Resp2 ABSC Respiratory Notes PDF

The Mechanics of Breathing Professor Brendan Corcoran ABSC Respiratory 01, 02 and 03 Areas covered  Phases of respiration  Respiratory muscles  Respiratory volumes  Dead space  Minute and alveolar ventilation  Oxygen requirements, body mass...

The Mechanics of Breathing Professor Brendan Corcoran ABSC Respiratory 01, 02 and 03 Areas covered  Phases of respiration  Respiratory muscles  Respiratory volumes  Dead space  Minute and alveolar ventilation  Oxygen requirements, body mass  Poiseuille’s law of gas flow.  Mechanics of breathing, resistance, compliance, elastance, lung recoil, surfactant  Pulmonary function tests  (control of ventilation) Function of the Respiratory System  Gas exchange utilising the following o Respiratory apparatus o Pulmonary Arterial Circulation- moving de-oxygenated (venous) blood from the right ventricle to the lung. o Pulmonary Venous Circulation- moving oxygenated blood back from the lung to the left atrium. o Cardiovascular system  Thermoregulation in certain species (i.e. dog)  Vocalisation and socialisation Lung Anatomy Species Differences While the system functions roughly the same in all terrestrial mammals, there are differences in lung anatomy that can affect respiratory function and performance.  All species have a lobar lung  Except for the horse, there are clear fissure lines between individual lobes  Except for the horse, all have 4 lobes in the right lung  Right cranial bronchus arises directly from trachea in sheep, cattle and pig, while in others it arises from the right main-stem bronchus. Lung Septae (microscopic level) The lung lobes are septated or divided into sections, but this is most apparent in the sheep (and human). This septation is only visible at the microscopic level and is different form the septation that can be seen grossly. In most domestic species septation is variable or minimal (as in the dog) and this has definitive physiological advantage as it allows gas movement between lung areas and so giving more efficient ventilation. Collateral ventilation is also possible at the smaller airway (bronchiolar- alveolar channels) and alveolar level (inter-alveolar pores), but is likely usually only used in disease situations or period of excess ventilation demands, such as exercise. In the dog it is suspected to be used under normal circumstance and may in part explain the dogs’ phenomenal capacity for aerobic exercise. 1 Lung Volumes Definitions  Tidal Volume; what you take in during each breath. This can vary depending on ventilator demand.  Inspiratory Capacity; the biggest breath (tidal volume) you can take.  Expiratory Reserve Volume; your maximal expiration. The horse, uniquely, and possibly other equids, exhales part of its ERV.  Residual Volume; what you cannot expired no matter how hard you try.  Functional Residual Capacity; what is left at the end of each breath (ERV & RV combined). Phases of Respiration General Principles Air travels down a pressure gradient created between the alveoli (alveolar pressure) and the mouth (atmospheric pressure). The pressure gradient required to move air in the system is very small. The thoracic cage is mechanically coupled to the lung by the pleural membranes and pleural space. By increasing the volume of the thorax, the volume of the lung increases and the alveolar pressure must then become sub- atmospheric; air then flows (pushed by atmospheric pressure) down this gradient. Pressure is indicated in either cmH2O, mmHg or kPa  Inspiration is active and is mainly driven by abdominal diaphragm movement o Onset of inspiration  Increased muscular activity  Slight fall in alveolar pressure (1 cmH2O ~1mmHg~ 0.13 kPa)  Pleural pressure becomes more negative (-10 cmH2O) o End of inspiration  Cessation of muscle activity  End-inspiratory pause (varies between species and for most it is imperceptible)  Expiration is always passive during resting breathing and only becomes active during diseases, exercise, excitement, stress, fever or any other circumstances that increases oxygen demand o Lung recoil (tendency to collapse inwards)  Lung elastance and alveolar surface tension o Thoracic wall recoil (tendency to expand outwards) o Abdominal recoil- minimal contribution  Pleural pressure becomes more positive, but is still sub- atmospheric (-5 cmH2O) Note; it is possible to tell the pleural pressure is negative. If you puncture the chest wall you can hear air being sucked in, the thoracic cage move outwards, and, if the hole is big enough, the lungs collapse. Easiest to see in a cadaver, but surgeons doing a thoracotomy report seeing the same. The tendency is for the