RESPIRATORY PHYSIOLOGY
Anatomy of Airways
Larynx (Fig. 1.1)
It is the organ of voice extending from root of tongue to trachea and lies opposite C3 to C6.
Distance between teeth and vocal cords is 12–15 cm and distance between vocal cords and carina is 10–15 cm.
It consists of 3 paired cartilages namely arytenoid, corniculate and cuneiform and three unpaired cartilages, namely thyroid, cricoid and epiglottis.
The glottis is the narrowest part in adults while subglottis (at the level of cricoid) is the narrowest part in children up to the age of 6 years. As subglottis is the narrowest part, the cuff of endotracheal tube can cause subglottic edema and stenosis, therefore the traditional approach had been not to use cuffed tube in children, however this approach is not followed in current day practice and cuffed tube can be used at any age except in prematures.
- Nerve supply: All muscles of the larynx are supplied by recurrent laryngeal nerve except cricothyroid which is supplied by external branch of superior laryngeal nerve.
- Sensory supply: Up to vocal cords by internal branch of superior laryngeal nerve and below vocal cords by recurrent laryngeal nerve.
- Arterial supply: By laryngeal branches of superior and inferior thyroid arteries.
Paralysis of Nerves of Larynx
- Paralysis of recurrent laryngeal nerve of one side has no serious consequences as it is compensated by other side. There is only hoarseness of voice.
- In bilateral partial paralysis of recurrent laryngeal nerve, the abductors (posterior cricoarytenoids) goes first (Semon's Law) causing the cords to be held in adduction producing respiratory stridor.
- In complete paralysis of recurrent laryngeal nerve, the cords are still held in adducted position due to tensing action of cricothyroid which is supplied by superior laryngeal nerve, causing severe respiratory distress, stridor, aphonia and complete obstruction. A tracheostomy is usually required for the management.
- In complete paralysis of both recurrent and superior laryngeal nerves, the cords are held in midposition (cadaveric position).
- Cords are also held in cadaveric position during anesthesia with muscle relaxants.
Trachea
- Length of the trachea is 10–12 cm.
- It starts from cricoid ring (C6) and ends at carina (T5). Anteriorly carina corresponds to second costal cartilage at the junction of manubrium with sternal body (angle of Louis).
- It consists of 16–20 incomplete rings.
- The diameter of trachea is 1.2 cm.
Bronchial Tree (Fig. 1.2)
At carina trachea divides into right and left main bronchus. Distance of carina from upper incisors is 28–30 cm.
Fig. 1.2: Bronchial tree. Right side: 1. Right main bronchus, 2. Right upper lobe bronchus, 3. Right middle lobe bronchus, 4. Right lower lobe bronchus, 5. Apical segment bronchus, 6. Posterior segment bronchus, 7. Anterior segment bronchus, 8. Lateral segment bronchus, 9. Medial segment bronchus, 10. Apical segment bronchus, 11. Medial basal bronchus, 12. Anterior basal bronchus, 13. Lateral basal bronchus, 14. Posterior basal bronchus. Left side: 1. Left main bronchus, 2. Left upper lobe bronchus, 3. Left lower lobe bronchus, 4. Apical upper lobe bronchus, 5. Posterior upper lobe bronchus, 6. Anterior upper lobe bronchus, 7. Superior bronchus, 8. Inferior bronchus, 9. Apical bronchus, 10. Anterior basal bronchus, 11. Lateral basal bronchus, 12. Posterior basal bronchus.
Further, the right main bronchus divides into right upper, middle and lower lobe bronchus and left main bronchus into left upper and lower lobe bronchus.
- Due to shorter, wider and less acute angle chances of endotracheal tube to be positioned on right side are more or other words the chances of endobronchial intubation are more on right side (Table 1.1).
- In children the angle of both right and left bronchus is same, i.e., 55° up to the age of 3 years.
Right upper lobe bronchus divides into apical, posterior and anterior segment bronchi.
Right middle lobe bronchus divides into lateral and medial segment bronchi.
Right lower lobe bronchus divides into apical, medial basal, anterior basal, lateral basal and posterior basal segment bronchi.
Left upper lobe bronchus divides into apical, anterior and posterior segment bronchi.
Left upper lobe bronchus also gives rise to lingular bronchus, which is further subdivided into superior and inferior segment bronchi.
Left lower lobe bronchus divides into apical, anterior basal, posterior basal and lateral basal segment bronchi.
Segmental bronchi along with lung parenchyma constitutes bronchopulmonary segments which are 10 in number on right side, viz.
- Upper lobe: Apical, posterior and anterior.
- Middle lobe: Lateral and medial.
