PreLec4: Ventilation and Gas Exchange

What is hypoxemia?

Decreased arterial blood oxygen (PaO2 less than 80 mmHg)

How is hypoxemia classifies by severity?

• Mild is PaO2 of 60-80 mmHg

• Moderate is PaO2 of 40-60 mmHg

• Severe is PaO2 of less than 40 mmHg

How is hypoxemia different from hypoxia/ischemia?

Hypoxemia is low arterial blood oxygen whereas hypoxia is decreased tissue oxygenation which can occur with or without pulmonary dysfunction

What are the main causes (etiologies) of hypoxemia?

• Hypoventilation

• Diffusion impairment at alveolar capillary membrane

• Shunting

• Ventilation perfusion mismatch

Why does hypoventilation cause hypoxemia?

Without enough fresh air reaching alveoli, oxygen cannot enter the blood efficiently

Why does diffusion impairment at alveolar capillary membrane cause hypoxemia?

Because it prevents oxygen from moving into blood effectively

Why does shunting cause hypoxemia?

Blood vessels move from right side of the heart to the left without oxygenating so there is less circulating the body

Why does ventilation perfusion mismatch cause hypoxemia?

Proper oxygenation requires matched airflow and blood flow; mismatching reduces the efficiency of gas exchange

What are the physiologic responses to hypoxemia?

• Increased ventialtion

• Pulmonary vasoconstriction

• Increased RBC production

• Tachycardia

What are the consequences of hypoxemia?

• Hyperventilation

• Shunting

• Hypercoagulability

• Potential impaired blood flow

What are the early neurologic manifestations of hypoxemia?

Early signs = brain acting weird

• Personality changes

• Restlessness

• Agitation

• Combative behavior

• Impaired coordination

• Poor judgment

What are the late neurologic manifestations of hypoxemia?

Later signs = brain shutting down

• Delirium

• Stupor

• Coma

What are the skin manifestations of hypoxemia?

• Pallor (pale because of low oxygen)

• Cyanosis (blue tint when really low)

What are the physiologic responses to hypoxemia?

Think: Breathe, squeeze, build, beat (body panics and tries to produce more things)

• Increased ventilation

• Pulmonary vasoconstriction

• Increased red blood cell production

• Tachycardia

What is clubbing?

Nail bed hypertrophy and bulbous enlargement of the fingertips

What condition leads to nail clubbing?

Chronic hypoxemia

What causes clubbing in chronic hypoxemia?

Clumping of platelets in nails beds that release growth factors and cause tissue hypertrophy

Is clubbing painful?

No, it is typically a painless change over time

What is the “Diamond Gap Sign”?

When the normal diamond shaped space between the entails of opposite fingers disappears due to clubbing angle of the nails

What is hypercapnia?

An increase in CO2 content of arterial blood (PaCO2 greater than 45 mmHg)

What is the normal PaCo2 range?

34-45mmHg

What are the contributing factors to hypercapnia?

• Increased CO2 production

• Disturbance in gas exchange

• Changes in neural control

• Decreased minute ventilation (RR x tidal volume)

What are the physiologic responses to hypercapnia?

Vasodilation of blood vessels, including those in the brain (Because CO2 dissolved in blood and forms carbonic acid which lowers pH, so the body responds by dilating blood vessels to increase blood flow to flush out CO2 and deliver more oxygen)

What are the consequences of hypercapnia?

• Diminished signs and hearing

• Drowsiness

• Mild narcosis

• Dizziness

• Confusion

• Headache

• Seizures

• Coma

Why do you have diminished sight and hearing because of hypercapnia?

Because the vasodilation increases blood flow to the brains therefore increasing intracranial pressure

Why do you feel drowsy, mild narcosis, and confusion during hypercapnia?

Because the extra CO2 in the brain makes it acidic and with extra acid in the CSF, it depresses neurons and makes you feel foggy

Why do you get a headache due to hypercapnia?

Because there is more blood in the skull and therefore more pressure

Why do you feel dizzy during hypercapnia?

Because the altered blood flow in the brain and low oxygen delivery make you feel lightheaded

Why could you have a seizure or go into coma when hypercapnia is prolonged?

