Respiratory acidosis and alkalosis › emp › studym › 99991725.pdf · Respiratory Alkalosis...

RESPIRATORY

ACIDOSIS AND

ALKALOSIS

Introduction

Body fluids must maintain a normal acid-base

balance to sustain life.

Acids are substance which can release hydrogen

ion whereas A base is an ion or a molecule that

can accept an H+.

H+ regulation is essential because the activities of

almost all enzyme systems in the body are

influenced by H+ concentration

Acid-Base homeostasis is concerned about the

proper balancing of acid and bases, also called

the pH

Hydrogen Ions

H+ is produced as a by-product of

metabolism.

[H+] is maintained in a narrow range.

Normal arterial pH is around 7.4.

A pH under 7.0 or over 7.8 is compatible

with life for only short periods.

pH

pH is a measure of how acidic or basic a

chemical solution is.

Normally, pH runs from 0 to 14.

pH = -log10[H+]

A normal [H+] of 40 nEq/L

corresponds to a pH of

7.40.

Because the pH is a

negative logarithm of the

[H+], changes in pH are

inversely related to

changes in [H+ ]

(e.g., a decrease in pH is

associated with an

increase in [H+ ]).

The body and pH.

Hydrogen Ion Regulation

The body maintains a narrow pH range by 3

mechanisms:

1. Chemical buffers (extracellular and intracellular) react instantly to compensate

for the addition or subtraction of H+ ions.

2. Respiratory mechanism- CO2 elimination is

controlled by the lungs.

3. Renal Mechanism - HCO3- elimination is

controlled by the kidneys.

Chemical Buffer System

Provide or remove H+ and stabilize the

pH.

Include weak acids that can donate

H+ and weak bases that can absorb

H+.

Change in pH, after addition of acid,

is less than it would be in the absence

of buffer.

Buffer System

Central Equation of Acid-

Base Physiology The hydrogen ion concentration [H+ ] in

extracellular fluid is determined by the

balance between the partial pressure of

carbon dioxide (PCO2) and the

concentration of bicarbonate [HCO3- ] in

the fluid.

This relationship is expressed as follows:

[H+] = 24 x (PCO2 / [HCO3 -] )

Acid base disorder

Normal acid base values

pH pCO2 HCO3-

Range 7.35- 7.45 36-44 22-26

Optimal

Value

7.4 40 24

Acid base disorder

Acidosis – process that increases [H+] by increasing PCO2 or by reducing [HCO3-]

decrease in the blood pH below normal range

Alkalosis – process that reduces [H+] by reducing

PCO2 or by increasing [HCO3-]

Elevation in blood pH above the normal range

Respiratory Acid base disorder

Respiratory acid-base disorders are those

abnormalities in acid-base equilibrium

initiated by a change in the arterial carbon

dioxide tension (PaCO2 )--the respiratory

determinant of acidity in the following

equation:

Kassirer-Bleich equation:

[H+] = 24 × PCO2 / [HCO3-]

(Remember this formula !!!!!)

Respiratory Acid base disorder

There are two respiratory acid-base

disorders:

Respiratory acidosis and

Respiratory alkalosis.

Respiratory Acidosis

Respiratory acidosis is the acid-base disturbance initiated by an increase in PaCO2

which decreases the pH below the normal range (pH< 7.35)

An increase in arterial pCO2 can occur by one of three possible mechanisms:

Presence of excess CO2 in the inspired gas

Decreased alveolar ventilation

Increased production of CO2 by the body


Overproduction of carbon dioxide is usually

matched by increased excretion (due to

increased alveolar ventilation)

Most cases of respiratory acidosis reflect a

decrease in alveolar ventilation.

Alveolar hypoventilation leads to an

increased PaCO2 (ie, hypercapnia).

The increase in PaCO2, in turn, decreases

the bicarbonate (HCO3–)/PaCO2 ratio,

thereby decreasing the pH.

