Chapter 42: Respiratory Insufficiency—Pathophysiology, Diagnosis, Oxygen Therapy

Written By: Razaullah Khattak (DPT Medical Professional)

Student Source Reference: Guyton and Hall Textbook of Medical Physiology

Chapter: 42

To read more check out Chapter 25:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-25-body-fluid-compartments.html

Useful Methods for Studying Respiratory Abnormalities

Respiratory insufficiency encompasses various pathological conditions where the lungs fail to perform adequate gas exchange. Understanding the underlying mechanisms, diagnostic methods, and physiological alterations in conditions such as chronic pulmonary emphysema, pneumonia, atelectasis, asthma, and tuberculosis is essential for effective patient management. Furthermore, evaluating blood gases, maximum expiratory flow rates, and applying oxygen therapy or artificial ventilation are critical skills in clinical physiology.

Useful Methods for Studying Respiratory Abnormalities

• Respiratory diagnostics measure vital capacity and tidal air.
• Tests evaluate functional residual capacity and dead space.
• Methods assess physiologic shunt and physiologic dead space.
• These tools aid the clinical pulmonary physiologist.

Study of Blood Gases and Blood pH

• Testing blood P_{O}_2}, P_{CO}_2}, and {pH} is essential.
• Rapid testing guides therapy for acute respiratory distress.
• Measurements require only a few drops of blood.
• Results are available within minutes at the bedside.

Determination of Blood pH

• Blood {pH} is measured using glass electrodes.
• The glass electrodes used are miniaturized.
• Generated voltage directly measures the blood {pH}.
• Results display on a voltmeter or chart.

Determination of Blood CO2

• Glass electrodes measure carbon dioxide in blood.
• {CO}_2 dissolves in sodium bicarbonate solution.
• {pH} relates to {CO}_2 via Henderson-Hasselbalch equation.
• A plastic membrane surrounds the miniature electrode.
• Sodium bicarbonate solution sits inside the membrane.
• Blood superfuses onto the outer plastic membrane surface.
• {CO}_2 diffuses from blood into bicarbonate solution.
• Measured {pH} allows calculation of blood {CO}_2.

Determination of Blood PO2

• Polarography measures oxygen concentration in fluids.
• Electric current flows to a small electrode.
• Applied voltage causes oxygen deposit on electrode.
• Current flow is proportional to oxygen concentration.
• Platinum negative electrodes are used in practice.
• Plastic membranes separate the electrode from blood.
• Membranes allow oxygen diffusion while blocking proteins.
• Combined devices measure {pH}, \{CO}_2, and P_{O}_2} together.

Measurement of Maximum Expiratory Flow

• Asthma increases airflow resistance during expiration.
• Maximum expiratory flow limits total expiratory rate.
• Increased effort cannot exceed maximum flow limit.
• Higher lung volumes allow higher maximum flow.

Mechanism of Expiratory Airflow Limitation

• Chest compression increases pressure on alveoli.
• External pressure also compresses the bronchioles.
• Bronchiolar collapse opposes air movement outward.
• Severe collapse prevents further airflow increase.
• Critical expiratory force establishes maximum flow rate.

Effect of Lung Volume on Maximum Expiratory Flow

• Healthy maximum expiratory flow exceeds 400 L/min.
• Flow rate decreases as lung volume diminishes.
• Expanded lungs hold airways open via traction.
• Smaller lung volumes relax structural elastic traction.
• Collapsed airways progressively reduce expiratory flow rates.

Abnormalities of the Flow-Volume Curve

• Constricted lungs show reduced total lung capacity.
• Constricted lungs show reduced residual volume.
• Peak expiratory flow cannot reach normal levels.
• Fibrotic diseases like tuberculosis constrict the lungs.
• Chest deformities like scoliosis restrict lung volume.
• Airway obstruction makes expiration harder than inspiration.
• Negative pleural pressure opens airways during inspiration.
• Trapped air increases total capacity over time.
• Trapped air increases residual volume over time.
• Obstructed airways collapse easily and reduce flow.
• Asthma causes severe partial airway obstruction.

Forced Expiratory Vital Capacity and FEV1

• Spirometers record forced expiratory vital capacity.
• Patient inspires maximally to total lung capacity.
• Patient exhales with maximum effort as rapidly as possible.
• Downslope distance represents total forced vital capacity.
• First-second forced expiratory volume is called FEV1.
• Normal FEV1/FVC ratio is about 80 percent.
• Airway obstruction reduces FEV1/FVC ratio significantly.
• Obstruction can drop FEV1/FVC below 20 percent.

Chronic Pulmonary Emphysema

• Pulmonary emphysema means excess air in lungs.
• Complex obstructive process caused by long-term smoking.
• Inhaled smoke irritates bronchioles causing chronic infection.
• Nicotine partially paralyzes respiratory epithelial cilia.
• Paralysed cilia fail to remove excess mucus.
• Irritation stimulates excess mucus secretion continuously.
• Inhibited alveolar macrophages fail fighting lung infections.
• Mucus and edema cause chronic small airway obstruction.
• Air entrapment overstretches alveoli during expiratory efforts.
• Infection and overstretching destroy alveolar walls.
• Wall destruction reaches 50 to 80 percent.