- Lower lobe: Apical (superior), medial basal (cardiac), anterior basal, lateral basal and posterior basal.
On the left side bronchopulmonary segments are 9 in number, viz.
- Upper lobe: Apical, posterior and anterior.
- Lingular lobe: Superior lingular and inferior lingular.
- Lower lobe: Apical, inferior basal, lateral basal and posterior basal.
These segmental bronchi further divide and redivide till terminal bronchioles. Further these terminal bronchioles lose their cartilage to form respiratory bronchiole which with alveolar duct and alveolar sac forms the respiratory unit. It is at this alveolar capillary membrane where gaseous exchange takes place. The thickness of alveolar capillary membrane is 0.3 mm.
Total number of alveoli is 300 million with surface area of 70 m2.
Alveolar epithelium has type I and type II cells. Type II cells secrete surfactant.
FOREIGN BODY ASPIRATION
Due to shorter, wider and less acute angle of right bronchus foreign body aspiration is more common on right side.
- In supine position most commonly involved segment is apical segment of right lower lobe.
- In lateral position, most commonly involved segment is posterior segment of upper lobe (right upper lobe in right lateral and left upper lobe in left lateral).
- In standing/sitting aspiration, most commonly aspiration occurs in posterior basal segment of right lower lobe.
Mucosa: Ciliated epithelium up to terminal bronchioles after which nonciliated epithelium.
Ciliary activity is inhibited by all inhalational agents except ether.
Arterial supply: Bronchial artery up to terminal bronchioles and beyond terminal bronchioles by pulmonary artery.
Nerve supply: Parasympathetic by vagus (causes bronchoconstriction).
REGULATION OF RESPIRATION
Mediated by:
- Pneumotaxic center in upper pons.
- Apneustic center in lower pons.
- Ventral group of neurons in medulla (expiratory group).
- Dorsal group of neurons in medulla (inspiratory group).
Normally pneumotaxic center has inhibitory effect on apneustic center which otherwise produces apneustic breathing or inspiratory spasm.
Normal respiration is maintained by expiratory and inspiratory neurons of medulla.
During inspiration stretch receptors in lung parenchyma, which are supplied by vagus get stimulated leading to inhibition of inspiratory group of neurons, and hence stopping the inspiration.
Expiration is normally passive and expiratory group of neurons comes into play only during active expiration.
These central respiratory centers are highly sensitive to changes in CSF pH, which in turn is influenced by pCO2 (partial pressure of carbon dioxide in blood).
Increase in pCO2 stimulates the respiration while decrease in pCO2 inhibits respiration. Other factors effecting respiratory centers are:
- Body temperature
- Hypoxia
- Exercise
- Pain
- Hypothalamus
- Cortex
Peripheral chemoreceptors: These are present in carotid body and aortic arch. Carotid body receptors are highly sensitive to hypoxia.
All inhalational agents (except nitrous oxide and minimum with ether) have depressant effect on ventilatory response to increased CO2 and hypoxia.
MUSCLES OF RESPIRATION
Inspiration
Diaphragm is the most important muscle of inspiration (moves 1.5 cm in quiet respiration and 6–10 cm in deep breathing).
External intercostals, pectoralis minor and scalene also assist in normal inspiration.
Pectoralis major, latissimus dorsi and sternomastoids are needed during deep inspiration.
Respiration in males is abdominothoracic while in children and females, it is thoracoabdominal.
Expiration
Expiration is normally passive. Forced expiration is mediated by internal intercostals and abdominal muscles.
During anesthesia with inhalational agents expiration is active, mediated by abdominal muscles.
AIRWAY RESISTANCE
For air to flow in lungs a pressure gradient must develop to overcome the airway resistance. This pressure gradient depends on airway caliber and pattern of airflow.
At laminar flows (which occurs below the main bronchi where velocity is less), resistance is proportional to flow rates but at turbulent flow (seen in trachea and main bronchi) resistance is square of flow rates. In other words, it can be said that maximum airway resistance to airflow occurs in trachea and then main bronchus and minimum in terminal bronchi.
VENTILATION/PERFUSION (V/Q)
Both ventilation and perfusion is more at bases as compared to apex but perfusion at base is comparatively higher decreasing V/Q ratio towards base (from 2.1 at apex to 0.3 at base, average 0.8).
This ventilation perfusion mismatch is responsible for producing alveolar dead space 7(i.e., alveoli are only ventilated but not perfused, wasting the oxygen in alveoli).
This V/Q mismatch creates alveolar to arterial oxygen difference [(A–a) pO2 difference] which is normally 3–5 mm Hg.