Types of Respiratory

Acidosis

Acute Respiratory Acidosis

Chronic Respiratory Acidosis

Acute Respiratory Acidosis

In acute respiratory acidosis, the PaCO2 is

elevated above the upper limit of the reference

range (over 45 mm Hg) with an accompanying

acidemia (pH <7.35).

failure in ventilation may be caused by

depression of the central respiratory center,

inability to ventilate adequately due to

neuromuscular disease, or airway obstruction

related to asthma or chronic obstructive

pulmonary disease (COPD) exacerbation.

The expected change in pH with respiratory

acidosis can be estimated with the following

equations:

Acute respiratory acidosis –

Change in pH = 0.008 × (PaCO 2 - 40)


In chronic respiratory acidosis, the PaCO2 is elevated above the upper limit of the reference

range, with a normal or near-normal blood pH

secondary to renal compensation and an elevated

serum bicarbonate (HCO3− >30 mm Hg).

Chronic respiratory acidosis may be secondary to

neuromuscular disorders, and severe restrictive

ventilatory defects as observed in interstitial fibrosis

and thoracic deformities



acidosis can be estimated with the following

equations:

Chronic respiratory acidosis –

Change in pH = 0.003 × (PaCO 2 - 40)

Compensatory mechanism for

Acute respiratory acidosis It is completed within 5-10 min from onset of

hypercapnia.

It originates exclusively from acidic titration by the

body’s protein buffers (hemoglobin, intracellular

proteins and phosphates, plasma proteins):

CO2 + H2O ↔ H2CO3 ↔ HCO3- + H+

H+ + Buf- ↔ HBuf

On average, plasma bicarbonate concentration

increases by about 1 mEq/L for each 10 mmHg

acute increment in PaCO2


Chronic respiratory acidosis

It requires 3-5 days of sustained hypercapnia for

completion.

It originates from renal acidification mechanisms (both

in the proximal and distal segments of the nephron) that

result in:

A transient increase in urinary net H+ excretion; and

A persistent increase in the rate of renal bicarbonate

reabsorption that maintains the increased plasma

bicarbonate level.

On average, plasma HCO3- concentration increases by

about 3 mEq/L for each 10 mm Hg increment in PaCO2

Respiratory Alkalosis

Respiratory alkalosis is the acid-base disturbance initiated by a reduction in PaCO2 which elevates the blood pH beyond the normal range (pH> 7.45)

This occurs when there is excessive loss of CO2 by alveolar hyperventilation.

Hypocapnia develops when a sufficiently strong ventilatory stimulus causes CO2 output in the lungs to exceed its metabolic production by the tissues.

As a result, partial pressure of CO2 and H+ conc. falls

Causes of Respiratory Alkalosis

By far, most cases of respiratory alkalosis reflect an

increase in alveolar ventilation.

moving into high altitude areas, where the low

atmospheric pressure of oxygen stimulates

increased ventilation

lung disease such as pneumonia

fever

pregnancy

decreased carbon dioxide production (e.g.,

sedation, skeletal muscle paralysis, hypothermia,

hypothyroidism) can cause respiratory alkalosis.

Types of Respiratory

Alkalosis

Acute Respiratory Alkalosis

Chronic Respiratory Alkalosis

Acute respiratory alkalosis

Acute respiratory alkalosis occurs rapidly.

During acute respiratory alkalosis, the person may lose consciousness

Compensation is completed within 5-10 min from onset of hypocapnia

It originates principally from alkaline titration by the body’s protein buffers (hemoglobin, intracellular proteins and phosphates, plasma proteins)

For every 10 mmHg drop in PCO2 in arterial blood, there is a corresponding 2 mEq/L drop in bicarbonate


alkalosis can be estimated with the following

equations:

Acute respiratory alkalosis:

Change in pH = 0.008 X (40 – PCO 2)

Chronic respiratory alkalosis

It is a long standing condition.

