Overview

• A heterogeneous chronic disease of the conducting airways.
• Characterized by intermittent and mostly reversible periods of bronchial obstruction, with bronchial hyper-reactivity, excessive mucus, and airway remodeling; in contrast, recall that COPD is characterized by constant, irreversible airway obstruction.
• Asthmatic patients experience episodes of cough, wheezing, dyspnea, and feelings of chest tightness.
• Characterized by increased total lung capacity because air is trapped in the lungs.
• The ratio of forced expiratory volume in one second to forced vital capacity is normal or low.
• The diffusing capacity of the lungs for carbon monoxide is normal or elevated.
• Ventilation to perfusion mismatches occur, which can lead to hypoxemia.
• Asthma is a heterogeneous disease with different etiologies and characteristics, and there are different ways to classify asthma types
  – Historically, asthma has been classified based on phenotypes, which are the observable characteristics – such as by the presence or absence of eosinophils.
  – Many researchers are now shifting towards classification based on endotypes, which are based on the pathophysiologic mechanisms that drive types of asthma – such as whether high levels of type 2 inflammation is involved.
• Diagnosis of asthma can include: pulmonary function tests (spirometry), methacholine bronchial challenge tests, and allergy tests.
• Treatments for asthma can be broad-based or more targeted, depending on the asthma type.

Types of Asthma

We organize these based on presence of granulocytes and the type of inflammation.

Eosinophilic asthma with high levels of type-2 inflammation

This group includes the most thoroughly studied subtypes of asthma.

• Type-2 inflammation is due to an imbalance towards T-helper 2 cells and Type-2 innate lymphoid cells and their cytokines (specifically, IL-4, IL-5, and IL-13) which promote IgE production from B cells.

• Early Onset with Allergy: because allergens are the triggers for asthma attacks, this subtype is sometimes called “Extrinsic asthma.”
  – Asthma attacks are IgE-mediated, and patients often have other allergies and history of rhinitis and atopic dermatitis. Click to review Hypersensitivity Type 1
  – Tends to be familial, is associated with early childhood respiratory viral infections (especially RSV and rhinovirus), and, is most common in boys.
  – Biomarkers include elevated eosinophils and serum IgE.
  – Common triggers include house dust mites, mold, pets, and pollen.

• Late-onset without atopy
  – Patients have fewer allergies, but often present with more severe asthma.
  – Often co-exists with chronic rhinosinusitis and nasal polyps.
  – Not familial, and tends to occur most often in adult females.
  – Biomarkers include elevated eosinophils, and, in a sub-set of patients, elevated levels of Staphylococcus aureusendotoxin-specific IgE.
  Be aware that, although most patients do not have elevated levels of IgE, it is thought that localized IgE in the airways occurs.
• Key triggers in this group include air pollution, especially diesel gas fumes and other irritants, as well as respiratory infections.

• Aspirin-exacerbated asthma (aka, aspirin-exacerbated respiratory disease – AERD)
• Form of asthma triggered by aspirin – it is often severe, and typically occurs in adults
  – Thus, some consider this to be a sub-sub-type of late-onset asthma.
  – Aspirin-exacerbated asthma is due genetic abnormalities that lead to dysregulation of arachidonic acid metabolism.

Next, let’s consider some other types of asthma.

Neutrophilic, type-2-low asthma

• Characterized by elevated levels of the cytokine IL-17 and oxidative stress.
• Neutrophilic asthma is usually adult-onset and is often severe.
  – Resistant to corticosteroids.
• Key at-risk populations include: smokers, obese people, and elderly people.

Mixed granulocytic asthma

• Characterized by elevated levels of T helper 2 and T helper 17 cytokines.

Paucigranulocytic asthma

• Characterized by non-inflammatory (or low inflammatory) changes in the bronchi: remodeling and hyper-reactivity.

Two additional asthma phenotypes that can overlap with those already listed:
Occupational, aka, work-related asthma

• Develops in response to exposure to workplace allergens or irritants (and, because it is occupation-related, tends to develop in adulthood).
  Exercise-induced/exacerbated asthma
• Triggered by exercise.
• Although the exact mechanisms are unclear, it is thought that exercise-related evaporation and water loss from the airway epithelia causes injury that leads to upregulation of pro-inflammatory mediators.