Because if there is severe CO2 buildup it overwhelms the brains ability to regulate pH and the neurons become unstable

What is hypoventilation?

Inadequate alveolar ventilation due to breathing being too shallow or too slow

What can cause hypoventilation?

Respiratory depression from neurologic issue, trauma, or pain

What does hypoventilation lead to?

Hypercapnia and respiratory acidosis

What is hyperventilation?

Excessive alveolar ventilationW from breathing too deep and too fast

What can cause hyperventilation?

Usually anxiety or stress

What does hyperventilation lead to?

Hypocapnia and respiratory alkalosis

What is acute respiratory failure?

A condition where the lungs fail to adequately oxygenate the blood or fail to adequately remove CO2

What defines hypoxemia respiratory failure?

A PaO2 level of less than 60 mmHg

What conditions can cause hypoxemia respiratory failure?

• COPD

• Interstitial lung disease

• Severe pneumonia

• Atelectasis

What defines hypercapnic respiratory failure?

A PaCO2 level of greater than 50 mmHg with pH of less than 7.25

What conditions can cause hypercapnic respiratory failure?

• Upper airway obstruction

• Laryngospasm

• Tumor obstruction

• Respiratory muscle failure

• Chest wall injury 

What is pleural effusion?

Excess fluid accumulation in the pleural space

What are the possible sources of pleural effusion fluid?

• Too much fluid in the interstitium (tissue space around the alveoli and small blood vessels)

• Too much fluid in the parietal pleura (Lines the chest wall)

• Too much fluid in the peritoneal cavity (Abdominal cavity where organs are)

• Decreased lymphatic drainage (normally lymph vessels drain pleural fluid)

What are the main types of pleural effusion?

• Transudative

• Exudative

• Emphyema

• Hemothorax

• Chulothorax

What is a Transudative pleural effusion?

Watery fluid that leaks into the pleural space

What is Exudative pleural effusion?

Fluid rich in which blood cells and plasma proteins 

What is empyema?

A pleural effusion containing pus (purulent fluid)

What is a Hemothorax?

A pleural effusion containing blood

What is a chylothorax?

A pleural effusion containing chyle (lymphatic fluid)

What is a pneumothorax?

Accumulation of gas in the pleural space, which causes loss of negative pressure and lung collapse

What are the main etiologies of pneumothorax?

• Rupture of the pleural space (loss of negative pressure means lung tissue shifts towards the hilum)

• Can occur spontaneously (bleb rupture), due to trauma or from a medical procedure

What is Atelectasis?

Incomplete expansion or collapse of a lung or portion of a lung

What are the three main types of Atelectasis by etiology?

• Compression

• Obstructive/Absorption

• Surfactant impairment/Adhesive

What is compression atelectasis?

Collapse caused by external pressure on the lung (from a tumor, fluid, air, abdominal distention, pleural effusion, pneurmothorax, etc)

What is obstructive (absorption) Atelectasis?

Collapse caused by blockage of airways or alveoli (from a mucus plug, tumor, foreign body, lung disease, hypoventilation from sedation, immobility, pain, etc)

What is surfactant impairment (adhesive) Atelactasis?

Collapse due to decreased production or inactivation of surfactant (seen in premature birth, pulmonary edema, and lung disease)

Who is at high risk for Atelectasis?

Surgical or immobile patients 

What ventilation-perfusion changes occur with Atelectasis?

• V/Q mismatch

• Shunting

• Hypoxemia

• Hypercapnia

• Vasodilation

• Hypotension

What are the clinical manifestations of Atelectasis?

• Dyspnea

• Tachycardia

• Cyanosis

• Diminished breath sounds (may also hear fine crackles or wheezing)

How is Atelectasis prevented?

• Early ambulation

• Deep breathing exercises

• Incentive spirometry

• Adequate pain control

How is Atelectasis treated?

• Remove the cause (obstruction or compression)

• Re expand lung with breathing techniques or interventions

What is pulmonary edema?

Excess fluid in the lungs, specifically in the airways and alveoli

What are the main etiologies of pulmonary edema?

• Heart disease (heart failures that increase pulmonary capillary pressure)

• Pulmonary capillary injury

• Obstruction of lymphatic vessels

What are key manifestations of pulmonary edema?