Physiologic Effects of Emphysema

• Bronchiolar obstruction greatly increases work of breathing.
• Compressive forces increase airway resistance during expiration.
• Alveolar wall loss decreases lung diffusing capacity.
• Impaired diffusion reduces blood oxygenation efficiency.
• Uneven obstruction causes abnormal ventilation-perfusion ratios.
• Low ventilation-perfusion ratio areas create physiologic shunts.
• High ventilation-perfusion ratio areas create physiologic dead space.
• Capillary loss increases pulmonary vascular resistance.
• High vascular resistance causes pulmonary hypertension.
• Pulmonary hypertension leads to right-sided heart failure.
• Patients develop severe chronic hypoxia and hypercapnia.
• Prolonged air hunger persists until patient death.

Pneumonia

• Pneumonia is inflammation with alveolar fluid accumulation.
• Bacterial pneumonia commonly arises from pneumococcal infection.
• Inflamed pulmonary membranes become highly porous.
• Fluid and blood cells leak into alveoli.
• Infection spreads continuously from alveolus to alveolus.
• Consolidation fills entire lobes with cellular debris.
• Localized pneumonia reduces ventilation while perfusion continues.
• Available surface area for gas exchange decreases.
• Decreased ventilation-perfusion ratio causes hypoxemia.
• Decreased ventilation-perfusion ratio causes hypercapnia.
• Aerated lung blood reaches 97 percent saturation.
• Unaerated lung blood remains 60 percent saturated.
• Aortic blood saturation drops to 78 percent.

Atelectasis

• Atelectasis refers to collapse of lung alveoli.
• It affects localized areas or whole lungs.
• Main cause is complete airway obstruction.
• Another cause is lack of alveolar surfactant.
• Airway Obstruction Causes Lung Collapse
• Mucus plugs block small or major bronchi.
• Solid objects or tumors obstruct major airways.
• Trapped air absorbs rapidly into pulmonary capillaries.
• Pliable lung tissue collapses following air absorption.
• Rigid tissue creates negative alveolar pressure instead.
• Negative pressure pulls fluid into alveolar spaces.
• Massive collapse fills entire lung with fluid.
• Lung collapse increases pulmonary vascular resistance.
• Compressed vessels increase resistance to blood flow.
• Local hypoxia causes pulmonary vasoconstriction.
• Blood shifts mostly toward the ventilated lung.
• Shifted blood flow prevents severe aortic desaturation.

Lack of Surfactant Causes Collapse

• Surfactant is secreted by alveolar epithelial cells.
• Surfactant reduces alveolar surface tension significantly.
• Reduced tension prevents alveolar collapse during respiration.
• Respiratory distress syndrome depresses surfactant secretion.
• High surface tension causes collapse in infants.
• Unmanaged atelectasis leads to infant suffocation death.

Asthma

• Asthma features spastic contraction of bronchiolar muscles.
• Muscle spasm causes extremely difficult breathing.
• Affects 3 to 5 percent of people.
• Caused by contractile hypersensitivity to air irritants.
• Allergic hypersensitivity causes asthma in young patients.
• Plant pollens act as common offending allergens.
• Smog irritants trigger asthma in older patients.

Mechanism of Allergic Asthma

• Allergic individuals produce excess IgE antibodies.
• IgE antibodies attach directly to mast cells.
• Mast cells reside near small lung bronchioles.
• Inhaled pollen binds to attached IgE antibodies.
• Activated mast cells release inflammatory chemical mediators.
• Released mediators include histamine and leukotrienes.
• Eosinophilic chemotactic factor and bradykinin are released.
• Mediators cause edema in small bronchiolar walls.
• Thick mucus secretes into the bronchiolar lumen.
• Smooth muscle spasm increases overall airway resistance.
• Expiratory forces compress partially occluded bronchioles.
• Severe obstruction occurs specifically during expiration.
• Asthmatic patients experience severe air hunger.
• Acute attacks increase functional residual capacity.
• Chronic asthma causes permanent barrel chest deformity.

Tuberculosis

• Tubercle bacilli invade lung tissue during infection.
• Macrophages enter and engulf invading tubercle bacteria.
• Fibrous tissue walls off infected lesions.
• Walled-off lesions are referred to as tubercles.
• Tubercle formation limits bacterial spread in lungs.
• Failed walling off spreads bacteria throughout lungs.
• Tissue destruction creates large pulmonary abscess cavities.
• Late stages feature widespread pulmonary tissue fibrosis.
• Fibrosis increases the work of breathing.
• Total vital capacity and breathing capacity decrease.
• Reduced membrane area diminishes pulmonary diffusing capacity.
• Abnormal ventilation-perfusion ratios worsen gas exchange.

