Chapter Outline
- 1. Introductory Anatomy: Respiratory System P Ramesh Menon
- 2. Physiology of the Respiratory System Lokesh Guglani
- 3. History Taking and Physical Examination Sushil K Kabra, Rakesh Lodha
- 4. Investigations in Pulmonology
- 4.1: Role of Chest Radiograph and Ultrasound in Respiratory Disorders in Children Manisha Jana, Ashu S Bhalla, Arun K Gupta
- 4.2: Microbiological Investigations Sushil K Kabra, Rakesh Lodha
- 4.3: Thoracocentesis Sushil K Kabra
- 4.4: Lung Puncture and Lung Biopsy Sushil K Kabra
- 4.5: Pulmonary Function Test Babak Ghalibafsabbaghi, Sushil K Kabra, Rakesh Lodha
- 4.6: Infant Pulmonary Function Test Babak Ghalibafsabbaghi, Sushil K Kabra
- 4.7: Impulse Oscillation Technique Sushil K Kabra
- 4.8: Plethysmography Sushil K Kabra
- 4.9: Bronchoscopy and Bronchoalveolar Lavage in Children Prawin Kumar, Sushil K Kabra
- 4.10: Ultrasound (Endobronchial Ultrasound)/Computed Tomography Guided Biopsy/Fine Needle Aspiration of Mediastinal Masses Anirban Mandal, Sushil K Kabra
- 4.11: Sweat Chloride Estimation Sushil K Kabra, Madhulika Kabra
- 4.12: Arterial Blood Gas Analysis Rakesh Lodha
- 4.13: Miscellaneous Investigations Sushil K Kabra2
Introduction
Respiration in humans essentially involves two processes. The first process is called external respiration and involves moving (breathing) air in and out of respiratory tract. It includes ventilation, which implies absorption of oxygen and release of carbon dioxide into the alveolar air. The second process is called internal (cellular respiration) and involves production of energy by oxidizing substrate molecules.1,2 In unicellular protozoa, both are synonymous and synchronous. Longer diffusion distances in multicellular organisms and terrestrial vertebrates implied that breathing (external) and ventilation (pulmonary) were different and distinct from cellular oxidation (internal) processes. This led to the evolution of respiratory structures from the pharyngeal diverticulum (laryngeal sphincter and phonation, tracheobronchial tree, and alveoli).
The respiratory tract, where external respiration occurs, is made up of the organs involved in the interchange of gases. It can be divided into two major parts—upper airways and lower airways.
Upper airways3-5 consist of the nose—external and internal (choanae) nares, vestibule, nasal cavities, conchae, septum, paranasal sinuses, valves, and the pharynx and larynx.
External nares are the proximal most part of respiratory tract and are lined by normal skin (stratified squamous epithelium) containing sweat, sebaceous glands, and hairs (vibrissae). It acts as the first filter of air into the respiratory tract. It extends into the vestibule and the nasal valve.
Nasal cavities: Distal to vestibule are the nasal cavities; in these, the skin is replaced by respiratory mucosa. The roof of nasal cavity is composed of the cribriform plate (ethmoid bone) and the sphenoid bone. The floor is constituted by the hard palate (palatine processes + bones) and the soft palate. The lining has ciliated columnar epithelium (goblet cells and serous glands). They produce a fluid containing lysozymes that is bactericidal.
There are three bony projections in each nasal cavity; the superior and middle conchae (turbinates) belong to ethmoid bone and inferior concha is a separate bone. The air conditioning functioning of the nose is carried out by the rich blood supply here. This helps to maintain optimal temperature of incoming air. The ciliated lining is characterized by metachronal waves. This moves trapped particles to the pharynx (throat) where it is swallowed. Within the lamina propria of the epithelia are swell bodies. These are venous plexi that engorge with blood causing occlusion of airflow between the conchae shelves; the phenomenon alternates between the sides about every 30 minutes.
Anterior nasal valve between the vestibule and the nasal cavity is the most constricted part of the upper airway in infants. Even partial blockage of nose may result in significant respiratory distress since young infants are preferable nose breathers.
Nasal septum extends from the external nares, being soft tissues and hyaline cartilage anteriorly and vomer (lower) and perpendicular plate of the ethmoid bone (upper) posteriorly 4separating the nasal cavity. Several vessels anastomose on the anterior septum and are called Little's area (Kiesselbach's plexus); this is a frequent site of epistaxis especially with nose picking.
