Practical Applications of Mechanical Ventilation Shaila Shodhan Kamat
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1Respiratory Physiology2

Anatomy of RespirationCHAPTER 1

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 is called external respiration.
  • Internal respiration: Combustion or biologic oxidation of nutrients by oxygen, to carbon dioxide and water at cellular level is called internal respiration.
 
RESPIRATORY ORGANS
The respiratory organs can be divided into:
  1. Upper airway
    • Nasal cavity
    • Oral cavity
    • Pharynx
  2. Lower airway
    • Larynx
    • Trachea
    • Bronchial tree
  3. Lungs.
 
Nasal Cavity (Fig. 1.1)
The nasal cavity has important functions (anesthetic significance):-
  1. Breathing through the nose:
    The adult patient breathes through the nose unless there is some form of an obstruction such as a nasal polyp.
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    Figure 1.1: Functions of nasal cavity
    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.
  2. 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.
  3. 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.
  4. 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
    • During treatment on a ventilator the importance of correct humidification and warming of the inspired gas has to be 5considered, as the gas is supplied through an endotracheal tube and not through the nose.
    • 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.
 
Larynx
The larynx protects the lower airway by closing the glottis (for example during swallowing). The extrapulmonary airway (larynx) is at its narrowest at the vocal cords in an adult and at the level of the cricoids in children. Any further narrowing at the vocal cords can give rise to considerable respiratory distress. The laryngeal mucosa can become oedematous due to anaphylactic reactions or postextubation. This can cause life-threatening problems.
 
Trachea
The trachea is a cartilaginous tube made up of 16–20 horseshoe shaped cartilage rings which are incomplete posteriorly. The trachea measures about 10–12 cm in length and 11–12.5 mm in diameter in an adult.
 
Anaesthetic Significance
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 always check the position of the endotracheal tube for accidental extubation or endobronchial intubation after any change in the position of the head.
 
Bronchial Tree (Figs 1.2 and 1.3)
The bronchial tree subdivides into 23 generations, the 23rd generation being alveoli. The total diameter of the airways increases considerably towards the periphery. The bronchioles begin in 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 more contains mucous producing cells. Upto the 16th generation the bronchi play no role in gas exchange, their only purpose is the 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.6
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Figure 1.2: Subdivisions of bronchial tree
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Figure 1.3: Generations of airways
 
Right Main Bronchus (Fig. 1.4)
The right main bronchus is wider and shorter than the left, being only 2.5 cm long. The angulations of both bronchi are not equal and it is 25° for right bronchus. In small children, under the age of three years, the angulations of the two main bronchi at the carina are equal on both sides.7
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Figure 1.4: Angle of the main bronchi
 
Clinical Applications
 
Adults
  • 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 either 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 bevelled end of the tube may get blocked off 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 this is required for thoracic anesthesia.
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.
 
MUCOCILIARY CLEARANCE
It is the most important cleansing mechanism of the peripheral airways. 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 to 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.8
 
Mucous Layers
The mucous layer covering the cilia consists of two layers:
  • Superficial gel layer (Fig. 1.5).
    An outer layer of thick, viscous mucous is designated to entrap dust and micro-organisms. 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.
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    Figure 1.5: Superficial gel layer
  • Fluid sol layer (pericilary fluid layer) (Fig. 1.6)
    An inner layer, surrounding the cilia, is 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.
 
Visco-mechanical Dissociation
Visco-mechanical dissociation occurs when: (Fig. 1.7)
  • 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.
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Figure 1.6: Inner fluid sol layer
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Figure 1.7: Visco-mechanical dissociation
 
Factors Affecting Mucociliary Clearance
  1. Toxic gases Toxic gases (NO2, SO2) and tobacco smoke have the same effect, depressing ciliary activity.
  2. Drugs used in anesthesia Anesthetic agents (thiopentone) and other drugs such as atropine or beta blockers also reduce the mucociliary clearance.
    • Anticholinergic drugs Anticholinergic drugs with dry anesthetic gases produce dry mucosa. This produces an inflammatory reaction producing 10excessive mucous giving rise to tracheitis, pulmonary collapse and bronchitis.
    • Volatile general anesthetics
      A volatile general anesthetic 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.
  3. Infections
    Infections attenuate the mucociliary clearance by way of a further ciliostatic effect.
  4. Tussive clearance
    Mucociliary transportation is enhanced further by coughing (tussive clearance) after increasing the pressure by closing the glottis. The 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.
 
