormal Forms of Hemoglobin
Mechanisms of Hypoxemia in Methemoglobinemia
Methemoglobinemias: Classification
Sulfhemoglobinemia
Carbon Monoxide (CO) Poisoning
Saturation Gap
Sources of Error While Measuring SpO₂
Point of Care (POC) Cartridges
Capnography and Capnometry
The Capnographic Waveform
Main-Stream and Side-Stream Capnometers
PEtCO₂ (EtCO₂): A Surrogate for PaCO₂
Factors Affecting PEtCO₂
Causes of Increased' PaCO₂-PEtCO₂ Difference
Bohr's Equation
Application of Bohr's Equation
Variations in EtCO₂
False-Positive and False-Negative Capnography
Capnography and Cardiac Output
Capnography as a Guide to Successful Resuscitation
Capnography in Respiratory Disease
Esophageal Intubation
Capnography in Tube Disconnection and Cuff Rupture
- Biphasic Capnograph

Acids and Bases

Intracellular and Extracellular pH
pH Differences
Surrogate Measurement of Intracellular pH
Preferential Permeability of the Cell Membrane
Ionization and Permeability
The Reason Why Substances Need to Be Ionized
The Exceptions to the Rule
The Hydrogen Ion (H+, Proton)
Intracellular pH Is Regulated Within a Narrow Range
A Narrow Range of pH Does Not Mean a Small Range of the H+ Concentration
The Earliest Concept of an Acid
Arrhenius's Theory
Bronsted-Lowry Acids
Lewis' Approach
The Usanovich Theory
Summary of Definitions of Acids and Bases
Stewart's Physico-Chemical Approach
The Dissociation of Water
Electrolytes, Non-electrolytes and Ions
Strong Ions
Stewart's Determinants of the Acid Base Status
Apparent and Effective Strong Ion Difference
Strong Ion Gap
Major Regulators of Independent Variables
Fourth Order Polynomial Equation
The Workings of Stewart's Approach

Buffer Systems

Generation of Acids
Disposal of Volatile Acids
Disposal of Fixed Acids
Buffer Systems
Buffers
Mechanisms for the Homeostasis of Hydrogen Ions
Intracellular Buffering
Alkali Generation
Buffer Systems of the Body
Transcellular Ion Shifts with Acute Acid Loading
Time-Frame of Compensatory Responses to Acute Acid Loading
Quantifying Buffering
Buffering in Respiratory Acidosis
Regeneration of the Buffer
Buffering in Alkalosis
Site Buffering
Isohydric Principle
Base-Buffering by the Bicarbonate Buffer System
Bone Buffering
Role of the Liver in Acid-Base Homeostasis

pH

Hydrogen Ion Activity
Definitions of the Ad-hoc Committee of New York Academy of Sciences, 1965
Acidosis and Alkalosis
The Law of Mass Action
Dissociation Constants
pK
The Buffering Capacity of Acids
- Buffering Power
The Modified Henderson-Hasselbach Equation
The Difficulty in Handling Small Numbers
The Puissance Hydrogen
Why pH?
Relationship Between pH and H+
Disadvantages of Using a Logarithmic Scale
pH in Relation to pK
Is the Carbonic Acid System an Ideal Buffer System?
The Bicarbonate Buffer System Is Open Ended
Importance of Alveolar Ventilation to the Bicarbonate Buffer System
Difference Between the Bicarbonate and Non-bicarbonate Buffer Systems
Measuring and Calculated Bicarbonate

Acidosis and Alkalosis

Compensation
Coexistence of Acid Base Disorders
Conditions in Which pH Can Be Normal
The Acid Base Map

Respiratory Acidosis

Respiratory Failure
The Causes of Respiratory Acidosis
Acute Respiratory Acidosis: Clinical Effects
Effect of Acute Respiratory Acidosis on the Oxy-hemoglobin Dissociation Curve
Buffers in Acute Respiratory Acidosis
Respiratory Acidosis: Mechanisms for Compensation
Compensation for Respiratory Acidosis
Post-hypercapnic Metabolic Alkalosis
Acute on Chronic Respiratory Acidosis
Respiratory Acidosis: Acute or Chronic?

Respiratory Alkalosis

Respiratory Alkalosis
Electrolyte Shifts in Acute Respiratory Alkalosis
Causes of Respiratory Alkalosis
Miscellaneous Mechanisms of Respiratory Alkalosis
Compensation for Respiratory Alkalosis
Clinical Features of Acute Respiratory Alkalosis