Comparison with healthy bronchi.

• Smooth muscle overgrowth, which contributes to hyper-reactivity and constriction of the airways.
• Possible inflammation and fibrosis with migration of eosinophils and/or neutrophils in the wall of the airway (depending on the type of asthma).
• The basement membrane is thickened.
• Respiratory epithelium displays goblet cell metaplasia.
• Excess mucus further narrowing the airway so that the lumen is much smaller than normal.
• Other hallmarks of asthma may be seen in the sputum:
  – Curschmann spirals are mucus plugs that comprise desquamated airway epithelial cells.
  – Charcot-Leyden Crystals are crystals formed from eosinophil proteins.

Treatments

Step-wise approach to treatment

• Short-acting beta-antagonists, such as Albuterol, are provide for quick relief during asthma attacks; they relax the smooth muscle lining the bronchi.
• Long-acting beta-antagonists, such as Salmeterol, may be used longer-term in conjunction with other drugs.
• Inhaled corticosteroids, such as beclomethasone, suppress airway inflammation and are first-line therapy for long-term asthma management.
• Leukotriene antagonists, such as montelukast, also reduce bronchoconstriction and inflammation, and are especially useful in exercise- and aspirin-exacerbated asthma.
• Anticholinergics, such as Ipratropium and Tiotropium, produce smooth muscle relaxation in the bronchi.
• Anti-IgE antibodies, such as Omalizumab, are effective in allergic asthma.
• Anti-IL-5 antibodies, such as Mepolizumab, are useful in severe eosinophilic asthma.
• Mast cell inhibitors, such as Cromolyn, prevent mast cells from releasing inflammatory mediators that lead to bronchospasm.
• Oral corticosteroids, such as prednisone, are used in severe asthma to reduce inflammation; because oral corticosteroids can have serious side effects, they are typically used for acute attacks.

Overview

• COPD is characterized by chronic, progressive, irreversible air flow obstruction
  – Leads to high lung volumes (functional residual capacity, residual volume, and total lung capacity).
• As a result of airway obstruction, the ratio of Forced Expiratory Reserve Volume in 1 second to Forced Vital Capacity is reduced
• COPD includes Chronic bronchitis, small airway disease, and emphysema.
• Asthma-COPD overlap syndrome: Symptoms of airway hyper-reactivity and airway obstruction.
• COPD can lead to V/Q mismatch, hypoxemia, pulmonary hypertension, and right heart failure (cor pulmonale).
• It is estimated that approximately 16 million people in the US have diagnosed COPD, and many are undiagnosed;.
• COPD is a leading cause of death worldwide.
• Previously thought of as a man’s disease, we now see high rates of COPD in both men and women; patients are typically over 65 years of age and are current or former smokers.
• Key risk factors are smoking, air pollution (including indoor pollution, such as the smoke from burning organic fuels for heating), and, genetics.
  – Alpha-1 antitrypsin deficiency is a known genetic cause of emphysema; this defect allows excess elastase in the lungs, which degrades alveoli.
• COPD is not curable
• Treatments are meant to relieve symptoms and slow progression; in addition to quitting smoking and limiting exposure to second hand smoke, patients may be prescribed bronchodilators, steroids, respiratory therapy, or supplemental oxygen. Prevention of respiratory infections via flu and pneumococcal vaccines are also important, as they prevent COPD exacerbations.
• Surgery may be appropriate for some patients.

Signs & Symptoms

COPD is the result of air trapping in the lungs; two key causes of air trapping are:
– Reduced elastic recoil in the lung parenchyma (which occurs in emphysema).
– Increased airway resistance (which occurs in chronic bronchitis and small airway disease, aka, SAD).

• Be aware that most patients have a combination of these disorders, which is why signs and symptoms often overlap – thus, the familiar concept of “pink puffers” vs. “blue bloaters” is an oversimplification.

