Acid-Base_Fars - Revisi.pdf

Acid-Base Balance

Normal Acid-Base Balance

Normal pH 7.35-7.45

Narrow normal range

Compatible with life 6.8 - 8.0 (7.0

6.8)

___/______/___/______/___

6.8 7.35 7.45 8.0

Acid Alkaline

pH & H+ Concentration

of Body Fluid Body Fluid H+ concentration

mEq/L

pH

Extracellular Fluids

Artery Vein Interstitial

4,0 x 10-5

4,5 x 10-5

4,5 x 10-5

7.40

7.35

7.35

Intracellular Fluid 1 x 10-3 s.d 4 x 10-5 6.0 7.4

Urine 3 x 10-2 s.d 1 x 10-5 4.5 8.0

Gastric Acid (HCl) 160 0.8

Handerson-Hasselbalch Equation

pH = - log10[H+]

pH = pK + log HCO3-

(0.03 x PCO2)

pH = pK + log HCO3-

(0.03 x PCO2)

Maintenance of Balance

Balance maintained by:

Buffering systems

Lungs

Kidneys

Buffer Systems

Prevent major changes in pH

Act as sponges

3 main systems 1. Bicarbonate-carbonic acid buffer

2. Phosphate buffer

3. Protein buffer

H+

H+ H+

Buffer Systems

1. Bicarbonate buffer - most important

Active in ECF and ICF

2. Phosphate buffer

Active in intracelluar (ICF) fluid

3. Protein buffer - Largest buffer store

Albumins and globulins (ECF)

Hemoglobin (ICF)

Bicarbonate-Carbonic Acid

Bodys major buffer (90%)

Also called as : Base Spare

Carbonic acid - H2CO3 (Acid)

Bicarbonate - HCO3 (Base)

1 20

pH = 7.4

H2CO3 HCO3

24 mEq/L 1.2 mEq/L

Bicarbonate-Carbonic Acid

Ratio important

Not absolute values

Person with COPD (CAL)

1 20

7.4

H2CO3 HCO3

48 mEq/L 2.4 mEq/L

Regulation

Key concept

Carbonic anhydrase equation

CO2 +H2O H2CO3 H+ + HCO3

Carbon Carbonic Bicarbonate

Dioxide Acid

(ACID) (BASE)

Acid

Substance that contains H+ ions

that can be released (H2CO3)

Carbonic acid releases H+ ions

Base

Substance that can accept H+ ions

(HCO3)

Bicarbonate accepts H+ ions

As CO2 increases, carbonic acid increases, H+ ions increase

pH drops.. becomes more acidic


Carbonic Bicarbonate

Acid

CO2 H2CO3 H+ HCO3

(pH Acidic

As HCO3 increases, H+ decreases

pH rises, becomes more alkaline


Carbonic Bicarbonate

Acid

CO2 H2CO3 H+ HCO3

(pH Basic >7.45)

Respiratory & Renal

Regulation

Lungs control CO2

Kidneys control HCO3

pH = kidneys (bicarbonate)

lungs (carbon dioxide)

Respiratory Regulation Mechanisms of control

Hyperventilation -- blow off CO2

Hypoventilation -- retain CO2

Regulation rapid...

Seconds to minutes

Measured by PaCO2 - Normal

35-45 mm Hg

Renal Regulation

Mechanism of control

Excretion or retention of

H+ or HCO3

Regulation.. Slow

Hours to days to change pH

Normal serum HCO3

22-26 mEq/L

Renal Regulation of Acid-Base

Acid Base Imbalance

Four Basic Types of Imbalance

Respiratory Acidosis

Respiratory Alkalosis

Metabolic Acidosis

Metabolic Alkalosis

Acid-Base Imbalances

Primary cause or origin:

Metabolic

Changes brought about by systemic

alterations (cellular level)

Respiratory

Changes brought about by

respiratory alterations


Compensation

Corrective response of kidneys and/or lungs

Compensated

Restoration of pH and 20 : 1 ratio

Uncompensated

Inability to adjust pH or 20 : 1 ratio

Compensation

When a primary acid-base disorder exists, the body

attempts to return the pH to normal via the other

half of acid base metabolism.

Primary metabolic disorder Respiratory compensation

Primary respiratory disorder Metabolic compensation

Compensation (continued)

Primary Disorder Compensatory Mechanism

Metabolic acidosis Increased ventilation

Metabolic alkalosis Decreased ventilation

Respiratory acidosis Increased renal reabsorption of HCO3-

in the proximal tubule

Increased renal excretion of H in the

distal tubule

Respiratory alkalosis Decreased renal reabsorption of HCO3-

in the proximal tubule

Decreased renal excretion of H+ in the

distal tubule


Ratio of 20 to 1 out of balance

Acidosis (acidemia)

pH falls below 7.35

Increase in blood carbonic acid or

Decrease in bicarbonate


Carbonic acid excess

Exhaling of CO2

inhibited

Carbonic acid builds up

pH falls below 7.35

Cause = Hypoventilation (see chart)

H2CO3


Normal

1 20

7.4

H2CO3 HCO3

24 mEq/L 1.2 mEq/L


1 13

7.21


Compensation: How?

