Why? To develop a foundation of knowledge of the physiology of the respiratory system that will enable systematic and considered approach to the care of the critically ill patient. To understand the impact of acute and chronic pathological processes on their presentation and treatment.
Anatomy Trachea Right & Left main bronchus Lobar, segmental and terminal bronchii Respiratory bronchioles Alveolar ducts Alveolar sacs Conducting airways Deadspace of ~150ml Gas moves by inspiration Respiratory airways. Just a few millimeters thick Contain 2-3L at rest = most of the lung volume Gas movement is by diffusion, Occurs within a second 1 16 23
Structure and innervation Thoracic structure –12 pairs of ribs. –Articulate with vertebrae posterior. –Drop inferior at anterior aspect at rest in expiration. –Articulate at the front with the sternum via costal cartilage. –Inferior aspect of thoracic cage bordered by diaphragm.
Innervation –Diaphragm: phrenic nerve C3-5 used in quiet respiration. –Intercostal nerves on inferior aspect of rib supplies the intercostal muscles which are used in forceful respiration. –Direct lung innervation limited because of paucity of fibres. –Sensitive to humoral influences.
respiratory centres in medulla chemoreceptors on aorta and carotid artery heart brain intercostal nerve to external intercostal muscles phrenic nerve to diaphragm diaphragm ribs Control of Respiration
V A /Q mismatch is an inequality in either ventilation or perfusion. Ratio of ventilation to perfusion varies Range from zero to infinity! Ideal ratio is 0.8 (0.5-2.0) Ventilation-perfusion mismatch and the alveolar-arterial pO 2 difference.
V/Q mismatch Ventilation increases gradually as you move from apex to base Perfusion poor in apices better at bases V/Q ratio decrease as you move down from apices A) Well-ventilated poorly perfused (V>Q) B) Underventilated well perfused (V<Q) C) Optimally ventilated + perfused (V=Q)
Ideal Alveolar Gas Equation. Clinically Useful Form: Complete Form:
A-a Difference P A O 2 - P a O 2 A-a = FiO2 – PaCO2 – PaO2 Normally up to +/- 25 mmHg –Because of normal anatomical shunt –Ventilation/Perfusion mismatching. A-a difference increases with pulmonary disease. 21 13 516 50 5 12 45 80 575 18 33357
a/A ratio Normally averages just over 0.8 a/A ratio falls with pulmonary disease. Lower limit normal: –young (room air) :0.74 –older (room air) :0.78 –Both groups (100% O 2 ):0.82
Oxygen carriage Is either carried bound to Hb or dissolved Oxygen dissociation curve Henry’s Law for dissolved blood –Amount of oxygen dissolved in the blood is equal to the partial pressure of oxygen –For each mmHg pO2, there is 0.003ml 02/100ml –Therefore at 100mgHg (alveolar pO2), there is 0.3ml O2/100ml blood.