• Decreased breath sounds, which some describe as “distant” sounds.
• Cough, sputum production, wheezing, and dyspnea.*
• Rhonchi; recall that rhonchi are low-pitched rattling sounds produced by airway secretions.
• Patients who are having difficulty breathing may adopt specific behaviors, such as pursed-lip breathing or tripod position:
  – Pursed-lip breathing slows down the breathing and increases airway pressure; incidentally, learning how to do pursed-lip breathing is also a component of respiratory therapy.
  – In the tripod position, patients brace their upper against their legs or the arms of a chair; this position helps to take some of the work out of breathing.
  – Cyanosis of the lips and nail beds, which are signs of hypoxia.
• Specific morphological changes may also be apparent; these are due to high lung volumes and hyperinflation of the chest.
  – Chronic hyperinflation leads to flattening of the diaphragmand shortening of its muscle fibers, which renders it less mechanically capable.
  – Thus, accessory respiratory muscles must do more work (including the scalenes, sternocleidomastoid, trapezius, abdominal muscles). It may be possible to palpate these hypertrophied muscles.
  – Many patients develop a “barrel-shaped” chest, in which the anterior-posterior diameter of the chest becomes larger.
  – Hoover’s respiratory sign is a pronounced inward movement around the costal margin during inspiration.
• Weight loss and muscle wasting, especially of the lower extremities.
• Peripheral edema
  – Edema is a potential manifestation of the systemic effects of COPD, which can include right heart failure and/or reduced renal flow.
• Be aware that COPD is associated with various cardiac, renal, and metabolic co-morbidities.
• Markers of systemic inflammation are often elevated.

• Exacerbations are defined as acute bouts of worse symptoms that require changes in treatment.
  – Exacerbations are often due to bacterial or viral infections, or exposure to air pollution.

Pathologic Changes

• Smoking leads to pulmonary dysfunction via multiple mechanisms:
  – Increased mucus production, impaired ciliary clearance, oxidative stress, and inflammatory cell recruitment.
• We draw the lower respiratory system, so that we can show how different portions are affected by these mechanisms, and we label the bronchi and bronchioles of the conducting zone, and the respiratory bronchioles and alveoli of the respiratory zone.

Chronic Bronchitis affects the bronchi, which are the larger airways of the lungs.
– Clinically defined as a chronic cough with sputum production that lasts for three or more months in two consecutive years.

• We show the histopathology of chronic bronchitis, from outermost to innermost:
  – For context, show some hyaline cartilage
  – Mucosa is characterized by hypertrophy and hyperplasia of mucous glands
  – Smooth muscle is also hypertrophied
  – Submucosa is infiltrated by inflammatory cells (specifically, CD8+ T-cells, neutrophils, and macrophages)
  – Respiratory epithelium has excessive goblet cells and possible squamous cell metaplasia
  – Respiratory cilia are impaired
  – The mucus, which is normally thin and runny, becomes abundant and viscous, thanks to mucus gland hyperplasia.
• Cumulatively, these changes produce a thicker bronchial wall, with limited ability to clear debris; thus, airway obstruction can occur.

Small Airway Disease affects the bronchioles.

• Small changes in bronchiole airflow have influence over total airflow, so this is where the most significant airway obstruction occurs.
• Small airway disease is characterized by mucus plugging, inflammation, and airway remodeling, which comprises fibrosis and thickening; together, these factors narrow the bronchiole lumen.
• Variation in V/Q can result from airway obstruction that leaves some alveoli under-ventilated and other alveoli holding trapped air.

Emphysema affects the respiratory bronchioles and alveoli.

• Inflammatory cells, which are recruited in response to cigarette smoke and other irritants, secrete elastase, which is an enzyme that breaks down alveolar walls.
  – Alpha-1 antitrypsin is a protective protein that inhibits elastase, which is why individuals with alpha-1 antitrypsin deficiency are more susceptible to emphysema.
• Oxidative stress from smoking damages pulmonary collagen and elastin.
• Elastase and oxidative damage break down the walls of the respiratory bronchioles and alveoli.
• Thus, there is less elastic recoil; thus, there is a reduction in the driving force to expel air during expiration.
• There is also loss of structural support, so the alveoli and alveolar ducts are prone to collapse and trap air.
• With the loss of respiratory zone structures, there is less surface area for gas exchange.*
• Because of variation in the disease process, emphysema is associated with variation in V/Q.