lung to recoil (combination of elastic fibres and alveolar surface tension) inwards and the chest wall to recoil (ligaments at the costovertebral and sternocostal articulations) outwards. The opposing forces reach a balance at 2 resting expiratory lung volume, the Functional Residual Capacity (FRC) where there is no movement of air (mouth and alveoli are both at atmospheric pressure). Otherwise the lung would completely collapse requiring massive driving pressure to initiate the next breath and to overcome airway and lung resistance (cross reference to how lecture on how birds and reptile ventilate without a diaphragm). The pressure changes during respiration also affect the mechanics of the trachea and major conducting airways, whose walls are retracted outwards during inspiration and compressed during expiration. The pressure gradients across the walls of the extra-thoracic airway (larynx and trachea) are the direct opposite of those affecting the intra-thoracic airways (trachea and bronchi). For example, during expiration the smaller airways collapse and the larger intra-thoracic airways would except for their structural rigidity. Tracheal collapse in small breed dogs is where this mechanism fails, with the airway being obstructed resulting in expiratory flow difficult (dyspnoea) and coughing. Note: the Horse (and presumably other equids) has a unique breathing style in that it exhales beyond the resting lung volume. This means that recoil energy stored in the lung can then be used to initiate respiration. This is an efficient way of initiating the next breath, but requires energy input during expiration. The advantage of this strategy is probably more efficient breathing in a large athletic animal with a disproportionally large abdomen (for example compared to ruminants). Respiratory Muscles Principal muscles-those that maintain resting breathing; Accessory muscles-those used only when necessary i.e. during hyperventilation.  Thoracic diaphragm (Principal Muscle o The most important inspiratory muscle generating tidal breathing o Even during exercise it contributes 70% of tidal volume o Animals can ventilate without the diaphragm using inspiratory intercostal and accessory muscles, but vital capacity is reduced o Effectively a bellows/piston-like mechanism that expands the caudal rib cage. The tendinous centre of the cupola moves caudally first (piston) and then the transverse and dorso-ventral dimensions increase (bellows), drawing air in. Excess caudal movement is prevented by increasing the intra-abdominal pressure. o Innervated by the phrenic nerves  External intercostal muscles (Accessory Muscle-making little contribution) o Only interosseous muscle type and only associated with inspiration, intermittently active during tidal breathing, but contributing to hyperventilation  Internal intercostal muscles (most important contributor to inspiration) o Interosseous component (Accessory Muscle) associated with expiration. o Interchondral component (Principal Muscle) associated with inspiration, and also active during resting respiration, making a small contribution, but much more important contribution to hyperventilation  Other “Accessory” Muscles- any muscle that can contribute to ventilation o Muscles running from the neck or vertebral column to the thoracic wall  Scaleneous, sternocleidomastoid etc. 3  Can be both active during resting respiration or only recruited during hyperventilation  Often used when there is orthopnoea (adopting a position to ease breathing)  Abdominal muscles (expiratory); transvers abdominis, rectus abdominis o Most important in expiration during hyperventilation or exercise with raised intra-abdominal pressure forcing the diaphragmatic cupola cranially. o Can hypertrophy in disease requiring active expiration (COPD horse; IPF dogs); “heave-line” Dead Space Dead Space is an important concept as it explains how not all air inhaled is available for gas exchange and is of crucial importance when there is significant lung disease or when using anaesthetic circuits.  Anatomical dead space (150ml in humans) is where no gas exchange can occur and includes the volume of the conducting airways (nose, trachea, bronchi etc). This means that of 450ml tidal volume only 300 contributes to ventilation.  Alveolar dead space exists when alveoli that are not perfused and maybe physiological, due to posture or caused by pathology (e.g. pneumonia).  Total dead space is then the combination of anatomical and alveolar dead space.  