Respiration is the uptake of oxygen by the body and the elimination of carbon dioxide. It can be divided into:
- External respiration:Ventilation and gas exchange at the level of lungs is called external respiration.
- Internal respiration:Combustion or biological oxidation of nutrients by oxygen to carbon dioxide and water at cellular level is called internal respiration.
The respiratory organs can be divided into:
- Upper airway
- Nasal cavity and Nasopharynx
- Oral cavity and Oropharynx
- Pharynx and Laryngopharynx
- Lower airway
- Bronchial tree
The respiratory tract includes the nasal cavity, the larynx, the trachea and the bronchial tree.
Nasal Cavity: Figure 1.1
The surface of the nasal cavity which includes turbinates and septum are covered with highly vascular mucosa that plays a major role in warming and humidification of the air.
Anaesthetic significance of nasal cavity functions are:
- Breathing through the nose:The adult patient breathes through the nose unless there is some form of an obstruction such as a nasal polyp. In normal subjects, the resistance created by the nasal passage is one and a half times greater than in mouth breathing. Deflection of the nasal septum may diminish the size of the nasal passage, reducing the size of the nasal endotracheal tube and increasing the airway resistance.
- Cleaning:Stiff hair, spongy mucous membrane and ciliated epithelium comprise a powerful defence against any organism. The hair present inside the nose nearest to the nostrils, clears the air of larger particles. The cilia are responsible for trapping and removing small foreign particles.
- Warming the inhaled air:The vascularity of mucosa helps to maintain a constant temperature. In the nasal cavity, there are a number of superficial, thin walled blood vessels which radiate heat and thereby warm the inspired air from 17°C to 37°C when it is passing through the nasal passage.
- Humidification of the inhaled air:The nasal cavity is kept moist by glandular secretions which also humidify the air. Relative humidity of air is 45–55% but the bronchi and alveoli require 95% for adequate functioning. The inspired air, which passes through the nose, is thus fully humidified.Anaesthetic significance of humidification:
- If the inhaled air does not pass through the nose, (for example when breathing through the mouth) partial drying of the mucous membranes of the lower airways occurs, making them more prone to infection.
The larynx protects the lower airway by closing the glottis (for example, during swallowing). Classic teaching has been that the adult larynx is cylindrical and the infant larynx is funnel shaped. The extrapulmonary airway (larynx) is at its narrowest at the vocal cords in an adult and at the level of the cricoid cartilage in children up to 10 years of age. However recent autopsy data has demonstrated that the narrowest part in approximately 70% of adults is also the subglottic region at the level of cricoid cartilage, but the opening is so large that commonly used endotracheal tubes are nearly always easy to advance past the glottic opening.
Any further narrowing at the vocal cords can give rise to considerable respiratory distress. The laryngeal mucosa can become oedematous in the postextubation period or due to anaphylactic reactions. This can cause life-threatening problems.
The trachea is a cartilaginous tube made up of 16–20 horseshoe shaped cartilage rings which are incomplete posteriorly and connected by tissue and smooth muscle (tracheal muscle) membrane. The trachea measures about 10–12 cm in length and 11–12.5 mm in diameter in an adult.
The trachea moves during respiration and with a change in position of the head. On deep inspiration, the carina can descend as much as 2.5 cm and the extension of the head can increase the length of the trachea by 25–30%. Therefore, the position of the endotracheal tube should always be checked for accidental extubation or endobronchial intubation after any change in the position of the head.
The bronchial tree subdivides into 23 generations, the 23rd generation being the alveoli. The total diameter of the airways increases considerably towards the periphery. The bronchioles begin at the 10th generation and their diameter measures less than 1 mm, the walls are free of cartilage, rich in smooth muscle fibres and the epithelium no longer contains mucous producing cells.
Up to the 16th generation, the bronchi play no role in gas exchange, their only purpose being transportation of air. The gas exchange zone begins with the respiratory bronchioles where the smooth muscle fibres become rarer and there is an increase in alveolar budding.
Right Main Bronchus: Figure 1.4
The right main bronchus is wider and shorter than the left, being only 2.5 cm long. The angulations of both the bronchi are not equal and it is 25° for right bronchus. In children under the age of three years, the angulations of the two main bronchi at the carina are equal on both sides.
Clinical Applications of Angle of the Main Bronchi
- Greater tendency for right endobronchial intubation:In adults, the right bronchus is more vertical than the left main bronchus, and hence, there is a greater tendency for endotracheal tubes or suction catheters to enter this lumen.
- Blocking bevel end of the tube:In the event of an endotracheal tube being inserted too far, the beveled end of the tube may get blocked because of it lying against the mucosa on the medial wall of the main bronchus.
- Difficult to occlude:The short length of the right bronchus also makes the lumen difficult to occlude when required during thoracic anaesthesia.
Children under the age of three years
Due to equal angulations of the two main bronchi at the carina, endotracheal tubes or suction catheters can enter either lumen.