Overview

• Acute respiratory distress syndrome (ARDS) occurs when the alveoli fill with fluid, which impairs gas exchange.
• Characterized by acute dyspnea, hypoxemia, and pulmonary infiltrates.
• Can lead to reduced lung compliance, increased pulmonary dead space, increased risk of pneumothorax, and pulmonary hypertension.
• ARDS has a mortality rate of approximately 40%.
• Treatment for acute respiratory distress syndrome requires treatment of the underlying causes.
• Mechanical ventilation restores airflow, but beware of potential complications, including: volutrauma (overdistention of the alveoli), alectectrauma (alveolar strain from repeated opening and closing), and, biotrauma (from migration of pro-inflammatory molecules and pathogens).
• Treatment includes fluid management measures, such as diuretics, to reduce left atrial filling pressure; some also recommend the use of neuromuscular blockades.
• Newborn respiratory distress syndrome (aka, infant respiratory distress syndrome, respiratory distress syndrome of the newborn) occurs when there is inadequate production of surfactant by premature lungs, leading to alveolar collapse; thus, treatment includes administration of synthetic surfactant and oxygen support. The risk of neonatal respiratory distress correlates with the degree of prematurity.

Pathophysiology: 3 stages
Exudative Phase: The initial response to injury, and occurs within the first 7 days after exposure.

• Inflammation damages the capillary endothelium and alveolar epithelium, and increases the permeability of these layers. We show neutrophils and their pro-inflammatory cytokines, but be aware that other cells of the innate immune system also play a role in barrier injury.

• Protein-rich fluid, activated neutrophils, and other pro-inflammatory mediators and cellular debris pass through the barrier and fill the alveolus.
• These infiltrating proinflammatory mediators damage the epithelial lining of the alveolus, leaving the basement membrane “denuded.”
• Furthermore, due to the loss of alveolar epithelial cells, surfactant production and fluid resorption is inhibited,which compounds fluid retention.
• Hyaline membranes from along the denuded basement membranes; these are formed by accumulating cellular debris and fibrin.
• The coagulation cascade is triggered by capillary endothelial damage, which leads to the formation of microthrombi in the vessels.
• As a result of these pathological processes, gas exchange is inhibited, dead space is increased, pulmonary hypertension occurs, and lung compliance decreases.

Proliferative Phase: barrier repair and fluid resorption and occurs days 7-21 after exposure to pulmonary injury.

• The alveolar epithelium and capillary endothelium barriers are re-established.
• Thus, surfactant production and fluid reabsorption resume
  – Epithelium sodium channels and aquaporins are inserted in the alveolar epithelium, and move fluids to the interstitium.
• Macrophages and lymphocytes remove apoptotic and inflammatory mediators, thus reducing further harm to the epithelia.
• As part of the healing and rebuilding process, fibroblasts and other interstitial cells proliferate to form a provisional extracellular matrix, which will eventually be removed by matrix metalloproteinases.
• However, in some individuals, pro-inflammatory and fibrotic forces overwhelm the healing and clearance process.

Fibrotic Phase: Fibrosis

• Extensive epithelial damage trigger over-proliferation and differentiation of fibroblasts and deposition of collagen, which leads to tissue fibrosis and destruction of the microvasculature.
• Thus, pulmonary dysfunction continues.