This A-a difference is increased in lung pathologies affecting alveoli such as pulmonary edema, acute respiratory distress syndrome (ARDS) and interstitial lung disease.
DEAD SPACE
Total dead space (also called as physiological dead space) = Anatomical dead space + Alveolar dead space.
Anatomical Dead Space
It is constituted by air which is not participating in diffusion. Therefore, it is constituted by air present in nose, trachea and bronchial tree (up to terminal bronchioles). Normally, it is 30% of tidal volume or 2 mL/kg or 150 mL.
Anatomical dead space is increased in:
- Old age
- Neck extension
- Jaw protrusion
- Bronchodilators
- Increasing lung volume
- Atropine (causes bronchodilatation)
- Anesthesia mask, circuits
- Intermittent positive pressure ventilation (IPPV) and positive end-expiratory pressure (PEEP)
Anatomical dead space is decreased by:
- Intubation (nasal cavity is bypassed and diameter of tube is less than airway diameter)
- Tracheostomy (upper airways and nasal cavity bypassed)
- Hyperventilation (decreasing lung volume)
- Neck flexion
- Bronchoconstrictors
Alveolar Dead Space
It is constituted by alveoli which are only ventilated but not perfused. It is 60–80 mL in standing position and zero in lying position (in lying position perfusion is equal in all parts of lung).
It is increased by:
- Lung pathologies affecting diffusion at alveolar capillary membrane such as interstitial lung disease, pulmonary embolism, pulmonary edema and ARDS.
- General anesthesia
- IPPV
- PEEP
- Hypotension
Anesthesia and Dead Space
- All anesthesia circuits, masks, humidifiers increase the anatomical dead space.
- Endotracheal tubes, tracheostomy decreases the anatomical dead space by bypassing the upper airways.
- All inhalational agents increase both anatomical and alveolar dead space. Anatomical dead space is increased because all these agents are bronchodilators. Alveolar dead space is increased because of hypotension (decreased perfusion) produced by these agents.
- Positions during anesthesia, especially lateral position causes more ventilation in upper lung (nondependent) and more blood flow in lower lung (dependent lung), thereby increasing the V/Q mismatch, and hence alveolar dead space. Other positions such as Trendelenburg, lithotomy also produces the V/Q mismatch.
- Anesthesia ventilation techniques such as IPPV, PEEP increase both anatomical and alveolar dead space. Anatomical dead space is increased by increasing lung volume and alveolar dead space is increased because of hypotension produced by IPPV and PEEP (increase in intrathoracic pressure produced by IPPV/PEEP decreases the venous return, cardiac output and hence hypotension).
OXYGEN AND CARBON DIOXIDE IN BLOOD
Oxygen
Normal oxygen uptake is 250 mL/min.
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Oxygen is mainly carried in blood attached to hemoglobin (1 g of Hb carries 1.34 mL of oxygen). Very less amount, 0.003 mL/dL/mm Hg is carried as dissolved fraction. Oxygen content of arterial blood is 20 mL/dL and that of venous blood is 15 mL/dL.
Oxygen Dissociation Curve (Fig. 1.3)
Normally, Hb is 97% saturated at normal partial pressure (pO2) of oxygen, which is 95–98 mm Hg.
At 60 mm Hg, saturation is still 90%. After this point, there is sudden drop in oxygen saturation leading to significant desaturation of Hb (cyanosis appears when pO2 fall below 50–60 mm Hg).
P50 is the partial pressure at which oxygen saturation is 50%. The partial pressure of oxygen for 50% saturation is 26 mm Hg. P50 is not affected by anesthetics.
- Bohr effect: Alkalosis shifts O2 dissociation curve to left and acidosis to right.
- Oxygen flux: It is the amount of oxygen leaving left ventricle/minute. It is 1,000 mL/min.
Shift of oxygen dissociation curve is seen with:
To left | To right |
|---|---|
Alkalosis | Acidosis |
Low pCO2 | High pCO2 |
Decreased 2, 3 DPG | Increased 2, 3 DPG |
Carbon monoxide poisoning | Hyperthermia |
Abnormal hemoglobins like methemoglobin, fetal hemoglobin, etc. | Inhalational anesthetics |
Hypophosphatemia | |
Hypothermia |
Carbon Dioxide
Transported in blood as:
- Bicarbonate (90%).
- Dissolved (0.0308 mmol/L/mm Hg): 5% of total.
- As carbamino compounds.
CO2 Dissociation Curve (Fig. 1.4)
It is relationship between pCO2 and CO2 content.
Deoxygenated blood has more CO2 content at a given pCO2, this is called as Haldane effect.
ABNORMALITIES OF CHEST MOVEMENTS
Paradoxical Respiration
Seen in flail chest.
