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 – death occurs Acidosis (acidemia) below 7.35 Alkalosis (alkalemia) above

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

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 12

The body produces more acids than bases Acids take in with foods Acids produced by metabolism of lipids and proteins Cellular metabolism produces CO 2. CO 2 + H 2 0 ↔ H 2 CO 3 ↔ H + + HCO

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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 15

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 CO 2 + H 2 0 ↔ H 2 CO 3 ↔ H + + HCO 3 - Body pH can be adjusted by changing rate and depth of breathing 18

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 19

Rates of correction Buffers function almost instantaneously Respiratory mechanisms take several minutes to hours Renal mechanisms may take several hours to days 20

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Acid base abnormalities- Causes Respiratory –abnormal processes which tend to alter pH because of a primary change in pCO 2 levels acidosis alkalosis Metabolic –abnormal processes which tend to alter pH because of a primary change in [HCO 3 - ] 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. 23

Compensation If underlying problem is metabolic, hyperventilation or hypoventilation can help : respiratory compensation. If problem is respiratory, renal mechanisms can bring about metabolic compensation. 24

Respiratory Acidosis Carbonic acid excess caused by blood levels of CO 2 above 45 mm Hg. Hypercapnia – high levels of CO 2 in blood 25

Compensation for Respiratory Acidosis Kidneys eliminate hydrogen ion and retain bicarbonate ion 26

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Respiratory Alkalosis Carbonic acid deficit pCO 2 less than 35 mm Hg (hypocapnea) Most common acid-base imbalance Primary cause is hyperventilation 29

Compensation of Respiratory Alkalosis Kidneys conserve hydrogen ion Excrete bicarbonate ion 30

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Diagnosis of Acid-Base Imbalances 1.Note whether the pH is low (acidosis) or high (alkalosis) 2.Decide which value, pCO 2 or HCO 3 -, is outside the normal range and could be the cause of the problem. If the cause is a change in pCO 2, the problem is respiratory. If the cause is HCO 3 - the problem is metabolic. 34

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. 35

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 ( ) pCO2 = 32 mm Hg ( ) 36

Diagnosis Metabolic acidosis With compensation 37

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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 CO 2 homeostasis.

Diagnosis PaO 2 <60 mmHg while breathing air, or a PaCO 2 >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 PaO 2 <60mmHg with normal or low PaCO 2  normal or high pH Most common form of respiratory failure Lung disease is severe to interfere with pulmonary O 2 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 PaCO 2 >50 mmHg Hypoxemia is always present pH depends on level of HCO 3 HCO 3 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 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