Causes of ARDS

• Direct and indirect causes of lung injury lead to acute respiratory distress.
• The most common causes are pneumonia (direct) and sepsis(indirect)
  – Account for more than half of all ARDS cases.
• Additional Direct Causes:
  – Aspiration of gastric contents
  – Pulmonary contusion
  – Near drowning
  – Vaping.
  Be aware that lung injury caused by vaping is sometimes called “EVALI” – “E-cigarette or Vaping Associated Lung Injury”, and is particularly associated with vaping fluids containing Vitamin E acetate (Vitamin E acetate is used in THC vaping products).
• Additional Indirect Causes:
  – Trauma
  – Repeated blood transfusion
  – Pancreatitis
  – Drug reactions or overdoses (ex: various narcotics, aspirin, tricyclic antidepressants).

Berlin Definition of ARDS
Establishes diagnostic criteria

• Onset of signs and symptoms must be *within the last 7 days of known clinical insult, with new or worsening symptoms during the last week.
• Chest X-ray or CT will show bilateral opacities consistent with pulmonary edema.
• Respiratory failure cannot be otherwise fully explained by cardiac failure or fluid overload.
• Presence of acute hypoxemia.
  – Based on the ratio of the partial pressure of arterial oxygen (PaO2) to the fraction of inspired oxygen (FiO2) (assessed while the patient is on a ventilator with a positive-end expiratory pressure (PEEP) of 5 or greater cm H2O).
  – Hypoxemia severity can be categorized as mild (201-300 mmHg); moderate (101-200 mmHg), or severe (100 mmHg or less).

DEFINITIONS

Bohr effect

• Low pH enhances hemoglobin oxygen dissociation

2, 3 Bisphosphoglycerate (BPG)

• Molecule localized in red blood cells
• Decreases hemoglobin’s oxygen affinity

DISSOCIATION CURVE

• % oxygen saturation vs. oxygen partial pressure (torr)
• Cooperative binding produces sigmoidal binding curve

Bohr Effect

• Decrease in blood pH shifts curve to the right
• Hemoglobin requires greater pO2 in peripheral tissues to reach 50% saturation
• Lowering pH decreases hemoglobin’s oxygen affinity

2,3 BPG

• Hemoglobin without 2,3 BPG: shifts curve to the left (hyperbolic like myoglobin)
• Adding 2,3 BPG: shifts curve back to the right.

BOHR EFFECT

Beta subunit

• Aspartate (-) and histidine (+)

T-form: histidine pKa = 8.0 (side chains close to each other)

• T-form favored when blood pH –> has a high affinity for H+

R-form: histidine pKa drops to 7.1 (side chains move apart)

• Histidine loses H+
• R-form favored when blood pH is high and H+ concentration is low

Carbon dioxide

• CO2 + H2O –> H2CO3 (carbonic acid) –> HCO3- (bicarbonate) + H+ (reversible)
• Carbonic anhydrase catalyzes carbonic acid formation
• Carbonic acid spontaneously loses proton to form bicarbonate
• Bicarbonate = blood buffer
• Increase in CO2 –> lowers blood pH –> favors T-form hemoglobin

Carbaminohemoglobin: CO2 binds N-terminal amino acids in hemoglobin

2,3 BPG

• Has strong negative charge: binds central cavity in hemoglobin
• Stabilizes the T-form (only binds T-form)

CARBON MONOXIDE

• Binds iron center with 220 times the affinity of O2 (irreversible)
• Permanently increases oxygen affinity of remaining heme groups for oxygen
• Decreases oxygen release in peripheral tissues

CLINICAL CORRELATION

Acetazolamide

• Carbonic anhydrase inhibitor used to treat altitude sickness
• Increases bicarbonate excretion by kidneys
• Makes blood more acidic, promotes oxygen release in peripheral tissues

High altitude conditions

• Individuals adapted to high altitude produce more 2,3 BPG
• Favors T-form hemoglobin and O2 release: more efficient O2 delivery

Tobacco smoke

• Smokers have elevated blood CO: hinders O2 delivery
• Can produce tissue hypoxia

Definitions

Cooperative binding

• Describes unique interactions between heme groups in hemoglobin
• Small movement of heme group propagates through hemoglobin’s 3D structure

HEMOGLOBIN STRUCTURE

R-form (relaxed)

• Alpha-alpha interactions: weak ionic and H-bonds form salt bridges
• Beta-beta interactions: no interactions; move apart upon oxygenation
• Alpha-beta dimers: strong hydrophobic interactions within each dimer