It requires 2-3 days of sustained hypocapnia for

completion.

Compensation originates from kidney by :

A transient decrease in urinary net H+

excretion

A persistent decrease in the rate of renal

bicarbonate reabsorption that maintains the

decreased plasma bicarbonate level.

For every 10 mmHg drop in PCO2 in arterial blood,

there is a corresponding 5 mEq/L drop in

bicarbonate ion.


alkalosis can be estimated with the following

equations:

Chronic respiratory alkalosis:



respiratory alkalosis Bicarbonate (HCO 3

-) falls 5 mEq/L for each

decrease of 10 mm Hg in the PCO 2; that is,

ΔHCO 3 = 0.5(ΔPCO 2)

The expected change in pH with respiratory alkalosis

can be estimated with the following equations:

Acute respiratory alkalosis:


Chronic respiratory alkalosis:


Identify?Respiratory Acidosis

or

Respiratory alkalosis

Arterial Blood Gas Analysis

An arterial-blood gas (ABG) test measures the

amounts of arterial gases, such as oxygen and

carbon dioxide.

An ABG test requires that a small volume of blood

be drawn from the radial artery with a syringe and

a thin needle,

Sometimes the femoral artery in the groin or

another site is used.

Getting an

arterial

blood gas

sample

Steps for

Successful

Blood Gas

Analysis

STEP 0 • Is this ABG report Authentic?

STEP 1 • ACIDEMIA or ALKALEMIA?

STEP 2• RESPIRATORY or METABOLIC?

STEP 3 • If Respiratory –ACUTE or CHRONIC?

STEP 4 • Is COMPENSATION adequate?

STEP 5 • If METABOLIC –ANION GAP?

Is this ABG authentic ? Henderson-Hasselbalch equation

pH = 6.1 + log HCO3-

0.03 x PCO2

pHexpected = pHmeasured = ABG is authentic

[H+] meq/l = 24 X (PCO2 / HCO3)

H+ ion pH100 7.00

79 7.10

63 7.20

50 7.30

45 7.35

40 7.40

35 7.45

32 7.50

25 7.60

Acidosis or Alkalosis?

pH < 7.35 acidosis

pH > 7.45 alkalosis

This is usually the primary disorder

Respiratory or Metabolic?

Is Primary Disturbance Respiratory or Metabolic?

pH & PCO2 or pH & PCO2 RESPIRATORY

pH & HCO3 or pH & HCO3 METABOLIC

In primary respiratory disorders, the pH and PaCO2

change in opposite directions

in metabolic disorders the pH and HCO3- change in the same direction.

If Respiratory –Acute or Chronic?

Acute respiratory disorder - ∆pH(e-acute) = 0.008 x ∆Pco2

Chronic respiratory disorder - ∆pH(e-chronic)= 0.003 x ∆pCO2

.08 change in pH ( Acute )

.03 change in pH ( Chronic )

10 mm Change PaCO2

=

Case: 6 year old male with progressive respiratory distress

Blood Gas Report

Measured 37.0o

C

pH 7.301

PaCO2 76.2 mm Hg

PaO2 45.5 mm Hg

Calculated Data

HCO3 act 35.1 mmol / L

O2 Sat 78 %

PO2 (A - a) 9.5 mm Hg D

PO2 (a / A) 0.83

Entered Data

FiO2 21 %

pH <7.35 :acidemia

Res. Acidemia : High PaCO2 and low pH

D CO2 =76-40=36

Expected D pH for ( Acute ) = .08 for 10

.08 × 3.6= 0.29

Expected ( Acute ) pH = 7.40 - 0.29=7.11

Chronic resp. acidosis

Chronic respiratory acidosis

With hypoxia due to hypoventilation

With renal compensation

HCO3 elevated

Respiratory acidosis and alkalosis › emp › studym › 99991725.pdf · Respiratory Alkalosis...

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