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Acid-Base Disturbance Department of Pathophysiology Shanghai Jiao-Tong University School of...

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Acid-Base Disturbance Acid-Base Disturbance Department of Pathophysiology Department of Pathophysiology Shanghai Jiao-Tong University School of Medicine Shanghai Jiao-Tong University School of Medicine
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PowerPoint Templatemain topics
Acid Base parameter/Arterial Blood Gases (ABGs)
Clinical Acid Base disorders
Mixed Acid/Base disorders
Acid Base disturbances
Acid Base balance
Acid-base balance refers to the mechanisms the body uses to keep its fluids close to neutral pH (that is, neither basic nor acidic) so that the body can function normally.
Arterial blood pH is normally closely regulated to between 7.35 and 7.45.
acids
bases
Any ionic or molecular substance that can act as a proton donor.
Strong acidHCl, H2SO4, H3PO4.
Weak acidH2CO3, CH3COOH.
Any ionic or molecular substance that can act as a proton acceptor.
Strong alkaliNaOH, KOH.
Lactic acid
Ketone bodies
Sulfuric acid
Phosphoric acid
Intracellular metabolism
Volatile acids
Fixed acids
Origin of acids
pH
- pH of ECF is between 7.35 and 7.45. Deviations, outside this range affect membrane function, alter protein function, etc.
- You cannot survive with a pH <6.8 or >7.7
Acidosis- below 7.35
Alkalosis- above 7.45
irregularities, heart failure, peripheral
vasodilation, drop in Bp.
Given that normal body pH is slightly alkaline and that normal metabolism produces acidic waste products such as carbonic acid (carbon dioxide reacted with water) and lactic acid, body pH is constantly threatened with shifts toward acidity.
In normal individuals, pH is controlled by two major and related processes; pH regulation and pH compensation. Regulation is a function of the buffer systems of the body in combination with the respiratory and renal systems, whereas compensation requires further intervention of the respiratory and/or renal systems to restore normalcy.
H load
intercellular NaHCO3/ H2CO3 Na2HPO4/NaH2PO4
organic acids
pH 6.1 log 24 /0.226·5.32
pH 6.1 log 24 / 1.2
pH 6.1 1.3
pH 7.4
: the factor which relates PCO2 to the amount of CO2 dissolved in plasma
plasma
RBC
Na+-H+ exchange of proximal tubule.
H+ secretion in collecting tubule is mediated by H+ ATPase pump in luminal membrane and a Cl-HCO3- exchanger in basolateral membrane. The H+ ATPase pump is influenced by aldosterone, which stimulates increased H+ secretion.
Hydrogen ion secretion in the collecting tubule is the process primarily responsible for acidification of the urine, particularly during states of acidosis. The urine pH may fall as low as 4.0.
Bicarbonate
Reabsorption
Excretion of titratable acids is dependent on the quantity of phosphate filtered and excreted by the kidneys, which is dependent on one's diet, and also PTH levels. As such, the excretion of titratable acids is not regulated by acid base balance and cannot be easily increased to excrete the daily acid load.
NH4+ excretion
The major adaptation to an increased acid load is increased ammonium production and excretion. Because the rate of NH4+  production and excretion can be regulated in response to the acid base requirements of the body.
The process of ammoniagenesis occurs within proximal tubular cells.
The generation of new HCO3¯ ions is probably the most important feature of this process.
Ammoniagenesis
Summary
uBuffers only provide a temporary solution.
uKidney: are the ultimate H+ ions balance. Slow acting mechanisms can eliminate any imbalance in H+ levels.
uLung: responds rapidly to altered plasma H+ concentrations, and keep blood levels under control until the kidneys eliminate the imbalance.
Acid base disturbance
Definition of acid-base disorders
An acid base disorder is a change in the normal value of extracellular pH that may result when renal or respiratory function is abnormal or when an acid or base load overwhelms excretory capacity.

Simple Acid-Base Disorders
Since PCO2 is regulated by respiration, abnormalities that primarily alter the PCO2 are referred to as respiratory acidosis (high PCO2) and respiratory alkalosis (low PCO2).
In contrast, [HCO3¯] is regulated primarily by renal processes. Abnormalities that primarily alter the [HCO3¯] are referred to as metabolic acidosis (low [HCO3¯]) and metabolic alkalosis (high [HCO3¯]).
Clinical disturbances of acid base metabolism classically are defined in terms of the HCO3¯  /CO2  buffer system.
Acidosis – process that increases [H+] by increasing PCO2 or by reducing [HCO3-]
Alkalosis – process that reduces [H+] by reducing PCO2 or by increasing [HCO3-]
Henderson Hasselbalch equation: 
Acid Base parameter
pH
pH is a measurement of the acidity of the blood, reflecting the number of hydrogen ions present.
pH = - log [H+]
pH7.45alkalosis
pH7.35acidosis
pH 7.35 - 7.45
Acid-base balance.