Paranasal sinuses are air spaces. Frontal, ethmoid, maxillary, and sphenoid bones that connect with the nasal cavities have these. Maxillary and ethmoid sinuses start developing at 10 weeks of gestation and are fully developed at birth. Sphenoid sinuses start developing by age of 3 years and are fully developed by 8 years. Frontal sinuses start developing by 7–8 years and are fully developed by early teens. Functions of sinuses include lightening of skull, production of mucus to enhance local defense, and as resonance chambers to amplify vocalizations. All the sinuses open into ostiomeatal complex except the sphenoid (into sphenoethmoidal recess) and posterior ethmoid cells (into superior meatus). The nasolacrimal duct opens into the inferior meatus. The entire respiratory passage functions as a unit (unified airway).6 Hence, afflictions of the nose and sinuses may also affect the trachea and bronchi, that is why, proper control of rhinitis in asthmatic child improves control of asthma. The space opening of nasal cavities in the superior portion of the pharynx through constrictions at the end are called choanae.
The pharynx is a funnel-shaped tubular structure forming proximal part of airways and gastrointestinal tract. Pharynx does not have rigid support; it is supported by various muscles including genioglossus, geniohyoid, sternohyoid, sternothyroid, and thyrohyoid muscles. These muscles are responsible for keeping it open by maintaining pharyngeal wall tension, and synchronizing activities with diaphragmatic contraction.7 Pharynx may get obstructed due to posterior displacement of the mandible during sleep in the supine position, flexion of the neck, external compression over the hyoid bone, negative intraluminal pressure during inspiration, and muscular paralysis or hypotonia due to any cause (e.g., anesthesia). During neonatal period, the oropharynx and the inlet of larynx at the level of the aryepiglottic fold is more easily collapsed due to neuromuscular or anatomical problems. In older children, muscle tone is impaired during sleep leading to compression of pharynx. This is reversible, and after awakening from sleep, the pharyngeal volume is maintained by resuming active tone of muscles. In infants, tongue is relatively larger than in adults. With loss of tone, tongue may cause upper airway obstruction.
Pharynx is divided into three parts:
- Nasopharynx: Part of pharynx above soft palate. It is lined with ciliated epithelium and is responsible for moving mucous and debris down towards esophagus so that they can be swallowed. In its posterior aspect, pharyngeal tonsils are located. Tonsils act as guard against microbial pathogens to airways. Eustachian tubes open in the nasopharynx and are responsible for equalization of air pressure within the middle ear. Due to smaller size and relatively straighter course in young children, bugs may be aspirated into middle ear and predispose them to frequent otitis media.
- Oropharynx: Part of pharynx between nasopharynx and laryngopharynx. It communicates with the oral cavity. This food and air passage overlap here. It is lined with stratified squamous epithelium. Its communication with oral cavity is lined by palatine and lingual tonsils.
- Laryngopharynx: Part of pharynx between hyoid bone to the larynx. There is division of airway and esophagus here. The laryngopharynx then continues as esophagus.
Larynx:8 It starts at glottis and continues distally as trachea. Glottis is covered with a flap of cartilage called epiglottis. Functions of larynx include protective, respiratory and phonatory. This is the sphincter of airways preventing aspiration of inhaled microbes/foreign bodies. It has evolved in humans for phonation as well. This anatomical evolution holds the secret of the development of communication, language, and human creativity (music and arts).
Larynx is cone shaped, narrowest at the subglottic cricoid ring, whereas in the adult it is cylindrical, narrowest at the glottis opening (vocal cord). It occupies a more superior and anterior position in neck—infant: C 1; 6 months old: C 3; adults: C 4–5.
Epiglottis is relatively larger in children. Infant epiglottis is omega (Ω) shaped and floppy, angled slightly more over the glottis and without much cartilage. During swallowing, glottis is closed by epiglottis and prevents aspiration. Any epiglottitis may lead to aspirations.
Vocal cords are important structure in larynx and their movements are regulated by various muscles and are responsible for phonation. Above the levels of vocal cords, larynx is lined with stratified squamous epithelium and lower part is lined with pseudostratified ciliated columnar epithelium. The cilia in this region beat up moving mucous and debris away from larynx. Stimulation of the larynx by ingested matter produces a strong cough reflex to protect the lungs. Choking occurs when food becomes lodged below glottis. Vocal cords are also slanted with the anterior commissure being more inferior in infants. A blindly placed endotracheal tube (ETT) may easily lodge in anterior commissure rather than in trachea. In addition, it is more difficult to maneuver an infant's epiglottis. This makes the use of a straight (Miller) laryngoscope blade better for visualization of the glottis in young infants during resuscitation.
