Transcript Acid base imbalance

Acid base imbalance
Objectives
• Define the terms acidosis and alkalosis.
• How to do blood gas interpretation
• Explain how the acid-base balance of the blood is
affected by C02 and HC03-, and describe the roles of the
lungs and kidneys in maintaining acid-base balance.
Acid-Base Balance
• It is the regulation of HYDROGEN
ions.
(The more Hydrogen ions, the more acidic the
solution and the LOWER the pH)
– The acidity or alkalinity of a solution
is measured as pH
4
HH equation
pH = 6.1 + log
[HCO3-]
0.03 x Pco2
Normal Arterial Blood Gas Values*
pH
PaCO2
PaO2
SaO2
HCO3¯
Base excess
** Age-dependent
7.35 - 7.45
35 - 45 mm Hg
70 - 100 mm Hg **
93 - 98%
22 - 26 mEq/L
-2.0 to 2.0 mEq/L
Types of Acids in the Body
-Volatile acids:
– Pco2 is most important factor in pH of body
tissues.
-Fixed Acids.
– Catabolism of amino acids, nucleic acids, and
phospholipids
-Organic Acids:
– Byproducts of aerobic metabolism, anaerobic
metabolism , during starvation, and diabetes.
– Lactic acid, ketones
Compensation=Buffer Systems
– Attempt to return the pH to normal or near
normal
• Provide or remove H+ and stabilize the pH.
• Include weak acids that can donate H+ and
weak bases that can absorb H+.
Compensation
-If the non primary system is in the normal range (CO2
35 to 45) (HCO3 22-26), then that system is not
compensating for the primary.
• For example:
– In respiratory acidosis (pH<7.35, CO2>45), if the HCO3 is
>26, then the kidneys are compensating by retaining
bicarbonate.
– If HCO3 is normal, then not compensating.
10
11
Acid-base Terminology
Acidemia: blood pH < 7.35
Acidosis: a primary physiologic process that,
occurring alone, tends to cause acidemia.
Examples: metabolic acidosis from decreased perfusion (lactic
acidosis); respiratory acidosis from hypoventilation.
Alkalemia: blood pH > 7.45
Alkalosis: a primary physiologic process that,
occurring alone, tends to cause alkalemia.
Examples: metabolic alkalosis from excessive diuretic therapy;
respiratory alkalosis from acute hyperventilation.
Primary Acid-base Disorders
•
•
•
•
Respiratory Acidosis
Respiratory Alkalosis
Metabolic Acidosis
Metabolic Alkalosis
Primary Acid-base Disorders:
Respiratory Acidosis
Respiratory acidosis - A primary disorder where the first change is
an elevation of PaCO2, resulting in decreased pH.
Compensation (bringing pH back up toward normal) is a
secondary retention of HCO3 by the kidneys; this elevation of
HCO3- is not metabolic alkalosis since it is not a primary process.
Primary Event
Compensatory Event
HCO3↑ HCO3↓ pH ~ --------- ↓ pH ~ --------↑PaCO2
↑ PaCO2
Respiratory Acidosis
• Mechanism
– Hypoventilation or Excess CO2 Production
• Etiology
– Pnumonia,Pneumothorax
– ARDS
– Respiratory Center Depression
– Inadequate mechanical ventilation
– Sepsis or Burns
– Neuromuscular Disease
Respiratory Acidosis (cont)
• Symptoms
•
•
•
•
Breathlessness,Restlessness
Lethargy and disorientation
Tremors, convulsions, coma
Skin warm and flushed due to vasodilation caused
by excess CO2
• Treatment
– Treat underlying cause
– Support ventilation
– Correct electrolyte imbalance .
Primary Acid-base Disorders:
Metabolic Acidosis
Metabolic acidosis - A primary acid-base disorder
where the first change is a lowering of HCO3-,
resulting in decreased pH.