Opposite regulating mechanism

Problem = depressed breathing,

build up of CO2 in blood

Response - Kidney retains HCO3

(Response .. Slow)


Carbonic acid deficit

Increased exhaling

of CO2

Carbonic acid decreases

pH rises above 7.45

Cause = hyperventilation (see chart)

H2CO3


Normal

1 20

7.4

H2CO3 HCO3

24 mEq/L 1.2 mEq/L


1 40

7.70


Compensation:

Problem = excess blowing off

of CO2

Result = decrease in carbonic

acid and increase in HCO3

Response: Kidney excretes excess

bicarbonate

Metabolic Acidosis

Base-bicarbonate deficit

Low pH (< 7.35)

Low plasma bicarbonate (base)

Cause = relative gain in H+ (lactic acidosis, ketoacidosis)

or actual loss of HCO3

(renal failure, diarrhea)


Normal

1 20

7.4

H2CO3 HCO3

24 mEq/L 1.2 mEq/L

Metabolic Acidosis Kidney failure (decrease in bicarbonate)

1 10

7.10

Metabolic Acidosis Lactic acidosis, keto acidosis (increase

acid no change in bicarbonate)

1 10

7.10

Metabolic Acidosis

Compensation:

Problem = low HCO3 (base) or high H+ ion (acid)

Response: Lungs hyperventilate

Get rid of CO2

(decrease PaCO2 and therefore raise level of HCO3)

Metabolic Alkalosis

Bicarbonate excess

High pH (> 7.45)

Loss of H+ ion or gain of HCO3

Most common causes vomiting,

gastric suctioning (NG tube)

Other: Abuse of antacids,

K+ wasting diuretics


Normal

1 20

7.4

H2CO3 HCO3

24 mEq/L 1.2 mEq/L

Metabolic Alkalosis

1 30

7.58

Metabolic Alkalosis

Compensation:

Problem = too much base

Response: Lungs compensate by

hypoventilating

Retain CO2, increase PaCO2

Increase acid level in blood

Assessing ABGs

pH 7.35 - 7.45

PaCO2 35 - 45 mmHg

HCO3 22 - 26 mEq/L

Base Excess -2 - +2 mEq/L

PaO2 80 - 100 mm Hg

O2 saturation 95 - 100 %

Interpreting ABGs 1. Start with pH

Normal?

Acidosis?

Alkalosis?

___/______/___/______/___ 6.8 7.35 7.45 8.0

Acidosis Alkalosis

Interpreting ABGs

2. Assess PaCO2

(respiratory value)

_____/________/______

35 45

Respiratory Respiratory

Alkalosis Acidosis (Note reversal)

(See Chart)

Interpreting ABGs

3. Evaluate metabolic indicators

Bicarbonate (HCO3) 22-26

and

Base excess (-2 to +2)

Interpreting ABGs HCO3

_______/_______/________

22 26

BE ______/_______/_________

-2 +2

Metabolic Metabolic

acidosis alkalosis

Interpreting ABGs

4. Determine level of compensation

Has the body tried to readjust the pH?

Uncompensated

Partly compensated

Compensated

Interpreting ABGs

Uncompensated

pH abnormal (high or low)

One component abnormal (high or low CO2 or HCO3)

The other component is normal

(The component not causing the acid-base imbalance is still normal)

Partly compensated

pH not normal (but moving toward normal)

Both CO2 and HCO3 are outside normal range

The component that was normal is changing in order to compensate

Interpreting ABGs

Compensated

pH normal

Other values abnormal in

opposite directions

One is acidotic the other alkaline

Interpreting ABGs

Determine amount of hypoxemia present

Normal PaO2 (adults - room air)

< 70 years = 80-100 mm Hg

70-79 = 70-100 mm Hg

Drops 10 mm Hg for each decade

Interpreting ABGs

Hypoxemia = < 70 mm Hg

(for adult < 70 years old)

Mild = 60-80 mm Hg

Moderate = 40-60 mm Hg

Severe = < 40 mm Hg

Interpreting ABGs

Oxygen saturation (pulse

oximetry)

95-100%

< 91% confusion

< 70% life threatening

Practice Problem

80 year old female with severe pneumonia, fever

pH = 7.25

PaCO2 = 55 mm Hg

HCO3 = 24 mEq/L

PaO2 = 65 mm Hg

O2 sat = 80%

Practice Problems

What is the problem?

Acidosis or alkalosis?

Respiratory or metabolic?

Compensated or not?

Level of hypoxemia?

Diagnoses?

Interventions?

Acid-Base_Fars - Revisi.pdf

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