It is the most important cleansing mechanism of the peripheral airways. The mucosa of the bronchial system contains ciliated and glandular epithelia. Throughout the respiratory tract, the continuous activity of the cilia is probably the single most important factor in the prevention of accumulation of secretions.
In the nose, the material is swept towards the pharynx, whereas in the bronchial tree, the flow is towards the entrance of the larynx. The coordinated movement of numerous cilia is capable of moving large quantities of material but their activity is greatly assisted by the mucous covering.
The mucous layer covering the cilia consists of two layers: A superficial gel layer, to which foreign particles and microorganisms adhere and a fluid sol layer surrounding the ciliae (periciliary fluid layer).
- Superficial gel layer: Figure 1.5An outer layer of thick, viscous mucous is designated to entrap dust and microorganisms. With each beat, the tips of the cilia just come in contact with the outer layer. Acting in unison, they set the outer mucous layer in motion and with gathering momentum, this flows towards the pharynx and larynx. The cilia cannot work without this blanket of mucous.
- Fluid sol layer (periciliary fluid layer): Figure 1.6An inner layer surrounding the cilia is made of thin, serous fluid that is required to lubricate the action of the ciliary mechanism. Ciliary movement consists of a rapid forward thrust followed by slow recoil which occupies about four-fifths of the cycle. Their action can be compared to that of a belt system of the platform on which the bags rest. The platform corresponds to the blanket of mucous and the propulsive force of the belt is represented by the action of the cilia.
Viscomechanical Dissociation: Figure 1.7
Viscomechanical dissociation occurs when:
- The periciliary fluid layer is too deep e.g. pulmonary oedema, overdose of mucolytics, etc.
- The periciliary fluid layer is too shallow e.g. dehydration, insufficient moistening of the administered gases during mechanical ventilation. When there is insufficient moisture within the airways, the transport function of the respiratory cilia stops rapidly.
- The composition of the mucous is pathologically altered due to abnormally tenacious mucous due to insufficient water content as in the case of mucoviscidosis.
Factors Affecting Mucociliary Clearance
- Toxic gases:Toxic gases (NO2, SO2) and tobacco smoke have the same effect of depressing ciliary activity.
- Drugs used in anaesthesia:Anaesthetic agents (thiopentone) and other drugs such as atropine or beta blockers also reduce the mucociliary clearance.
- Anticholinergic drugs:Anticholinergic drugs with dry anaesthetic gases produce dry mucosa. This produces an inflammatory reaction producing excessive mucous giving rise to tracheitis, pulmonary collapse and bronchitis.
- Volatile general anaesthetics:A volatile general anaesthetic not only slows the propelling mechanism but also limits the production of suitable mucous.
- Beta stimulation:Beta adrenergic substances, sympathetic stimulation and theophyllines stimulate mucociliary clearance.
- Infections:Infections attenuate the mucociliary clearance by way of a further ciliostatic effect.
- Tussive clearance:Mucociliary transportation is enhanced further by coughing (tussive clearance). After increasing the pressure by closing the glottis, sudden opening of the glottis at maximum pressure leads to enormous local airflow in large airways enabling great masses of mucous to be removed suddenly.
The alveoli are composed of the alveolar epithelium, the epithelial basement membrane and the capillary endothelium. Altogether these layers are referred as ‘alveolar capillary membrane’. It measures 1 µ in thickness, thus representing a short diffusion distance for gas exchange between the alveolar space and the capillary space.
Oxygen moves from the inspired air to the deoxygenated venous blood. Carbon dioxide moves in the opposite direction from the venous blood to the air in the lungs. This movement is carried out by passive diffusion, which means oxygen crosses passively through a membrane from a greater concentration in the lungs to a lower concentration in venous blood. Carbon dioxide crosses through the membrane in the opposite direction by the same principle.
The alveolar epithelium begins in the alveolar ducts and consists of flat epithelial cells (type I cells) and alveolar granulocytes (type II cells), which produce surfactants. Foreign particles that gain access to the alveolar space are removed by alveolar macrophage by phagocytosis.
Control of Ventilation
Regulation of gas exchange is possible because the level of ventilation is carefully controlled. Respiration is largely an involuntary process involving rhythmic impulses from the higher centre (control of breathing) in the brain which are passed on through efferent pathways to the muscles of respiration.
There are two types of control:
- Voluntary control: This is initiated by the cerebral cortex.
Basic Elements of Respiratory Control: Figure 1.8
There are three basic elements of the respiratory control system:
- Sensor:The sensor gathers information and feeds it to the central controller.
- Central controller:The central controller, which is in the brain, coordinates the information from various sensors, and in turn, sends impulses to the respiratory muscles.
- Effectors:Effectors are respiratory muscles which cause ventilation. By changing ventilation, the respiratory muscles reduce the output of the sensors (negative feedback).