The trachea starts below the larynx. Entering the mediastinum at the thoracic inlet, it is anatomically an inverted U-shaped (cross section) cylinder joined posteriorly by smooth muscle bands (trachealis). Though it has support of cartilages, it may be compressed and occlude airways with external pressure of 50 cmH2O. This major conducting airway acts as a reinforced tube with compliance (C-ring) and elasticity (trachealis) unlike any other tissue in the body (see later). It divides into right and left bronchi at the level of the 5th thoracic vertebra. The right main bronchus is aligned in such a manner that inhaled objects are found there more often than left even when the infant is supine or sitting (wider, shorter, and more vertical than the left). Both bronchi enter the lungs at the hila and divide subsequently into segmental bronchi, subsegmental bronchi (up to 5th generation), smaller bronchi (about 15 generations), terminal bronchioles, respiratory bronchioles (3 generations), and alveolar ducts and sacs (Fig. 1).
As an introduction to the clinical application of tracheal anatomy, it is important to understand the concept. A detailed discussion is beyond the scope of this chapter. At present, plaster of Paris cast based static measurements of the airway are usually reported for anatomical norms. The dynamic modulus is a frequency-dependent material property. In general, measurements are conducted by either of the two methods—(i) An oscillatory strain is imposed and the resulting oscillatory stress is measured or (ii) An oscillatory stress is imposed and the resulting oscillatory strain is measured. Knowing the dynamic mechanical properties is important for understanding dynamic loads, e.g., how much the trachea deforms due to impact loading, or due to applied deformations at the frequency of breathing, running or whatever. Other mechanical tests characterize the dynamic mechanical properties, such as stress relaxation test or creep test. The clinical application of the concept for an advanced student is in helping to understand the conducting system for gases (the cartilage rings interspersed with soft tissue like a corrugated tube in a mechanical ventilator), dead space ventilation and determining the timing and site of tracheostomy in an electively ventilated infant/child (non-emergency situation).9-11 Ultrasound assessment is being done to assess the trachealis muscle thickness in chronically ventilated neonates and infants to assess the deformation of developing trachea in infants.10
The bronchial tree branches into ten functional bronchopulmonary segments on either side (Fig. 2). Each bronchopulmonary segment is an autonomous unit for airflow and blood supply. That means that in case of damage by trauma or infection of a specific segment (e.g., lobar pneumonia), flow may be reduced to that specific segment (only). Vasoconstriction of the blood vessels and bronchoconstriction of the bronchioles occurs.
Bronchioles are the smallest branches of the bronchi in the lungs.12 Cartilage in the walls extends up to the 11th generation (diameter ~ 1 mm); subsequently the air passages are directly embedded and held open by the lung parenchyma and patency depends on the lung volume. Beyond the terminal bronchiole, nutrition is derived from pulmonary circulation. Each terminal bronchiole delivers air to a single pulmonary lobule. Beyond the 16th generation, respiratory (functional) zone begins (TRU, terminal respiratory unit).
Bronchiolar cells: These are tall, cupolar cells extending up to the tips of the cilia of the adjacent cells. Apparently, the mucosal lining has a scalloped appearance under light microscopy. These cells, for unknown reasons, are mostly present in bronchioles of 1 mm or less in diameter. Bronchiolar cells are very active metabolically producing serous secretions rich in mucolytic and proteolytic enzymes.
They probably have a role in liquefying the viscous mucous 6secreted elsewhere in the bronchial tree. Serous cells are more numerous than mucous cells in the distal regions. Clara cells in the distal portions of terminal bronchioles secrete surface-active lipoproteins. The neuroepithelial body contains a number of neurosecretory13 cells (bronchial Kulchitsky cells) that are positive for dopamine. There is no direct sympathetic innervation of smooth muscles of bronchi. However, functional β2 receptors of undefined physiological role are found in bronchiolar smooth muscles. During anoxic or hypoxic episodes, catecholamines are locally released. The smooth muscle of the bronchiolar walls may respond. This would lead to a dilation of bronchioles. Parasympathetic stimulation, on the other hand, via the vagus nerve may cause bronchoconstriction. Nonadrenergic noncholinergic (NANC) nervous system with vasoactive intestinal polypeptide (VIP), nitric oxide (NO) and substance P as probable mediators is also found in the bronchioles. The peripheral airways contribute to total airways resistance (50%) more than in adults (20%): Resistance to airflow is inversely proportional to the fourth power of the radius of the airway. One mm of concentric edema in a newborn trachea (radius approximately 2 mm) increases resistance about 16 times.
Alveolus is a cul-de sac. A functional outpouching from the wall of the respiratory bronchiole or alveolar duct numbering about 300 million in the lungs (150 million/lung). There are contractile cells in alveolar interstitium, which can contract and reduce sac volume. They also have alveolar pores (of Kohn) between sacs which equalizes pressure gradient between sacs. The alveolocapillary thickness14 is usually less than 0.4 µm. There are two types of cells lining alveolus called type 1 and type 2 pneumocytes.