T-form (tense)

• 3D structure changes between oxygenated and deoxygenated states

HEME SITE

T-form (tense)

• Heme site: when O2 leaves, iron center moves out of porphyrin plane & proximal histidine moves away from iron center
• Small movement in heme group makes O2 binding unfavorable

SALT BRIDGES

• Salt bridges break and reform upon oxygen binding –> peptide wiggle-room
• Alpha chain salt bridges:
  – Alpha1: arginine carboxy terminus (-) and arginine side group (+)
  – Alpha2: lysine (+) and aspartate (-)
• Salt bridges regulate cooperativity: iron centers move into porphyrin planes

DISSOCIATION CURVE

• % oxygen saturation vs. oxygen partial pressure (torr)
• Cooperative binding produces sigmoidal binding curve
• After hemoglobin reaches 50% saturation: saturation increases rapidly (steepest point of curve)
• Hemoglobin O2 affinity rapidly increases at half saturation

Globular proteins

• Compact proteins that are approximately spherical in shape

Hemoproteins

• Specialized proteins that have prosthetic heme group

Prosthetic groups

• Non-protein molecules that are essential to biological function

HEME GROUP STRUCTURE

• Porphyrin ring with iron center (Fe2+)

Fe2+ coordinates 6 bonds:

• 1-4. Four planar nitrogen atoms (of porphyrin ring)
  5. Proximal histidine
  6. Oxygen

MYOGLOBIN

• Skeletal and cardiac muscle
• Reservoir for oxygen
• Single polypeptide with 8 alpha helix segments
• One heme group
• Distal histidine holds oxygen in place

HEMOGLOBIN

• Red blood cells
  – Supplies body’s tissues with oxygen
• 4 polypeptides (instead of one): each subunit resembles myoglobin structure
  – Tetramer: with 2 alpha-beta dimers
• Strong hydrophobic interactions: stabilize alpha-beta dimer
• Weak ionic and H-bonds: between dimers

T-form hemoglobin

• “Taught” or “tense” form: polypeptides restricted in movement
• Deoxygenated form: oxygen affinity is low

R-form hemoglobin

• “Relaxed” form: weaker ionic and H-binds between dimers
• Oxygenated form: oxygen affinity is high

COOPERATIVE BINDING

• Conformational change between T-form and R-form hemoglobin
• Myoglobin does not exhibit cooperative binding: only one oxygen binding site
  – One oxygen binds hemoglobin subunit
  – Binding disrupts inter-dimer bonds: causes conformational change
  – Change in 3D structure increases oxygen affinity of remaining subunits

DISSOCIATION CURVE

• % oxygen saturation vs. oxygen partial pressure (torr)
• Cooperative binding produces sigmoidal binding curve
• pO2 in body’s tissues: 30 torr
• pO2 in lungs: 100 torr

Hemoglobin

• Half saturated at 30 torr (body’s tissues): responds to O2 availability

Myoglobin

• Hyperbolic curve (simpler binding pattern corresponds to single heme)
• High affinity for O2
• Binding properties correspond to role in oxygen in storage (not oxygen delivery)
• Early curve: exercising muscle; plateau: muscle at rest

CLINICAL CORRELATION

Fetal hemoglobin

• Dissociation curve to the left of adult hemoglobin
• Greater affinity for O2: O2 transfer from maternal hemoglobin to fetus

OVERVIEW

Medulla

• The medulla is the primary brainstem mediator of respiration.
• Via the dorsal respiratory group (DRG), the dorsal (posterior) medulla controls sensory integration.
  • For its location, think: solitary tract nucleus.
• Via the ventral respiratory group (VRG), the ventral (anterior) medulla controls motor output.
  • For its location, think: nucleus ambiguus.

Phrenic nerve

• C3, C4, C5 supply the phrenic nerve, which innervate the diaphragm: C3, C4, C5 “keep the diaphragm alive”.