By intubating and connecting by extended tubes to an anaesthetic machine the dead space is increased, compromising ventilation. This can be a particular problem large patients, the best example being horses. The problem is mainly overcome by keeping tubing as short as possible and using positive pressure ventilation. Minute and Alveolar Ventilation While the total amount (minute ventilation) of gas entering the system is important to sustain adequate ventilation it is only that portion that is delivered to the alveoli (alveolar ventilation) that can contribute to gas exchange. Even then not all gas in the alveoli is of benefit unless there is adequate perfusion of the alveolar capillaries.  Minute Ventilation = Tidal volume (VT) x breaths per minute (bpm).  Alveolar Ventilation = (VT – anatomical dead space) x breaths per minute.  Only a portion of each breath contributes to gas exchange. Interestingly, resting respiratory rate is roughly the same in domestic species (cat, dog, horse, cattle) irrespective of their size; slightly lower in large species and slightly higher in small species. Oxygen Requirement and Body Mass A factor that needs to be considered in veterinary respiratory physiology, because of the large number of species that are studied is mass-specific oxygen consumption  Mass-specific oxygen consumption is defined as “oxygen consumption per unit body mass”  It is lower in large mammals but also dependent on body size and energetics 4  o Mouse 10x Cow; Horse 2x Cow  Small mammals have a higher metabolic rate and the reason is unknown o Not necessarily because they need to generate more heat  A major implication for MSOC is artificial ventilation and requirements during anaesthesia. o O2 per unit body mass delivered must be increased in small animals o But must also consider arterial O2 tension achieved (extrapolated from SPO2 if PCV and Hb concentration are normal), and also maintain normalised expired CO2 levels Poiseuille’s Law of Gas Flow Gas delivery to the alveoli is not just a function of volume but is also affected by the rules that govern gas movement through a series of tubes (airways). The use of Poiseuille’s Law can help in understanding the mechanisms that govern gas movement under pressure and how it is affected by the anatomy of the respiratory system. The main caveat, however is that the respiratory system is not a laminar flow system.  Poiseuille’s Law describes flow in a laminar system where flow (V) is o Proportional to the pressure (P) gradient o Proportional to the fourth power of the radius (r4) o Inversely proportional to the tube length (l) o This works well for understanding airflow through the upper airway and the large airways bronchi, but not at the level of the respiratory bronchioles or alveoli. o However, flow mechanics in the distal lung are not of major importance as most gas movement at that level is by simple diffusion.  What is the relevance for larger airways? o A trivial change in lumen diameter can cause a significant increase in airway resistance, but only when lumen diameter has narrowed to a critical point. o The effective distance over which a gas can be can be moved if there is a change in diameter.  What can be done to improve flow? o The pressure gradient must be increased, but there is a finite limit to increasing pressure.  But the issue of turbulence is a confounding factor. o Anatomy of the tracheobronchial tree, breed specific anatomy, the effect of disease o In a turbulent system, pressure varies with the square-root of flow such that to maintain flow you must square the pressure. The Mechanics of Ventilation In a simple breathing system there would be a series of connecting tubes, through which air is drawn using a simple bellows mechanism, but the mammalian respiratory system is much more complex and the system has to overcome a series of mechanical factors in order to ventilate the alveoli. These factors can be significantly affected by body conformity, oxygen demand (normal physiological requirements) and by disease.  Components of Mechanics are o Resistance (R) with the upper airways contributing 50% of the total 5  o Compliance (C) of the lung and chest wall; how distensible under set pressure. o Elastance of the lung; the level of tendency to recoil when under pressure. o Inertia of the tissues and of the air column is trivial and of no consequence during normal breathing or disease. But it can be an issue in intubated patients.  Consideration has to be given as to how Airways, Lung, Thoracic Cage individually or collectively affect R and C  Remember inspiration is active and expiration is passive during quite breathing (except for the biphasic breathing pattern of the horse) Respiratory Resistance (R)  The pressure needed to generate flow (movement of air)  A combination of airway (Raw), tissue and chest wall resistance, with Raw being the most important component.  