Treatment: IPPV.
Tracheal Tug
This is downward movement of trachea during deep inspiration. It is seen in:
- Deep anesthesia (by inhalational agents).
- Upper airway obstruction (this is the main reason of tracheal tug at the end of anesthesia as airway can get obstructed by secretions).
Mechanism: During deep anesthesia and partially curarized patient diaphragm is not supported by costal margins. Also larynx is not supported by neck muscles so strong contraction of central part of diaphragm pulls the trachea downwards.
Sigh
It is deep inspiratory hold.
Hiccup
Intermittent clonic spasm of diaphragm of reflex origin.
Causes
- Light anesthesia.
- Gastric and bowel distension.
- Diaphragm irritation by touching diaphragm in upper abdominal surgeries.
- Uremia.
Treatment
- Increase the depth of anesthesia.
- Muscle relaxants.
- Pharyngeal stimulation by nasal catheter, Valsalva maneuver, CO2 inhalation.
- Drugs such as amyl nitrate.
- Ether.
- For intractable hiccups, phrenic nerve block may be required.
HYPOXIC PULMONARY VASOCONSTRICTION
This is a protective mechanism. Whenever there is hypoxia, there occurs vasoconstriction in these hypoxic areas leading to shunting of blood to well perfused area, decreasing the V/Q mismatch. All inhalational agents blunt this hypoxic pulmonary vasoconstriction (HPV) response thereby increasing the shunt fraction (maximum with halothane).
PULMONARY FUNCTION TESTS
Lung Volumes (Fig. 1.5)
- Tidal volume: Volume of gas inspired or expired in each breath during normal quiet respiration.
It is 400–500 mL (10 mL/kg).
- Inspiratory reserve volume: It is the maximum volume of gas which a person can inhale from end-inspiratory position.
It is 2,400–2,600 mL.
- Inspiratory capacity: It is the maximum volume which can be inhaled from end expiratory position, i.e., it is inspiratory reserve volume + tidal volume.
It is 2,500 (IRV) + 500 (TV) = 3,000 mL
- Expiratory reserve volume: Maximum volume of gas that can be exhaled after normal expiration.
It is 1,200–1,500 mL.
- Vital capacity: It is the maximum amount of gas that can be exhaled after maximum inhalation, i.e., it is IRV + TV + ERV.
It is 4,200–4,500 mL (75–80 mL/kg).
- Residual volume: It is volume of gas still present in lungs after maximal expiration.It is 1,200–1,500 mL.
Fig. 1.5: Lung volumes.
(TV: tidal volume;IRV: inspiratory reserve volume; IC: inspiratory capacity; ERV: expiratory reserve volume; RV: residual volume; FRC: functional residual capacity; VC: vital capacity; TLC: total lung capacity)
Note: All these lung volumes are approximately 5% less in females (except residual volume).
- Maximum breathing capacity: Maximum volume of air that can be breathed/minute. It is 120–170 L/min (normally it is measured for 15 seconds and expressed as L/min).
- Minute volume: It is tidal volume × respiratory rate.It is 500 × 12 = 6,000 mL/min.
- Total lung volume: IRV + TV + ERV + RV.It is 5,500–6,000 mL.
- Functional residual capacity (FRC): It is the volume of gas in lungs after end expiration. It is ERV + RV. It is 2,400–2,600 mL.
During general anesthesia FRC decreases by 15–20%.
Simple Bedside Test
- Breath-holding time: It is very simple and useful bedside test. Normal is > 25 seconds. Patients with breath-holding time of 15–25 seconds are considered borderline cases and breath-holding time <15 seconds indicate severe pulmonary dysfunction.
- Match test: Person is asked to blowoff match stick from a distance of 15 cm. A person with normal pulmonary reserve will blow off the matchstick from this distance.
- Tracheal auscultation: If breath sounds are audible for more than 6 seconds it denotes significant airway obstruction.
- Able to blow a balloon.
- Spirometry by pocket size microspirometers can now be performed on bed side.
Spirometry
It is the instrument used to measure following lung volumes:
- Tidal volume
- Inspiratory reserve volume
- Inspiratory capacity
- Expiratory reserve volume
- Vital capacity
It cannot measure residual volume, therefore any lung volume which requires measurement of residual volume, i.e., functional residual capacity and total lung volume cannot be measured by spirometry.
Forced Spirometry (Timed Expiratory Spirogram)
- Forced vital capacity (FVC): Expiration is performed as hard as possible. It is 4,200–4,500 mL (75–80 mL/kg).