Hypoxia and Oxygen Therapy

• Pulmonary abnormalities cause cellular hypoxia throughout body.
• Oxygen therapy efficacy depends on underlying cause.

Classification of Hypoxia Causes

• Low atmospheric oxygen causes extrinsic blood hypoxemia.
• Neuromuscular disorders cause hypoventilation and hypoxia.
• Increased airway resistance causes pulmonary hypoventilation.
• Decreased pulmonary compliance reduces adequate alveolar ventilation.
• Abnormal ventilation-perfusion ratios impair blood oxygenation.
• Diminished respiratory membrane diffusion causes tissue hypoxia.
• Right-to-left cardiac shunts cause venous-to-arterial hypoxia.
• Anemia decreases overall blood oxygen transport capacity.
• Circulatory deficiency reduces systemic oxygen delivery rate.
• Tissue edema impairs localized oxygen tissue delivery.
• Cyanide poisoning blocks cellular oxidation enzymes.
• Vitamin deficiencies impair cellular oxygen utilization capacity.

Inadequate Tissue Capability to Use Oxygen

• Cyanide blocks cytochrome oxidase enzyme activity completely.
• Blocked enzymes prevent cellular oxygen usage entirely.
• Beriberi impairs steps in cellular oxygen utilization.
• Vitamin B deficiency compromises carbon dioxide formation.

Effects of Hypoxia on the Body

• Severe hypoxia causes generalized cellular death.
• Mild hypoxia causes depressed mental activity.
• Severe mental depression leads directly to coma.
• Hypoxia reduces working capacity of muscles.

Oxygen Therapy Efficacy

• Tents deliver oxygen-enriched air to patients.
• Masks deliver pure oxygen at high concentrations.
• Intranasal tubes supply supplemental oxygen directly.
• Supplemental oxygen fully corrects atmospheric hypoxia.
• Supplemental oxygen significantly helps hypoventilation hypoxia.
• Oxygen therapy does not clear excess P_{CO}_2}.
• Supplemental oxygen increases alveolar P_{O}_2} to 600 mmHg.
• Alveolar-capillary diffusion gradient increases by 800 percent.
• Therapy increases dissolved oxygen in anemia cases.
• Oxygen therapy offers no benefit for cyanide poisoning.

Cyanosis

• Cyanosis means bluish discoloration of skin.
• Excessive deoxygenated hemoglobin causes skin blueness.
• Dark purple deoxygenated blood shines through capillaries.
• Appears when deoxygenated hemoglobin exceeds 5 g/dL.
• Anemic patients rarely exhibit clinical cyanosis signs.
• Polycythemia patients frequently show apparent cyanotic symptoms.

Hypercapnia

• Hypercapnia means excess carbon dioxide in fluids.
• Hypoventilation and circulatory deficiencies cause hypercapnia.
• Low atmospheric oxygen does not cause hypercapnia.
• Diffusion defects rarely cause significant arterial hypercapnia.
• Carbon dioxide diffuses 20 times faster than oxygen.
• Incipient hypercapnia triggers immediate compensatory alveolar ventilation.
• Hypoventilation impairs oxygen and carbon dioxide equally.
• Circulatory deficiency leads directly to tissue hypercapnia.
• Blood carries carbon dioxide better than oxygen.
• High alveolar P{CO}_2} causes severe dyspnea.
• Extreme P_{CO}_2} levels cause lethargy and coma.
• Very high P_{CO}_2} depresses respiratory centers.
• Respiratory depression creates a fatal vicious cycle.

Dyspnea

• Dyspnea is mental anguish from inadequate ventilation.
• Commonly described as severe air hunger.
• High carbon dioxide levels trigger dyspneic sensations.
• Increased respiratory muscle effort causes dyspneic feelings.
• Neurogenic dyspnea stems entirely from emotional states.
• Psychological fear of suffocation induces acute dyspnea.

Artificial Respiration
Resuscitators

• Resuscitators supply pressurized air or pure oxygen.
• Intermittent positive pressure forces air into lungs.
• Masks or endotracheal tubes connect equipment to patients.
• Passive exhalation occurs between positive pressure cycles.
• Pressure limits prevent structural lung tissue damage.
• Pressure limits set to 12-15 cm \{H}_2{O}.

Tank Respirator (Iron Lung)

• Patient body rests inside an airtight tank.
• Patient head protrudes through flexible collar outside.
• Motorized diaphragm alters pressure inside the tank.
• Inward diaphragm movement forces passive lung expiration.
• Outward diaphragm movement causes active lung inspiration.
• Negative pressure reaches -10 to -20 cm {H}_2{O}.
• Positive pressure reaches 0 to +5 cmH}_2{O}.

Hemodynamic Effects of Artificial Ventilation

• High lung pressure exceeds general body pressure.
• Elevated intrathoracic pressure impedes venous blood return.
• Excessive respirator pressures decrease total cardiac output.
• Continuous positive pressure above 30 mmHg causes death.

Disclaimer: This content is for educational purposes only and not a substitute for professional medical advice.