Type 1 pneumocytes are very thin (about 0.2 µm), line the alveoli and cover about 95% of gas exchange surface. They do not replicate and have a turnover time of about 4–6 weeks. They meet at the tight junctions but permit free passage of macrophages and polymorphs in response to chemotactic stimuli. Type 2 pneumocytes are the stem cells for type 1 and are found singly or in small groups between the type 1 cells. These cells contain lamellar bodies and produce surfactant proteins A, B, C, and D that form a thin coat of lipoprotein complex on the inner surface of the alveolus.15 Surfactant consists of dipalmitoyl phosphatidylcholine (>50%), unsaturated phosphatidylcholines (25%), phosphatidyl glycerol (5–10%), glycerol (5%), and other proteins (up to 10%). A high proportion of its fatty acids are saturated (i.e., contain two palmitic acid residues), rendering it surface active. In premature infants, there is decreased palmitic acid producing a qualitative change in pulmonary surfactant.
Surfactant increases pulmonary compliance, reduces work of breathing by reducing alveolar edema and atelectasis. It also acts as a hydrophobic, anticorrosive, anti-inflammatory, and microbicidal lubricant.
Pulmonary Circulation
The pulmonary arteries and bronchioles travel to the center of lobule whereas the pulmonary veins are found between lobules. The bronchial circulation arteries arise from intercostal arteries or from the aorta and supplies blood to mediastinal structures and airways up to terminal bronchioles. This blood supply is not critical as lungs can function without it as in lung transplantation.
Lymphatic drainage is there up to 11th generation bronchioles. The superficial and deep pulmonary lymphatics from these bronchioles anastomose at the hilum. Beyond the 11th generation, there are no identifiable lymphatic vessels and interstitium acts as a fluid pump, draining fluid to terminal lymph vessels at the level of respiratory bronchiole. There is a large flux of fluid from the parietal to visceral pleura, absorbed by the pleural lymphatics. Normally, very small amount of fluid remains in pleural space.
Muscles of Respiration
These include diaphragm and intercostal muscles.
Diaphragm
The diaphragm is the most important muscle used for participating in respiration. It is attached to base of the sternum, the lower parts of the rib cage, and the spine. As the diaphragm contracts, it increases the length and diameter (together with the intercostals) of the chest cavity and thus expands the lungs. Due to the obtuse angle of insertion on the rib cage and the small area of apposition, the flat diaphragm of the infant is designed inefficiently to suck in air.16 During quiet respiration, much of breathing in infants is diaphragmatic. At best, the efficiency of diaphragm in a term, healthy baby without any respiratory problems, lying supine, is about 8%. The near perpendicular attachment of the diaphragm makes the compliant chest wall move in with each inspiratory effort in a baby lying supine.
Intercostal Muscles
The intercostals, which aid respiratory effort in adults as a reserve, are relatively inefficient in young infants. In infancy, the chest wall is nearly three times as compliant as the lung and that by the second year of life, chest wall stiffness increases to the point that the chest wall and lung are nearly equally compliant, as in adulthood. Stiffening of the chest wall may play a major role in developmental improvements in respiratory system function such as the ability to passively maintain resting lung volume and improved ventilatory efficiency afforded by reduced rib cage distortion.17
Anatomical differences relevant in pediatric resuscitation:18
- Babies come in various sizes ranging from: 0.5–50 kg. An intensive care unit pediatric crash cart must be bigger with a wider range of equipment (sizes) than other carts
- Airway sizes may vary unpredictably among pediatric patients of same age and weight. So at least three differently sized ETTs should be ensured before airway protection
- Infants have minimal oxygen reserve (with greater oxygen consumption), so risk of rapid oxygen deficiency is greater
- Young pediatric patients are more likely to be completely uncooperative. Use of sedation and/or paralysis is often a necessity for effective interventions
- Infant head is usually large and remains flexed in the supine position with or without pillow. Keeping head extended may result in extubation while flexion may lead to bronchial intubation
- Inhalational agents can quickly affect the sensitive upper airway of the infant resulting in collapse. Oral airways may be quite useful and appropriate size should always be available
- Young children (esp. 12–24 months of age) are at risk of foreign body (food, coins) aspiration
- Asymptomatic gastroesophageal reflux is very common in infants. It usually resolves spontaneously. Some regurgitation occurs in all infants. Proton pump inhibitors may be added as acid aspiration prophylaxis if significant reflux was preexistent or not known.
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