Acidosis or alkalosis with complete compensation.
A mixed acidosis and alkalosis, both events have opposite effects on pH, may also have a normal pH.
PaCO2
The amount of carbon dioxide dissolved in arterial blood.
Normal: 4.39 6.25kPa33 46 mmHg
Average: 5.32 kPa40 mmHg
Respiratory acidosis: > 46 mmHg (> 6 .25kPa)
Respiratory alkalosis: <33 mmHg (< 4.39 kPa)
The PaCO2 reflects the exchange of this gas through the lungs to the outside, so it is called “respiratory parameter”.
SB, AB
These two parameters are designed for HCO3¯ concentration in plasma.
SB is measured under “standard condition”, AB is measured under “actual condition”. The difference between two cases is that the former rules out the respiratory effect on HCO3¯ concentration measurement, but the later does not.
HCO3¯
Metabolic acidosis: <22 mmol/L
Metabolic alkalosis: > 27 mmol/L
[Standard Bicarbonate: Calculated value. Similar to the base excess. It is defined as the calculated bicarbonate concentration of the sample corrected to a PCO2 of 5.3kPa (40mmHg).
BE (base excess)
The base excess indicates the amount of excess or insufficient level of bicarbonate in the system. (A negative base excess indicates a base deficit in the blood.) A negative base excess is equivalent to an acid excess.
Normal: -3 to +3 mmol/L
Metabolic acidosis: < -3 mmol/L
Metabolic alkalosis: > +3 mmol/L
Base excess (BE) is the mmol/L of base that needs to be removed to bring the pH back to normal when PCO2 is corrected to 5.3 kPa or 40 mmHg. During the calculation any change in pH due to the PCO2 of the sample is eliminated, therefore, the base excess reflects only the metabolic component of any disturbance of acid base balance.
AG (anion gap)
Anion gap = Na+ - [Cl¯ + HCO3¯]
Based on the principle of electrical neutrality, the serum concentration of cations (positive ions) should equal the serum concentration of anions (negative ions).
However, serum Na+ ion concentration is higher than the sum of serum Cl¯ and HCO3¯ concentration.
Na+ = Cl¯ + HCO3¯ + unmeasured anions (gap).
Normal: 122mmol/L (10 - 14 mmol/L)
These “undetermined anions” are generally accounted for by negatively charged proteins, phosphate, sulfate and organic anions. Except for a few relatively uncommon circumstances, an increase in the AG is synonymous with the accumulation of nonvolatile acids in body fluids, and suggests metabolic acidosis.
pH—Determine Acidosis versus alkalosis
Determine Metabolic
SB (standard bicarbonate)
BE (base excess)
AG — Helpful in Metabolic Acidosis
Helpful in mixed acid-base disorders
Once the acid-base disorder is identified as respiratory or metabolic, we must look for the degree of compensation that may or may not be occurring. This compensation may be complete (pH is brought into the normal range) or partial (pH is still out of the normal range but is in the process of moving toward the normal range.)
In pure respiratory acidosis (high PaCO2, normal [HCO3¯], and low pH) we would expect an eventual compensatory increase in plasma [HCO3¯] that would work to restore the pH to normal. Similarly, we expect respiratory alkalosis to elicit an eventual compensatory decrease in plasma [HCO3¯].
A pure metabolic acidosis (low [HCO3¯], normal PaCO2, and a low pH) should elicit a compensatory decrease in PaCO2, and a pure metabolic alkalosis (high [HCO3¯], normal PaCO2, and high pH) should cause a compensatory increase in PaCO2.
All compensatory responses work to restore the pH to the normal range (7.35 - 7.45)
Pathogenesis of Acid Base disorders
Metabolic acidosis
Consume
Primary [HCO3]
Normal AG
Metabolic alkalosis
Fixed acids
Exclusion
Generation
Feature HCO3-BB,SB,AB,BE() H2CO3 PaCO2 AB>SB
Blood HA + HCO3-A-+ H2CO3 plasma protein, RBC Hb
buffering (No compensation to acute
CO2 + H2O repiratory acidosis)
(Kussmaul Respiration)
[K+ ]e
ammonia production, this result in HCO3¯ reabsorption.
Bone Ca3(PO4)2 + 4H+3Ca2+ + 2H2PO4-
Results PaCO2, HCO3- recovery BB,SB,AB,BE(+)
In general, respiratory compensation results in a 1.2 mmHg reduction in PCO2  for every 1.0 meq/L reduction in the plasma HCO3- concentration down to a minimum PCO2 of 10 to 15mmHg.  