BRAINSTEM CIRCUITRY

Ventral respiratory group (VRG)

• Within the medulla, anteriorly, lies the ventral respiratory group (VRG), which lies within the ventrolateral medulla.
  • It provides innervation for motor output.
  • It is involved in the activation of both inspiration and expiration.

Dorsal respiratory group (DRG)

• Within the dorsal medulla (in the solitary tract nucleus), lies the dorsal respiratory group (DRG).
  • It provides sensory integration.
  • It receives sensory input related to the inspiration phase of respiration.

PERIPHERAL CHEMORECEPTORS

Peripheral chemoreceptors act on the dorsal respiratory group

• Peripheral innervation involves the aortic bodies (shown here in the arch of the aorta) and the carotid bodies in the carotid bifurcation.
  • The carotid and aortic bodies are chemoreceptors.
  • They respond to levels of arterial oxygen and carbon dioxide levels and blood acidity.

Innervation to the brainstem respiratory center

• Both cranial nerves 9 and 10 (we treat them jointly for simplicity) pass through the jugular foramen within the skull base across from the brainstem to innervate the dorsal respiratory group.

SECONDARY INSPIRATORY MUSCLES

Key structures

• Anterior face, tongue, pharynx, and larynx.

Innervation

• Cranial nerves 9, 10, and 12 innervate the secondary inspiratory muscles (again, we treat them jointly for simplicity).

VRG innervation of CNs 9 and 10

• The ventral respiratory group acts upon these cranial nerves.

PRIMARY INSPIRATORY MUSCLES

Key structures

• Thoracic cage, diaphragm, and intercostal muscles.

Innervation

• The ventral respiratory group innervates C3, C4, C5 motor neurons in the anterior horn of the spinal cord gray matter, which supply the phrenic nerve, which innervates the diaphragm (again: C3, C4, C5 “keep the diaphragm alive”).

Intercostal nerves

• Intercostal nerves innervate the intercostal muscles.

DETAILED ANATOMY OF THE DRG & VRG

Dorsal respiratory group (DRG)

• The dorsal respiratory group lies within the solitary tract nucleus of cranial nerves 9 and 10.

Ventral respiratory group (VRG)

Simplification of VRG microanatomy

• First, add nucleus ambiguus of CNs 9 and 10.
  • This will help us continue to recall the important of CNs 9 and 10 and thus the medulla itself in respiratory control.
• There are many subnuclei that constitute the ventral respiratory group; we’ll only address the Bötzinger nuclei, here.
• The Bötzinger complex lies within the superior aspect of the ventral respiratory group (some authors distinguish it from the ventral respiratory group, entirely).
• The pre-Bötzinger complex is considered the “respiratory pacemaker.”
  • Notably, it contains mu receptors, which makes it sensitive to opioids.
  • Thus, we can see one of the ways in which opioids (such as morphine) can depress our drive to breathe.

CLINICAL CORRELATION: ONDINE’S CURSE

• Ondine’s curse is the clinical eponym for the failure of automatic breathing during sleep.
  • It typically occurs from lower medullary or high cervical spinal cord lesions.
  • These patients are dependent on a ventilator when they sleep to survive.

PONTINE RESPIRATORY CONTROL CENTERS

Apneustic center

• The apneustic center is a nonspecific region in the posterior lower pons.
  • It promotes apneusis: a prolonged inspiratory pause.
  • It comprises diffuse lower pontine nuclei.

Pneumotaxic center

• The pneumotaxic center (aka the pontine respiratory group).
  • It prevents apneusis: it promotes regular breathing.
  • The pneumotaxic center comprises the medial parabrachial nucleus and the Kölliker-fuse nucleus.

Functions

• Whereas these centers where formerly thought to be well-defined and to perform unique functions, now they are understood to be diffuse and their functions are no longer thought to be unique (the pneumotaxic center is not the only site to prevent apneusis, for instance).