Alveoli
The alveoli are composed of the alveolar epithelium, the epithelial basement membrane and the capillary endothelium. The total of all these layers is referred to as alveolar capillary membrane. It measures 1µ in thickness, thus representing a short 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 in alveolar ducts consists of flat epithelial cells or type I cells and alveolar granulocytes or 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 a largely involuntary process 11involving rhythmic impulses from the higher center of 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.
  • Involuntary or automatic rhythm control: This involves medulla, pons, limbic system (emotional response), hypothalamus (temperature regulation) and other subcortical structures.
 
BASIC ELEMENTS OF RESPIRATORY CONTROL
There are three basic elements of the respiratory control system (Fig. 1.8)
  • 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.
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Figure 1.8: Basic elements of the respiratory control system
By changing ventilation the respiratory muscles reduce the output of the sensors (negative feedback).
 
Brainstem
The normal automatic process of breathing originates in 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 center. 12The respiration is normally initiated and controlled by neural output from the respiratory centre.
The three main groups of neurons (Fig. 1.9) are:
  • Medullary respiratory centre
  • Apneustic centre
  • Pneumotaxic centre
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Figure 1.9: Respiratory centre or pacemaker of ventilation
  1. 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 center has been divided into two different parts:
  • Inspiratory centre
    Dorsal group: Mainly 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 peripheral chemoreceptor to the inspiratory area. In addition, vagal nerves transmit sensory signals from the lung that help to control inflation and the rate of breathing.
  • Expiratory centre
    Ventral group: Both inspiratory (I) and expiratory (E) neurons situated in the reticular substance under the floor of 4th ventricle. 13Expiratory neurons control the descending spinal cord pathways to the motor neurons innervating the muscle of expiration (these are somatic motor nerves, not autonomic nerves).
    Reciprocal Innervations for Respiratory Muscles (Fig. 1.10)
    The 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.
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    Figure 1.10: Reciprocal innervations for respiratory muscles
    Possible organization of the respiratory centre (Fig. 1.11) are as follows.
  1. Apneustic Centre
    The apneustic center is situated in the lower two third of the pons and provides the initial stimulus which begins inspiratory activity in the inspiratory center in the medulla. The apneustic center 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 center and the apneustic centre, leads to slower and deeper breathing.
    If the vagus nerves on both sides are also divided, a state of inspiratory spasm appears due to uninhibited activity of the inspiratory center in the medulla, termed as apneusis. This is interrupted by expiratory gasps called apneustic breathing. The 14inference is that uninhibited action of the apneustic center causes prolonged activation of the inspiratory center in the medulla, but its action can be interrupted by afferent vagal impulses and to a lesser extent by the pneumotaxic centre.
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    Figure 1.11: Possible organization of the respiratory center
  2. Pneumotaxic Centre
    The pneumotaxic respiratory center, situated in the upper third of the pons, is capable of inhibiting the apneustic center. Stimulation of this centre results in tachypnoea. 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.
 
Cortex
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:
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  1. The neural and the muscular components (Fig. 1.12)
    The activity of the whole system depends on the initial excitation from both respiratory and non respiratory 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 center. Inspiratory activity causes nerve impulses to pass up the brain stem to the pneumotaxic center. These excite the pneumotaxic centre.
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    Figure 1.12: Neural and the muscular components
    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.
  2. 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 ventilation VA and perfusion Q are well matched and proportional to the metabolic rate.
  3. Diffusion across alveolar membrane
    The gas transfer takes place by a process of diffusion across the alveolar membrane.