The units of R are Pressure/volume/time (cmH2O/l/s or kPa/l/s) and reflect the ratio of pressure to flow.  Raw- 50% upper airway, 50% trachea/bronchi. This is important when considering if an animal is an obligate (sheep) or semi-obligate (cat) nasal breather or has major anatomical changes to the upper airway (brachycephalic dogs).  Raw is high at low lung volume and lowers as lung volume increases. This means more energy is needed to initiate inspiration than is required to sustain it and is one of the problems the horse overcomes with biphasic breathing.  The major contributors to airway resistance are conditions causing upper airway obstruction, such as, brachcephalia, laryngeal disorders, tracheal collapse and bronchoconstriction (asthma) Respiratory Compliance (C) Compliance is effectively a measure of how stiff a structure is and therefore how easy or hard it is to deform that structure by putting it under pressure. In respiratory physiology and medicine we are primarily interested in the compliance of the lung, but chest wall compliance can also be affected by disease.  Compliance is the volume change that occurs per unit pressure (l/cmH2O or l/kPa) and is the force that tends to obstruct expansion of the lung.  Total compliance (CTot) effects are a combination of lung tissue (CL) and chest wall (CW) compliance. Respiratory compliance is a reciprocal relationship such that; 1/CL = 1/CTot - 1/CW. Per unit lung volume, compliance is roughly similar between species.  The compliance effect is such that lung has a tendency to collapse inwards and the thoracic cage has a tendency to move outwards. It is the balance between these tendencies that maintains the lung at FRC and the pleura are the structures that link the lung and chest wall together. When the thoracx is opened to atmosphere (break the “pleural seal”) this relationship can be clearly seen, with the lung collapsing and the rib cage springing outwards.  Dynamic Compliance (Cdyn) is a measure of the volume change with pressure difference between the end-inspiratory and end-expiratory point and can be 6  used as index of peripheral airway calibre and patency or indirect measure of respiratory resistance.  Any disease that affects the lung parenchyma reduces CL and any disease affecting the pleural, rib cages and diaphragm affects CW. Lung Elastance (E) Elastance is best thought of as being analogous to the opposite of Compliance.  The lungs recoil due to their elastic properties and elastance is the tendency of the lung to return to its resting size (volume) once peak inspiration has been achieved.  Lung elastance is counter-balanced by chest wall elastance, otherwise lung would collapse completely the pressures required to re-open the smaller airways and alveoli would be unattainable (cross-reference to respiration in birds and reptiles) Surfactant While it is easy enough to consider the breathing operates because of a series of mechanical events, these forces alone would not be sufficient to ventilate but for the presence for surfactant in the small bronchi, respiratory bronchioles and alveoli. The total cross sectional area of the lung in humans would cover the surface area of a tennis court. To be able to expand such an area in the confined space of the thoracic cage would not be possible with the maximal pressure differences that can be generated by the respiratory muscles. To also help overcome this problem the system does not allow the lung to empty completely (FRC) and cartilage in the major conducting airways prevents them from collapsing during expiration (anatomic dead space).  Surfactant is a complex material containing phospholipids and a number of apoproteins. It is produced by the Type II alveolar cells and it lines the alveoli and smallest bronchioles. Its presence is necessary for alveolar expansion when the newborn take their first breath. It reduces surface tension throughout the lung contributing to its general compliance, stabilizes the alveoli, reducing the likelihood of alveolar collapse which have a greater tendency to collapse compared to large alveoli. The best analogy is the water bubbles generated by detergents; the bubbles do not collapse because the detergent (surfactant) reduces their surface tension according to LaPlace’s law.  LaPlace’s Law best explains how surfactant works where Pressure (P) = 2 x surface tension (T) /radius of the sphere (r).  