RESPIRATORY MODULE

RESPIRATORY MODULE

FAWAD AHMAD RANDHAWA

MBBS ( King Edward Medical College)M.C.P.S; F.C.P.S. ( Medicine)

F.C.P.S. ( Endocrinology)Assistant Professor of Endocrinology

King Edward Medical University

Objectives

At the end of this session the participants should be able to:• Grasp a clear concept governing acid base homeostasis and

Clinical implementation of acid base concepts• Identify basic pathophysiology of respiratory failure

Learning outcomes

• Basic terminologies • Normal buffers of the body• Respiratory component of acid base homeostasis • Compensatory mechanisms in acid base disorders• Definition of respiratory failure with its types

The Body and pH

• Homeostasis of pH is tightly controlled• Extracellular fluid = 7.4• Blood = 7.35 – 7.45• < 6.8 or > 8.0 death occurs• Acidosis (acidemia) below 7.35• Alkalosis (alkalemia) above 7.45

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Terms

• Acid– Any substance that can yield a hydrogen ion (H+) or hydronium ion when

dissolved in water– Release of proton or H+

• Base– Substance that can yield hydroxyl ions (OH-)– Accept protons or H+

Terms

• Buffer– Combination of a weak acid and /or a weak base and its salt– What does it do?

• Resists changes in pH

Terms• Acidosis

– pH less than 7.35

• Alkalosis– pH greater than 7.45

• Note: Normal pH is 7.35-7.45

Acid-Base Balance

• Function– Maintains pH homeostasis– Maintenance of H+ concentration

• Potential Problems of Acid-Base balance– Increased H+ concentration yields decreased pH– Decreased H+ concentration yields increased pH

Blood Buffer Systems

• Why do we need them?– If the acids produced in the body from the catabolism of food and

other cellular processes are not removed or buffered, the body’s pH would drop

– Significant drops in pH interferes with cell enzyme systems.

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Small changes in pH can produce major disturbances

• Most enzymes function only with narrow pH ranges• Acid-base balance can also affect electrolytes (Na+, K+, Cl-)• Can also affect hormones

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The body produces more acids than bases

• Acids take in with foods• Acids produced by metabolism of lipids and proteins• Cellular metabolism produces CO2.

• CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3-

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Control of Acids

1. Buffer systems

Take up H+ or release H+ as conditions change

Buffer pairs – weak acid and a base

Exchange a strong acid or base for a weak one

Results in a much smaller pH change

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Regulation of pH• Weak acids good buffers since they can tilt a

reaction in the other direction• Strong acids are poor buffers because they make

the system more acid

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Respiratory mechanisms

• Exhalation of carbon dioxide• Powerful, but only works with volatile acids• Doesn’t affect fixed acids like lactic acid• CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3

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• Body pH can be adjusted by changing rate and depth of breathing

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Kidney excretion

• Can eliminate large amounts of acid• Can also excrete base• Can conserve and produce bicarb ions• Most effective regulator of pH• If kidneys fail, pH balance fails

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Rates of correction

• Buffers function almost instantaneously• Respiratory mechanisms take several minutes to hours• Renal mechanisms may take several hours to days

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Acid base abnormalities- Causes

• Respiratory – abnormal processes which tend to alter pH because of a primary

change in pCO2 levels• acidosis• alkalosis

• Metabolic– abnormal processes which tend to alter pH because of a primary

change in [HCO3-]

• acidosis• alkalosis

Acid-Base Imbalances

• pH< 7.35 acidosis• pH > 7.45 alkalosis• The body response to acid-base imbalance is called

compensation• May be complete if brought back within normal limits• Partial compensation if range is still outside norms.

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Compensation

• If underlying problem is metabolic, hyperventilation or hypoventilation can help : respiratory compensation.

• If problem is respiratory, renal mechanisms can bring about metabolic compensation.

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Respiratory Acidosis

• Carbonic acid excess caused by blood levels of CO2 above 45 mm Hg.

• Hypercapnia – high levels of CO2 in blood

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Compensation for Respiratory Acidosis

• Kidneys eliminate hydrogen ion and retain bicarbonate ion

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Respiratory Alkalosis

• Carbonic acid deficit• pCO2 less than 35 mm Hg (hypocapnea)• Most common acid-base imbalance• Primary cause is hyperventilation

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Compensation of Respiratory Alkalosis

• Kidneys conserve hydrogen ion• Excrete bicarbonate ion

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Diagnosis of Acid-Base Imbalances

1. Note whether the pH is low (acidosis) or high (alkalosis)

2. Decide which value, pCO2 or HCO3- , is outside the normal range

and could be the cause of the problem. If the cause is a change in pCO2, the problem is respiratory. If the cause is HCO3

- the problem is metabolic.

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3. 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.

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Example

• A patient is in intensive care because he suffered a severe myocardial infarction 3 days ago. The lab reports the following values from an arterial blood sample:• pH 7.3• HCO3- = 20 mEq / L ( 22 - 26)• pCO2 = 32 mm Hg (35 - 45)

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Diagnosis

• Metabolic acidosis• With compensation

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RESPIRATORY FAILURE

Definition

• Inability of the lung to meet the metabolic demands of the body.

• This can be from failure of tissue oxygenation and/or failure of CO2 homeostasis.

Diagnosis

• PaO2 <60 mmHg while breathing air,

• or a PaCO2 >50 mmHg.

Respiratory system includes:

CNS (medulla) Peripheral nervous system (phrenic nerve) Respiratory muscles Chest wall Lung Upper airway Bronchial tree Alveoli Pulmonary vasculature

Potential causes of Respiratory Failure

RESPIRATORY FAILURE- TYPE 1

• PaO2 <60mmHg with normal or low PaCO2 normal or high pH

• Most common form of respiratory failure• Lung disease is severe to interfere with

pulmonary O2 exchange, but over all ventilation is maintained

• Physiologic causes: V/Q mismatch and shunt

Causes of Hypoxemic Respiratory failure

• Caused by a disorder of heart, lung or blood.

Respiratory Failure -Type II

• PaCO2 >50 mmHg

• Hypoxemia is always present• pH depends on level of HCO3

• HCO3 depends on duration of hypercapnia

• Renal response occurs over days to weeks

Take Home Message

• Body is naturally tuned up to maintain pH of the blood between 7.35-7.45

• This maintenance is done with the help of buffers• Buffers are of two types:

– Chemical and physiological

• The process of correcting any acid base disorder is called compensation

Take Home Message

• Compensation can be either respiratory or metabolic(kidneys)

• Respiratory failure are of two types • pCO2 helps to classify the type of respiratory failure