Metabolic Acidosis

The Pathogenesis of Metabolic Acidosis
The pH, PCO₂ and Base Excess: Relationships
The Law of Electroneutrality and the Anion Gap
Electrolytes and the Anion Gap
Electrolytes That Influence the Anion Gap
The Derivation of the Anion Gap
Calculation of the Anion Gap
Causes of a Wide-Anion-Gap Metabolic Acidosis
The Corrected Anion Gap (AGc)
Clues to the Presence of Metabolic Acidosis
Normal Anion-Gap Metabolic Acidosis
Pathogenesis of Normal-Anion Gap Metabolic Acidosis
Negative Anion Gap
Systemic Consequences of Metabolic Acidosis
Other Systemic Consequences of Metabolic Acidosis
Hyperkalemia and Hypokalemia in Metabolic Acidosis
Compensatory Response to Metabolic Acidosis
Compensation for Metabolic Acidosis
Total CO₂ (TCO₂)
Altered Bicarbonate Is Not Specific for a Metabolic Derangement
Actual Bicarbonate and Standard Bicarbonate
Relationship Between ABC and SBC
Buffer Base
Base Excess
Ketosis and Ketoacidosis
Acidosis in Untreated Diabetic Ketoacidosis
Acidosis in Diabetic Ketoacidosis Under Treatment
Renal Mechanisms of Acidosis
L-Lactic Acidosis and D-Lactic Acidosis
Diagnosis of Specific Etiologies of Wide Anion Gap Metabolic Acidosis
Pitfalls in the Diagnosis of Lactic Acidosis
Renal Tubular Acidosis
Distal RTA
Mechanisms in Miscellaneous Causes of Normal Anion Gap Metabolic Acidosis
Toxin Ingestion
Bicarbonate Gap (the Delta Ratio)
Urinary Anion Gap
Utility of the Urinary Anion Gap
Osmoles
Osmolarity and Osmolality
Osmolar Gap
Abnormal Low Molecular Weight Circulating Solutes
Conditions That Can Create an Osmolar Gap

Metabolic Alkalosis

Etiology of Metabolic Alkalosis
Pathways Leading to Metabolic Alkalosis
Maintenance Factors for Metabolic Alkalosis
Maintenance Factors for Metabolic Alkalosis: Volume Contraction
Maintenance Factors for Metabolic Alkalosis: Dyselectrolytemias
Compensation for Metabolic Alkalosis
Urinary Sodium
Diagnostic Utility of Urinary Chloride (1)
The Diagnostic Utility of Urinary Chloride (2)
Diagnostic Utility of Urinary Chloride (3)
Some Special Causes of Metabolic Alkalosis
Metabolic Alkalosis Can Result in Hypoxemia
Metabolic Alkalosis and the Respiratory Drive

The Analysis of Blood Gases

Normal Values
- Venous Blood Gas (VBG) as a Surrogate for ABG Analysis
Step 1: Authentication of Data
Step 2: Characterization of the Acid-Base Disturbance
Step 3: Calculation of the Expected Compensation
The Alpha-Numeric (a-1) Mnemonic
The Metabolic Track
The Respiratory Track
Step 4: The 'Bottom Line': Clinical Correlation
- Clinical Conditions Associated with Simple Acid-Base Disorders
- Mixed Disorders
Acid-Base Maps

Factors Modifying the Accuracy of ABG Results

Electrodes
Accuracy of Blood Gas Values
The Effects of Metabolizing Blood Cells
Leucocyte Larceny
The Effect of an Air Bubble in the Syringe
Effect of Over-Heparization of the Syringe
The Effect of Temperature on the Inhaled Gas Mixture
Effect of Pyrexia (Hyperthermia) on Blood Gases
Effect of Hypothermia on Blood Gases
Plastic and Glass Syringes

Case Examples

Patient A: A 34 year-old man with Metabolic Encephalopathy
Patient B: A 40 year-old man with Breathlessness
Patient C: A 50 year-old woman with Hypoxemia
Patient D: A 20 year-old woman with Breathlessness
Patient E: A 35 year-old man with Non-resolving Pneumonia
Patient F: A 60 year-old man with Cardiogenic Pulmonary Edema
Patient G: A 72 year-old Drowsy COPD Patient
Patient H: A 30 year-old man with Epileptic Seizures
Patient I: An Elderly Male with Opiate Induced Respiratory Depression
Patient J: A 73 year-old man with Congestive Cardiac Failure
Patient K: A 20 year-old woman with a Normal X-ray
Patient L: A 22 year-old man with a Head Injury
Patient M: A 72 year-old man with Bronchopneumonia
Patient N: A 70 year-old woman with a Cerebrovascular Event
Patient O: A 60 year-old man with COPD and Cor Pulmonale
Patient P: A 70 year-old smoker with Acute Exacerbation of Chronic Bronchitis
Patient Q: A 50 year-old man with Hematemesis
Patient R: A 68 year-old man with an Acute Abdomen
Patient S: A young woman with Gastroenteritis and Dehydration
Patient T: A 50 year-old woman with Paralytic Ileus
Patient U: An 80 year-old woman with Extreme Weakness
Patient V: A 50 year-old man with Diarrhea
Patient W: A 68 year-old woman with Congestive Cardiac Failure
Patient X: An 82 year-old woman with Diabetic Ketoacidosis
Patient Y: A 50 year-old male in Cardiac Arrest
Patient Z: A 50 year-old Diabetic with Cellulitis

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