• Dyspnea

• Hypoxemia

• Increased work of breathing

• Inspiratory crackles on auscultation

• Dullness to percussion

• Pink, frothy sputum (because capillaries under high pressure leak RBCs into the alveoli)

• Hypoventilation and hypercapnia

What is aspiration?

The passage of foreign fluids or particles into the lungs

What happens when aspiration occurs?

It causes bronchial obstruction and inflammation

What are the main risk factors and etiologies for aspiration?

• ALOC

• Advanced age

• Neuromuscular dysphagia

• GI dysfunction (GERD, tube feeding, motility disorders)

• COPD

What is pulmonary embolism?

An occlusion of the pulmonary vascular bed by blood borne objects that travel from the systemic venous system, through the right heart, into the pulmonary circulation

What are the main etiologies of pulmonary embolism?

• Thromboembolus (blood clot usually)

• Fat embolus

• Foreign body embolus

• Air embolus

• Amniotic fluid embolus

What are the major risk factors for thromboembolic pulmonary embolism?

• Immobility

• Surgery

• Smoking

• Hormone replacement therapy (Like oral brith control)

What are the three main factors that contribute to thrombus formation?

• Venous stasis

• Vascular injury

• Hypercoagulability

What happens after a thrombus dislodges?

It becomes an embolus that travels to and occludes part of the pulmonary circulation

What are the pathophysiologic consequences of pulmonary embolism?

• Hypoxemic Vasoconstriciton

• Decreased surfactant

• Release of neurohumoral and inflammatory mediators

• Pulmonary edema

• Atelectasis

(Essentially, the clot itself blocks blood flow, but the mediators that are activated by the body thinking there is damage, is what spreads more damage by tightening the vessel, leaking fluid and weakening alveoli)

What are the clinical manifestations of pulmonary embolism?

• Tachypnea (most common, happens right away because low oxygen tells the chemoreceptors to breathe more)

• Dyspnea (sometimes happens later if the embolism is severe because CO2 builds up in the brain and depresses the brains respiratory drive)

• Chest pain

• V/Q mismatch (ventilation but no perfusion to some ares of the lungs - dead space)

• Pulmonary infarction

• Pulmonary hypertension

• Right ventricular failure

• Shock

What are the main prevention measures for pulmonary embolism?

• Bed exercises

• Early ambulation

• Pneumatic calf compression

• Prophylactic low dose anticoagulation (medications like heparin that reduce bloods tendency to clot)

What is pulmonary hypertension?

Elevation of pressure in the pulmonary vascular system (pulmonary artery pressure greater than 25)

What are the main etiologies of pulmonary hypertension?

• Problems originating in the heart

• Problems originating in the lungs

• Pulmonary embolism

• Profound hypoxemia caused shunting

• Vascular constriction in the pulmonary circulation

What are common manifestations of pulmonary hypertension?

• Fatigue

• Chest discomfort

• Tachypnea (Pathologic sign, your body is breathing faster to try to get more oxygen)

• Dyspnea (No matter how fast you breath you still feel breathless because of the lack of oxygen flow to the lungs)

What is for pulmonale?

Right sided heart failure that results from pulmonary disease or chronic pulmonary hypertension

What is the main cause of cor pulmonale?

Increased pulmonary arterial pressure that raises right ventricular workload

What happens to the right ventricle in cor pulmonale?

Right ventricular hypertrophy (increase in size from workload), dilation (chamber enlargement) or both 

What are the key etiologies of cor pulnonale?

• Chronic lung disease

• Pulmonary hypertension

What are common manifestations of cor pulmonale?

• Systemic venous congestion (right side is failing so blood backs up in the veins)

• Jugular venous  (neck veins become distended because venous blood cant drain well into the failing right heart)

• Hepatosplenomegaly (means enlarged liver and spleen because the congested venous blood fills the organs)

• Peripheral edema (Extra fluid leaks out of the tissues because of the high venous pressure)

• Dyspnea (the original problem was in the lungs so this is still present)

• Pulmonic and tricuspid valve murmurs  (high pressures stretch the valves so they dont close properly)

• Polycythemia with chronic lung disease (chronic low oxygen stimulates the bone marrow to make more RBCs which thickens blood)