Compensation (bringing pH back up toward normal)
is a secondary hyperventilation; this lowering of
PaCO2, Renal excretion of hydrogen ions & K+
exchanges
Primary Event Compensatory Event
↓ HCO3↓ pH ~ -----------PaCO2
↓HCO3-
↓ pH ~ -----------↓ PaCO2
Metabolic Acidosis (cont)
• Symptoms
– Kussmaul’s respiration
– Lethargy, confusion, headache, weakness
– Nausea and Vomiting
– Lab:
• pH below 7.35
• Bicarb less than 22
• Treatment
– treat underlying cause
– monitor ABG, I&O, VS, LOC
Sodium Bicarb?
Metabolic Acidosis
• Etiology
– Conditions that increase acids (lactic acid or
ketones)
•
•
•
•
Renal Failure
DKA
Starvation
Lactic acidosis
– Loss of bicarbonate through diarrhea or renal
dysfunction
– Accumulation of acids Failure of kidneys to
excrete H+
Anion Gap
Metabolic acidosis is conveniently
divided into elevated and normal
anion gap (AG) acidosis.
AG = Na+ - (Cl- + HCO3)
Normal AG is typically 12 ± 4 mEq/L. If AG is calculated
using K+, the normal AG is 16 ± 4 mEq/L
Metabolic acidosis and the anion gap
1. Normal gap
1.
Renal “HCO3”
losses
Proximal RTA
Distal RTA
2. GI “HCO3”
losses
Diarrhea
2. Increased gap
1.  Acid
prod
Lactate
DKA
Ketosis
Toxins
Alcohols
Salicylates
Iron
2.  Acid
elimination
Renal disease
MUDPILES
•
•
•
•
•
•
•
•
M
U
D
P
I
L
E
S
Primary Acid-base Disorders:
Metabolic Alkalosis
Metabolic alkalosis - A primary acid-base disorder
where the first change is an elevation of HCO3-, resulting
in increased pH.
Compensation is a secondary hypoventilation (increased
PaCO2), Compensation for metabolic alkalosis is less predictable
than for the other three acid-base disorders.
Primary Event
↑ HCO3↑ pH ~
PaCO2
Compensatory Event
↑HCO3↑ pH ~
------------
PaCO2
↑
---------
Metabolic Alkalosis
• Risk Factors/Etiology
– Acid loss due to
• vomiting
• gastric suction
– Loss of potassium due to
• steroids
• diuresis
– Antacids (overuse of)
Metabolic Alkalosis (cont)
• Symptoms
–
–
–
–
Hypoventilation (compensatory)
Dysrhythmias, dizziness
Paresthesia, numbness, tingling of extremities
Hypertonic muscles, tetany
– Lab:
–
–
pH above 7.45, Bicarb above 26
CO2 normal or increased w/comp
Hypokalmia, Hypocalcemia
• Treatment
– treat underlying cause
– I&O, VS, LOC
– give potassium
Primary Acid-base Disorders:
Respiratory Alkalosis
Respiratory alkalosis - A primary disorder where the first
change is a lowering of PaCO2, resulting in an elevated pH.
Compensation is a secondary lowering(excreting)HCO3 by the
kidneys.
