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