This means that at a constant surface tension (T), small alveoli will generate bigger pressures (P) within them than will large alveoli (r) and so if you reduce the surface tension using surfactant (or a detergent) less pressure is needed to keep the sphere (alveolus) open, or conversely if you reduce the surface tension, you need less pressure to open alveoli and less chance of them collapsing during expiration.  The lack of surfactant is one of the main reasons new-born premature infants are kept on a ventilator (as well as under-development of respiratory muscles). Its production is also reduced by lung inflammation and fibrosis. For the former even when the inflammation is resolved surfactant production does not immediately return to normal. In the case of fibrosis, production is 7  irretrievably lost. Under these circumstances the individual must breath with greater effort, or benefits from positive pressure ventilation. Pulmonary Function Tests-PFT PFT is crucially important in human respiratory medicine and routinely used to assist diagnosis, measure degree of respiratory disability, monitor response to treatment and monitor disease progression. However, most viable and economical tests rely on patient compliance and the ability of the patient to follow instructions while the tests are being performed. That means their application in veterinary medicine is severely limited. While testing can be done under anaesthesia, that in itself affects the value of the tests, but several tests are under development that can be applied to co-operative animals. However, less useful data is produced than if the animal could carry out respiratory manoeuvres on command.  Minimal use in Veterinary Medicine as most requires patient compliance, active participation and a capacity to carry out respiratory manoeuvres. This also limits their use in paediatrics.  Tidal Volume-Flow Loop Recordings have been used in cats and can identify marked bronchoconstriction, but use in detecting mild to moderate disease would be of greater clinical utility.  Barometric whole body plethysmography (BWBP) has been successfully used in the cat to investigate airway resistance in asthma and is now being used examine respiratory disability in brachycephalic dogs. The equipment is both expensive and complicated and limited to a few specialist centres, including the Dick Vet Hospital for Small Animals.  Trans-pulmonary pressure measurements have been used in horse with COPD. This uses a pleural balloon and can be carried out conscious. This has primarily been used as a research tool, as it does not contribute enough to be a routine diagnostic tool.  PFT in human patients is a complete specialist discipline in itself and includes tests such as spirometry (peak expiratory flow (PEF), lung volumes, and forced expiratory volume in 1 second (FEV1)), gas diffusion methods and whole body plethysmography for measuring compliance and resistance. Control of Ventilation (see associated lectures) Cross-reference to lectures on gas exchange, blood gas analysis and acid-base status, since arterial CO2 is the main activator of respiratory drive.  The respiratory centres in the hindbrain are very primitive structures and include the medullary & central chemoreceptor (& pneumotaxic, apneustic centres). Effectively breathing is autonomic and in evolutionary terms primitive.  However there is conscious control of breathing which is controlled by the cerebral cortex and mainly contributes to breath holding, vocalisation.  The neural control of breathing and respiratory reflexes such as cough, sneezing and bronchoconstriction involves activation of stretch and rapidly adapting (irritant) receptors and c-fibre afferent fibres and their receptors and these systems use the vagus.  The stretch receptors in the airways and lungs apply the brake on inspiration (Hering-Breuer Reflex) and are the cause of the end-inspiratory pause. 8   Rapidly adapting receptors and c-fibre afferents mainly contribute in the control of breathing where there is airway injury (induce cough) or bronchoconstriction and are otherwise fairly inactive during normal tidal breathing.  Direct motor innervation of respiratory muscles is provided by the bilateral phrenic nerve to the diaphragm and the intercostal nerves to the intercostal muscles. Joint mechanoreceptors can also stimulate ventilation, an important consideration during movement such as exercise. AB3 Respiratory Section RE14 Clinical Applications & Pathobiology This lecture will use clinical examples to illustrate the importance of understanding respiratory anatomy and physiology in assisting understanding of clinical problems. The lecture will be an overview while the clinical detail will be covered in the Dog and Cat Course in Semester of third year. 9