For example, if an acid load lowers the plasma HCO3- concentration to 9 meq/L, then:
Degree of HCO3- reduction is  24 (optimal value) – 9 = 15.
Therefore, PCO2 reduction should be  15 × 1.2 =  18.
 
Winter's Formula
To estimate the expected PCO2 range based on respiratory compensation, one can also use the Winter's Formula which predicts: PCO2 = (1.5 × [HCO3-]) + 8 ± 2
Therefore in the above example, the PCO2 according to Winter's should be
(1.5 × 9) + 8 ± 2 = 20-24
Another useful tool in estimating the PCO2 in metabolic acidosis is the recognition that the pCO2 is always approximately equal to the last 2 digits of the pH.
Compensatory Responses: Metabolic Acidosis
Feature HCO3-BB,SB,AB,BE() H2CO3 PaCO2 AB<SB
Blood limited effect on alkali HCO3- enter RBCCO2 diffuse in plasma
Buffering OH-+ H2CO3(HPr)HCO3-(Pr-)+ H2O HCO3-HBuf H2CO3Buf
Lung PHH+ deceased breathing
CO2 exhalation PaCO2 no compensation
ICF H+K+ exchange, [K+]
Buffering oxygen dissociation curve left shift, glucolysis , H+
Kidney excrete the excess load of HCO3¯
Results H2CO3HCO3- recovery chronicBBSB BE(-)
On average the pCO2 rises 0.7 mmHg for every 1.0 meq/L increment in the plasma [HCO3-].
For example, if an alkali load raises the the plasma HCO3- concentration to 34 meq/L, then:
Degree of HCO3- elevation is  34 – 24 (optimal value)= 10.
Therefore, PCO2 elevation should be  0.7 × 10 =  7.
Then PCO2 measured should be 40 (optimal value) +7 = 47mmHg.
Compensatory Responses: Metabolic Alkalosis
Effects of acidosis
Respiratory Effects
Hyperventilation ( Kussmaul respirations) Shift of oxyhaemoglobin dissociation curve to the right Decreases 2,3 DPG levels in red cells, which opposes the effect above. (shifts the ODC back to the left) This effect occurs after 6 hours of acidemia.
Cardiovascular Effects
Central Nervous System Effects
Cerebral vasodilation leads to an increase in cerebral blood flow and intracranial pressure (occur in acute respiratory acidosis) Very high pCO2 levels will cause central depression
Other Effects
Increased bone resorption (chronic metabolic acidosis only) Shift of K+ out of cells causing hyperkalemia (an effect seen particularly in metabolic acidosis and only when caused by non organic acids) Increase in extracellular phosphate concentration
Increased rate and depth of breathing ("Kussmaul breathing")
Decreased heart rate (bradycardia)
Respiratory Effects
Shift of oxyhaemoglobin dissociation curve to the left (impaired unloading of oxygen The above effect is however balanced by an increase in 2,3 DPG levels in RBCs. Inhibition of respiratory drive via the central & peripheral chemoreceptors
Cardiovascular Effects
Central Nervous System Effects
Cerebral vasoconstriction leads to a decrease in cerebral blood flow (result in confusion, muoclonus, asterixis, loss of consciousness and seizures) Only seen in acute respiratory alkalosis. Effect last only about 6 hours. Increased neuromuscular excitability ( resulting in paraesthesias such as circumoral tingling & numbness; carpopedal spasm) Seen particularly in acute respiratory alkalosis.
Other Effects
Causes shift of hydrogen ions into cells, leading to hypokalemia.
Mixed acid base disorders
The simple, or primary, acid-base disorders (respiratory and metabolic acidosis and alkalosis) evoke a compensatory response that produces a secondary acid-base disturbance and reversion of the blood pH towards (rarely to) normal; e.g., a simple metabolic acidosis will result in a secondary respiratory alkalosis, both of which will ordinarily be reflected in the patients’ acid-base-related analytes in blood. When two primary acid-base disturbances arise simultaneously in the same patient, the complex is called a mixed acid-base disorder. If three primary disturbances occur together, the patient is described as having “triple acid-base disorder.”
More than one acid base disturbance present. pH may be normal or abnormal.
A 50 year old insulin dependent diabetic woman was brought to the ED by ambulance. She was semi-comatose and had been ill for several days. Current medication was digoxin and a thiazide diuretic for CHF.
Lab results
Serum chemistry: Na 132, K 2.7, Cl 79, Glu 815,
Lactate 0.9 urine ketones 3+
ABG: pH 7.41 PCO2 32 HCO3¯ 19 pO2 82  
Case study

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