Acid-Base Physiology
n
n
n
n
n
n
n
n
pH (puissance Hydrogen)
Acids and pH
Acid Buffering
Acid-Base Balance in the Body
Carbonic Acid-Bicarbonate Buffering
Total CO2
Base Excess
Base Excess of ECF
Effects of Acid-Base Balance on Oxygen
n
n
Buffer in the Blood
n
A buffer is any substance that can reversibly
bind hydrogen ions. The general form of the
buffering reaction is:
pH= -log [H+]
A normal pH in the human body is between 7.35 -7.45.
Buffers Present in the Blood
1. Proteins - High concentration - Very important
1
Proteins Buffer
System
Buffers Present in the Blood
2. Phosphate Buffer System
n
n
Plays a role in buffering intracellular and
tubular fluid
Extracellular fluid - low concentration - NOT
important
Buffers Present in the Blood
3. Bicarbonate Buffer System
Bicarbonate
Buffer System
2
Henderson-Hasselbalch
Equation
pH = pK + log HCO3- / H2CO3
or pH = pK + log HCO3- / 0.03 x PCO2
n
n
n
Renal Mechanisms
0.03 is the solubility coefficient for carbon dioxide and pK
is a constant whose value is 6.1.
A normal value for bicarbonate = 24 mEq/l and for
carbon dioxide = 40mmHg.
Therefore, pH = 6.1 + log 24 / 0.03 x 40
or pH = 6.1 + log 24 / 1.2
or pH = 6.1 + 1.3 or pH = 7.4
New
Bicarbonate
Bicarbonate
Re-absorption
3
Clinical Acid-Base Disturbances
n
n
n
n
n
n
n
Approach to the Abnormal Blood Gas
Metabolic Acidosis
Metabolic Alkalosis
Respiratory Acidosis
Respiratory Alkalosis
Therapy of Respiratory Acidosis
Therapy of Metabolic Acidosis
Eight Primary Blood Gas
Classifications
Nomenclature
Physiologic condition
Values
Classification
Acidosis
pH <7.35
Alkalosis
pH >7.45
1. Acute ventilatory failure (acute
respiratory acidosis)
Hypoxia
PaO2 <60
Hyperoxia
PaO2>100
Alveolar
Hyperventilation
Ventilatory Failure
pH PaCO2 HCO3v
^
N
2. Chronic ventilatory failure
(compensated respiratory acidosis)
vN
^
^
PaCO2 <35
3. Acute alveolar hyperventilation
(acute respiratory alkalosis)
^
v
N!
PaCO2 >50
4. Chronic alveolar hyperventilation
^N
(compensated respiratory alkalosis)
v
v
4
RENAL
COMPENSATION
Respiratory
Acidosis
Basic Causes of Abnormalities
n
Respiratory
Alkalosis
Acute ventilatory failure:
a. acute airway obstruction
b. severe pneumonia/pulmonary edema
c. neuromuscular disorders
d. CNS depression (drugs, CNS event)
e. ventilator dysfunction
n
Chronic ventilatory failure:
a. chronic lung diseases
5
Basic Causes of Abnormalities
n
Acute alveolar hyperventilation:
a. anxiety焦慮
b. hypoxia氧過少
c. drugs (aspirin, catecholamines,
progesterone)
d. sepsis敗血症
e. hepatic encephalopathy腦病變
RENAL
COMPENSATION
Eight Primary Blood Gas
Classifications
Classification
5. Acute metabolic acidosis
6. Chronic metabolic acidosis
7. Acute metabolic alkalosis
8. Chronic metabolic alkalosis
pH PaCO2 HCO3v
N!
v
v N!
v!
v
^
N!
^!
^N
^
^
Metabolic
Acidosis
6
Respiratory
Compensation
Effect of
hyperventilation
Basic Causes of Abnormalities
n
Metabolic acidoses:
a. ketoacidoses (diabetic/alcoholic)
b. renal failure
c. lactic acidosis
d. rhabdomyolysis橫紋肌溶解
e. toxins (methanol, ethylene glycol, salicylates,
paraldehyde)
阿斯匹靈類
f. diarrhea
g. HCl administration
7
Metabolic
Alkalosis
Respiratory
Compensation
Basic Causes of Abnormalities
n
Metabolic alkaloses:
a. vomiting/ NG suctioning嘔吐/ᆄ胃管抽取
b. diuretic therapy
c. mineralocorticoid activity (Cushing's
syndrome, exogenous steroids)
Eight Primary Blood Gas
Classifications
Classification
1. Acute ventilatory failure (acute
respiratory acidosis)
pH PaCO2 HCO3v
^
N
2. Chronic ventilatory failure
(compensated respiratory acidosis)
vN
^
^
3. Acute alveolar hyperventilation
(acute respiratory alkalosis)
^
v
N!
v
v
4. Chronic alveolar hyperventilation
^N
(compensated respiratory alkalosis)
8
Eight Primary Blood Gas
Classifications
Classification
5. Acute metabolic acidosis
6. Chronic metabolic acidosis
7. Acute metabolic alkalosis
Questions
pH PaCO2 HCO3v
N!
v
v N!
v!
v
^
N!
^!
^N
^
^
1.
2.
3.
4.
8. Chronic metabolic alkalosis
What are the three main blood buffer systems
and how do they operate?
How does the body try to compensate for acidbase disturbances?
If you know the blood pH of a patient, what
extra information does a knowledge of his/her
pCO2 provide?
What are the different types of acid-base
disturbance and how does use of a nomogram
help in interpretation?
•Questions
5.
6.
In what situations is the determination of
pO2 clinical useful?
What are the advantages of obtaining
pO2 and pCO2 by transcutaneous
measurement?
9