Primary Event
HCO3↑ pH ~ ------↓ PaCO2
Compensatory Event
↓HCO3-
↑ pH ~ -------↓ PaCO2
Respiratory Alkalosis
Etiology
– Hyperventilation due to Conditions that stimulate
respiratory center
»
»
»
»
»
»
»
extreme anxiety, stress, or pain
Fever
overventilation with ventilator
hypoxia
salicylate overdose
hypoxemia (emphysema or pneumonia)
CNS trauma or tumor
Respiratory Alkalosis (cont)
• Symptoms
–
–
–
–
–
–
Tachypnea or Hyperpnea
Complaints of SOB, chest pain
Light-headedness, syncope, coma, seizures
Numbness and tingling of extremities
Difficult concentrating, tremors, blurred vision
Weakness, paresthesias, tetany
– Lab findings
– pH above 7.45
– CO2 less than 35
Respiratory Alkalosis (cont)
• Treatment
•
•
•
•
•
Monitor VS and ABGs
Treat underlying disease
Assist patient to breathe more slowly
breathe in a paper bag or apply rebreather mask
Sedation
Metabolic Acid-base Disorders:
summary
METABOLIC ACIDOSIS
↓HCO3- & ↓ pH
- Increased anion gap
• lactic acidosis; ketoacidosis; drug poisonings (e.g., aspirin,
ethylene glycol, methanol)
- Normal anion gap
• diarrhea; some kidney problems (e.g., renal tubular acidosis,
interstitial nephritis)
METABOLIC ALKALOSIS
↑ HCO3- & ↑ pH
Chloride responsive (responds to NaCl or KCl therapy): contraction
alkalosis, diuretics, corticosteroids, gastric suctioning, vomiting
Chloride resistant: any hyperaldosterone state (e.g., Cushing’s
syndrome, Bartter’s syndrome, severe K+ depletion)
Respiratory Acid-base Disorders:
summary
RESPIRATORY ACIDOSIS ↑PaCO2 & ↓ pH
Central nervous system depression (e.g., drug overdose)
Chest bellows dysfunction (e.g., Guillain-Barré syndrome,
myasthenia gravis)
Disease of lungs and/or upper airway (e.g., chronic obstructive lung
disease, severe asthma attack, severe pulmonary edema)
RESPIRATORY ALKALOSIS
↓PaCO2 & ↑ pH
Hypoxemia (includes altitude)
Anxiety
Sepsis
Any acute pulmonary insult (e.g., pneumonia, mild asthma attack, early
pulmonary edema, pulmonary embolism)
Mixed Acid-base Disorders are Common
In chronically ill respiratory patients, mixed disorders are
probably more common than single disorders, e.g., RAc +
MAlk, RAc + Mac, Ralk + MAlk.
In renal failure (and other conditions) combined MAlk + MAc
is also encountered.
Always be on the lookout for mixed acid-base disorders.
They can be missed!
Expected changes in pH and HCO3- for a 10-mm Hg change in
PaCO2 resulting from either primary hypoventilation (respiratory
acidosis) or primary hyperventilation (respiratory alkalosis):
ACUTE
CHRONIC
Resp Acidosis
pH ↓ by 0.07
HCO3- ↑ by 1*
pH ↓ by 0.03
HCO3- ↑ by 3 - 4
Resp Alkalosis
pH ↑ by 0.08
HCO3- ↓ by 2
* Units for HCO3- are mEq/L
pH ↑ by 0.03
HCO3- ↓ by 5
Predicted changes in HCO3- for a directional
change in PaCO2 can help uncover mixed
acid-base disorders.
a)
A normal or slightly low HCO3- in the presence of hypercapnia
suggests a concomitant metabolic acidosis, e.g., pH 7.27,
PaCO2 50 mm Hg, HCO3- 22 mEq/L. Based on the rule for
increase in HCO3- with hypercapnia, it should be at least 25
mEq/L in this example; that it is only 22 mEq/L suggests a
concomitant metabolic acidosis.
b)
A normal or slightly elevated HCO3- in the presence of
hypocapnia suggests a concomitant metabolic alkalosis, e.g.,
pH 7.56, PaCO2 30 mm Hg, HCO3- 26 mEq/L. Based on the
rule for decrease in HCO3- with hypocapnia, it should be at
least 23 mEq/L in this example; that it is 26 mEq/L suggests a
concomitant metabolic alkalosis.
Diagnosis of Acid-Base Imbalances
1. Look at the pH
• is the primary problem acidosis (low) or alkalosis (high)
2. Check the CO2 (respiratory indicator)
• is it less than 35 (alkalosis) or more than 45 (acidosis)
3. Check the HCO3 (metabolic indicator)
• is it less than 22 (acidosis) or more than 26 (alkalosis)
4. Which is primary disorder (Resp. or Metabolic)?
• If the pH is low (acidosis), then look to see if CO2 or HCO3 is
acidosis (which ever is acidosis will be primary).