- Forced expiratory volume (FeV): It is the volume of gas expired in 1 second (FeV1), 2 seconds (FeV2), 3 seconds (FeV3) measured from the start of expiration after full inspiration (forced vital capacity). Normal person can exhale 83–85% of FVC in 1 second (so FeV1 is 83–85%), 93% in 2 seconds (FeV2 is 93%) and 97% in 3 seconds (FeV3 is 97%).FeV1/FVC ratio is important as this ratio is decreased in obstructive lung diseases. It is expressed as percentage.
- Peak expiratory flow rate: Normal 500–600 L/min.
- Forced mid-expiratory flow rate: Measures flow rate during 25–75% of exhalation.
Flow Volume Loops
These are more sensitive and informative in detecting pulmonary diseases than conventional spirometry. Modern microprocessor controlled recording spirometers automatically generate these flow volume loops.
Body Plethysmography, Helium Dilution, Nitrogen Washout
These techniques are employed for measuring functional residual capacity, residual volume and total lung capacity.
Helium dilution technique is also used to measure closing capacity, which is the volume at which airway closes. Normally it is 1 liter less than functional residual capacity. If functional residual capacity falls below closing capacity there will be significant hypoventilation and V/Q abnormalities.
Body plethysmograph is also used to measure airway resistance (normal = 2.5 cm H2O/L/second).
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Lung Compliance
It is volume change per unit of pressure.
- Lung compliance: 0.2 L/cm H2O
- Chest wall compliance: 0.2 L/cm H2O
- Total compliance: 0.1 L/cm H2O (lung and chest wall compliance act in opposite direction).
Blood Gas Analysis
Described in Chapter 40, page 323.
Pleural Pressures
- Normal intrapleural pressure is –3 to –5 cm H2O.
- During inspiration, it becomes more negative up to –7 cm H2O.
- During expiration, it is +1 to +2 cm H2O.
Fitness for Surgery and Pulmonary Functions
- Patients with FVC <20 mL/kg and FeV1 <15 mL/kg require appropriate preoperative preparation before surgery such as chest physiotherapy, antibiotics, bronchodilators, etc.
- Patient with FeV1<10 mL/kg and history of dyspnea at rest or on minimal activity should be subjected to only lifesaving operations.
Three most important criteria to indicate severe respiratory compromise are:
- Dyspnea at rest or on minimal activity.
- FeV1 <15 mL/kg (normal 65 mL/kg).
- pO2 <60 mm Hg (or oxygen saturation <90%) on room air.
PHYSICS RELATED TO ANESTHESIA
Boyle's Law
At a constant temperature, volume of gas is inversely proportional to the pressure.
Charle's Law
At a constant pressure, volume of gas is directly proportional to temperature.
Graham's Law
The rate of diffusion of gas is inversely proportional to square root of their molecular weight.
An ideal gas should follow all the above said laws.
Partial Pressure of Gas
It is the pressure exerted by each gas in a gaseous mixture.
Vapor
Vapor is the gaseous state of liquid.
Avogadro Number
The number of molecules contained in one gram molecular weight of any compound. It is 6.23 × 1023.
Critical Temperature
It is the temperature below which a gas can be stored in liquid form.
Flow of Gases
Flow may be laminar or turbulent.
Laminar
Laminar flow is produced when the gas pass through straight tube. Flow is smooth. Laminar flow is more dependent on viscosity.
At laminar flow, Hagen–Poiseuille law is applicable which states that flow rate is directly proportional to pressure gradient and fourth power of radius of tube and inversely proportional to viscosity and length.
Q = Flow rate
P2 – P2 = Pressure gradient (P1 and P2 are pressure at each end of tube)
η = Viscosity
l = Length
Turbulent
Turbulent flow is produced, if flow rate is very high or if gas passes through bends, constrictions. Flow is rough. Reynolds number 12must exceed to 2,000 for turbulence. Turbulent flow is more dependent on density.
VENTURI PRINCIPLE
When a fluid or gas passes through a tube of varying diameter, the pressure exerted by fluid (lateral pressure) is minimum where velocity is maximum (pressure energy drops where kinetic energy increases: Bernoulli's law).
This principle is very much utilized in anesthesia particularly with Venturi masks. By increasing flow rate (velocity) through narrow constriction subatmospheric pressure can be created in vicinity of enthralling air to mix in fixed proportion (through pores in mask) with oxygen. Jet ventilation and suction apparatus also works on this principle.
POYNTING EFFECT
Mixing of liquid nitrous oxide at low pressure with oxygen at high pressure (in Entonox) leads to formation of gas of nitrous oxide. Therefore, oxygen and nitrous oxide both are present in gaseous state in Entonox cylinder.