BREATHING PATTERNS

We can use breathing patterns in comatose patients to localize the level of the CNS lesion, as follows:

Patterns

• Cerebral hemispheric lesions cause Cheyne-Stokes respirations.
  – Illustrate Cheyne-Stokes respirations as periods of hyperpnea (deep breathing) with apneas (cessation of breathing)
• Midbrain lesions cause hyperventilation.
  • Illustrate hyperventilation as rapid, deep breathing.
• High pontine lesions cause apneustic breathing.
  • Illustrate apneustic breathing as periods of long inspiratory pauses before release of air.
• Low pontine lesions cause cluster breathing.
  • Illustrate cluster breathing as irregular clusters of breaths.
• Medullary lesions cause ataxic breathing.
  • Illustrate ataxic breathing as a completely irregular breathing pattern.

Limitations

• Although these localizations are notoriously unreliable, they still give us a simple heuristic to follow when we examine comatose patients, which is essential.

VENTILATION-TO-PERFUSION RATIOS

• The ventilation/perfusion (V/Q) ratio is an indication of how well alveolar ventilation matches pulmonary capillary perfusion.
  • Due to gravitational forces, the V/Q ratio ranges regionally in the lung, from 3.0 at the apex to 0.6 at the base
• Clinicians and physiologists typically use the average value for the entire lung as a reference point.
• Healthy average alveolar ventilation rate is approximately 4 liters of air/minute
• Blood flow rate is approximately 5 liters of blood/minute.
• A healthy V/Q for the entire lung is 0.8.

Healthy V/Q

• Inspired air flows through the tracheobronchial tree and to the thin-walled, sac-like alveoli
• Pulmonary capillaries are in close physical proximity
• When the V/Q is 0.8, ventilation and perfusion are well matched and optimal gas exchange occurs (not perfect, which would be V/Q = 1; we’ll use the average for the entire lung).
• The partial pressures of oxygen and carbon dioxide in the alveoli and pulmonary blood flow equilibrate (the specifics of gas exchange are discussed elsewhere).

V/Q mismatches, aka, defects, occur along a spectrum.

• Two extremes of mismatches are shunts and dead space:
  • Shunts occur when the rate of alveolar ventilation is zero; V/Q = 0
  • Dead spaces occur where the rate of blood flow is zero; V/Q = infinity
• Notice that the normal V/Q of 0.8 is between the two extremes (a V/Q of 1 would be perfect, but we are using the average value)
• Mismatches between 0 and 0.8 reflect defects in alveolar airflow
• Mismatches greater than 0.8 reflect defects in pulmonary capillary blood flow

Shunt

• Most extreme alveolar ventilation defect.
• Occurs when air flow is blocked and the alveoli remain unventilated.
• Unventilated alveoli cannot participate in gas exchange with the pulmonary capillaries, which remain de-oxygenated.
• As a result, the partial pressures of oxygen and carbon dioxide of the pulmonary blood remain equal to that of mixed venous blood.

Airway obstruction is a common cause of shunts

• Because airflow ceases, alveolar collapse, aka, atelectasis, can occur in all or a portion of the lung.
  • Without proper ventilation, the partial pressure of arterial oxygen is reduced (aka, hypoxemia).
    In the case of shunts, hypoxemia cannot be reversed by administration of concentrated oxygen because oxygen does not reach the pulmonary blood flow.

Reduced ventilation

• Alveoli are partially ventilated, so some gas exchange occurs with the nearby pulmonary capillary.
• When ventilation is less than perfusion, the partial pressure of pulmonary blood carbon dioxide increases because it is held within the body, and the partial pressure of oxygen decreases because the normal amount of fresh oxygen is not available.

Reduced perfusion

• Reduced perfusion causes an increased ventilation-to-perfusion ratio.
• The partial pressure of pulmonary blood carbon dioxide decreases, while the partial pressure of oxygen increases.

Alveolar dead space

• Absence of blood perfusion produces a ventilation-to-perfusion ratio equal to infinity.
• No gas exchange occurs.
• As a result, the partial pressures of oxygen and carbon dioxide of alveolar gas remain equal to that of inspired air.
• Pulmonary embolism is a common cause of alveolar dead space
  • To minimize “wasted” ventilation, bronchiolar constriction diverts air from non-perfused alveoli.