The normal automatic process of breathing originates from impulses that come from the brainstem. The cortex can override these centres if voluntary control is desired. Nerve cells which are situated in the pons and medulla are responsible for the automatic rhythm of breathing. These cells are arranged in functional groups known as the ‘respiratory centre’. The respiration is normally initiated and controlled by neural output from the respiratory centre.
Possible organisation of the respiratory centre is as follows: Medullary respiratory centre, apneustic centre and pneumotaxic centre: Figures 1.9 and 1.10
- Medullary respiratory centre:The medullary centre is situated in the reticular formation beneath the caudal end of the floor of the 4th ventricle. The medullary centres have connections with the higher centres, the reticular activating system and the hypothalamus. The medullary centre has been divided into two different parts, the inspiratory and expiratory centre.
- Inspiratory centre:Dorsal group: Comprises mainly of inspiratory (I) neurons lying more caudal and deep to the expiratory centre. Inspiratory neurons control the descending spinal cord pathways to the motor neurons innervating the muscles of inspiration. Vagal and glossopharyngeal nerves transmit signals from the peripheral chemoreceptors to the inspiratory area. In addition, vagal nerves transmit sensory signals from the lungs that help to control inflation and the rate of breathing.
- Expiratory centre:Ventral group: Expiratory (E) neurons are situated in the reticular substance under the floor of 4th ventricle. Expiratory neurons control the descending spinal cord pathways to the motor neurons innervating the muscles of expiration (these are somatic motor nerves, not autonomic nerves).
- Apneustic centre:The apneustic centre is situated in the lower two thirds of the pons and provides the initial stimulus which begins inspiratory activity in the inspiratory centre of the medulla. The apneustic centre also acts as a central station for vagal inhibitory impulses. The section of the brain at the junction of the upper one third and lower two thirds of the pons, separating the pneumotaxic centre and the apneustic centre, leads to slower and deeper breathing.If vagus nerves on both sides are also divided, a state of inspiratory spasm appears due to uninhibited activity of the inspiratory centre in the medulla, termed as ‘apneusis’. This is interrupted by expiratory gasps called ‘apneustic breathing’. The inference is that, uninhibited action of the apneustic centre causes prolonged activation of the inspiratory centre in the medulla, but its action can be interrupted by afferent vagal impulses and to a lesser extent by the pneumotaxic centre.
- Pneumotaxic centre:The pneumotaxic respiratory centre, situated in the upper third of the pons, is capable of inhibiting the apneustic centre. The pneumotaxic centre transmits signals continuously to the inspiratory area. The purpose of these impulses is to inhibit the inspiratory signal before the lungs become overinflated. Some investigators believe that the role of this centre is the ‘fine tuning’ of the respiratory rhythm because normal rhythm can exist in the absence of this centre. The pneumotaxic centre has no inherent rhythm but seems to act by controlling the other centres.Reciprocal innervations for respiratory muscles: Figure 1.11The inspiratory and expiratory neurons exhibit reciprocal innervations (mutually inhibitory) and are not generally active at the same time. The expiratory area is quiescent during normal quiet breathing because ventilation is then achieved by active contraction of the respiratory muscles (chiefly the diaphragm), followed by passive relaxation of the chest wall.
Breathing is under voluntary control to a considerable extent and the cortex can override the function of the brainstem within limits. It is not difficult to halve the arterial PaCO2 by hyperventilation, although the consequent alkalosis may cause tetany with contraction of the muscles of the hand and foot (carpopedal spasm).
Components for the Normal Functioning of Respiratory System
There are three major components for the normal functioning of the respiratory system:
- The neural and the muscular components: Figure 1.12The activity of the whole system depends on the initial excitation from both respiratory and nonrespiratory sources and also from the chemical stimuli such as the arterial carbon dioxide tension. Inspiration is initiated by the action of the apneustic centre and the somatic afferent impulses exciting the inspiratory centre. Inspiratory activity causes nerve impulses to pass up the brainstem to the pneumotaxic centre. These excite the pneumotaxic centre.In other words, once inspiration is in progress, the activity of the inspiratory centre is inhibited by the action of impulses from the pneumotaxic centre and from the pulmonary stretch receptors via the vagus nerve. This follows the inhibition of inspiration and allows expiration to take place.The neural output is influenced by input from the carotid (PaO2) and central (PaCO2, H+) chemoreceptors, proprioceptive receptors in the muscles, tendons and joints, and impulses from the cerebral cortex. These impulses are governed by information from different receptors in the body.
- The inherent properties of the lung i.e. elastance and resistance:Normal gas exchange occurs if inspired gas is transmitted through structurally sound unobstructed airways to patent, adequately perfused alveoli. Normally alveolar ventilationA and perfusionare well matched and proportional to the metabolic rate.
- Diffusion across alveolar membrane:The gas transfer takes place by a process of diffusion across the alveolar membrane.