• If the pH is high (alkalosis), then look to see if CO2 or HCO3 is
alkalosis (which ever is alkalosis is the primary).
• The one that matches the pH (acidosis or alkalosis), is the primary
disorder.
4. Look at the value that doesn’t
correspond to the observed pH
change. If it is inside the normal
range, there is no compensation
occurring. If it is outside the normal
range, the body is partially
compensating for the problem.
37
Case Study #1
HPI:
A 5 month-old girl presents with a one day history of irritability
and fever. Mother reports three days of “bad” vomiting and
diarrhea.
Home meds:
Acetaminophen and ibuprofen for fever
PE:
BP 70/40, HR 200, R 60, T38.3 C. Irritable, sunken eyes and
fontanelle, skin feels like Pillsbury Dough Boy
Case Study #1
Place IV line
Bolus 40 ml/kg of isotonic saline
Reassessment (HR 170, RR 40, BP 75/40)
Serum studies
Sodium 164 mEq/L
Chloride 139 mEq/L
Potassium 5.5 mEq/L
Bicarbonate 12 mEq/L
pH 7.07
pCO2 11
pO2 121
HCO3 8
BUN 75 mg/dL
Creatinine 3.1 mg/dL
Glucose 101 mg/dL
Case Study #1
What is the most likely explanation of
this patients acidosis?
Case Study #1
Metabolic acidosis and the anion
gap
Anion Gap
[Na+] – ([HC03-] + [Cl-])
164 - (12+139 ) = 13
Case Study #2
Metabolic acidosis and the anion gap
1. Normal gap
1.
Renal “HCO3” 2. GI “HCO3”
losses
losses
Proximal RTA
Distal RTA
Diarrhea
2. Increased gap
1.  Acid prod 2.  Acid elimination
Lactate
DKA
Ketosis
Toxins
Alcohols
Salicylates
Iron
Renal disease
Summary:
Clinical and Laboratory Approach to
Acid-base Diagnosis
Determine existence of acid-base disorder from arterial blood
PH Check serum HCO3,CO2; if abnormal, there is an acidbase disorder. If the anion gap is significantly increased, there
is a metabolic acidosis.
Examine pH, PaCO2, and HCO3- for the obvious primary acidbase disorder and for deviations that indicate mixed acid-base
disorders
Summary:
Clinical and Laboratory Approach to
Acid-base Diagnosis (cont.)
Use a full clinical assessment (history, physical
exam, other lab data including previous arterial blood
gases and serum electrolytes) to explain each acidbase disorder.
Treat the underlying clinical condition(s); this will
usually suffice to correct most acid-base disorders.
Clinical judgment should always apply
Acid-base Disorders:
Test Your Understanding
State whether each of the following statements is true or false.
a)
from
Metabolic acidosis is always present when the measured serum CO2 changes acutely
24 to 21 mEq/L.
b)
with
In acute respiratory acidosis, bicarbonate initially rises because of the reaction of CO2
water and the resultant formation of H2CO3.
c)
above
If pH and PaCO2 are both above normal, the calculated bicarbonate must also be
normal.
d)
An abnormal serum CO2 value always indicates an acid-base disorder of some type.
e)
The compensation for chronic elevation of PaCO2 is renal excretion of bicarbonate.
f)
acid-
A normal pH with abnormal HCO3- or PaCO2 suggests the presence of two or more
base disorders.
g)
A normal serum CO2 value indicates there is no acid-base disorder.
h)
Normal arterial blood gas values rule out the presence of an acid-base disorder.
Acid-base Disorders:
Test Your Understanding - Answers
a) false
b) true
c) true
d) true
e) false
f) true
g) false
50