Applied Respiratory Physiology Flashcards
Summarize the functions of the lungs
- Gas exchange
- Phospholipid production (surfactant)
- Metabolism and de-activation of certain compounds (Angiotensin)
- Synthesis of prostaglandins and histamine
- Intrinsic component of the immune system
- Reservoir for blood 500 - 900 ml
Compare the VO2 in adults versus infants
Adults: 3 - 4 ml/kg
Infants: 7 - 9 ml/kg
Define and classify respiratory failure
Type 1: PaO2 < 8 kPa | PaCO2 N or low
Type 2: PaO2 < 8kPa | PaCO2 > 6.5
How many mmHg, cmH20, hPa are in 1 kPa
1 kPa = 7.5 mmHg = 10 cmH2O = 10 hPa
Where is the respiratory center
In the Medulla oblongata
Where are chemoreceptors found and what are the different chemoreceptors sensitive to?
PERIPHERAL CHEMORECEPTORS
Carotid bodies (@ bifurcation of common carotid a)
- Afferents carried by glossopharyngeal n.
Aortic bodies (aortic arch) - Afferents carried by vagus n.
STIMULATED BY
- Low PaO2 (tension NOT content)
- High PaCO2 (20% of hypercapnic response)
- Acidaemia (pH<7.35 - carotid bodies only)
- Hypotension
CENTRAL CHEMORECEPTORS
Ventral surface of the Medulla
1. CSF pH only caused by increased CO2 in CSF as H+ and HCO3 ions can’t diffuse in or out
Explain why chronic CO2 retention (COPD) leads to reduced sensitivity to PaCO2
Kidneys reabsorb and regenerate HCO3 –> compensatory metabolic acidosis –> reduced stimulation of peripheral chemoreceptors
HCO3 is actively secreted into the CSF to buffer the change in pH –> pH returns to normal and the central chemoreceptors are no longer stimulated.
Therefore these patients are largely dependent on hypoxic drive (peripheral chemoreceptors)
How do opioids and general anaesthetics affect the ventilatory response to PaCO2
Decreased response
What is the formula for minute ventilation and what is normal minute ventilation. What is the effect of opioids and anaesthetic agents on MV
MV = RR x Vt
100 ml/kg/min
± 6 - 8 L/min
Opioids and anaesthetic agents all reduce VT and hence MV
Define anatomical dead space and alveolar volume
Alveolar volume - the volume of air which reaches perfused alveoli
Dead Space - The volume of inspired air that plays no part in gas exchange
What are the different types of dead space and how do they relate
Anatomical dead space - the volume of the first 16 generations of the tracheobronchial tree which form the conducting airways
Alveolar dead space - The total volume of ventilated alveoli that are unable to take part in gas exchange due to impaired perfusion (V/Q mismatch).
Physiological dead space - The total dead space that is the sum of the anatomical dead space and the alveolar dead space
DSphys = DSanat + DSalv
Define alveolar ventilation
This is the alveolar minute ventilation:
Alveolar MV = (tidal vol - phys. dead space) x RR
Define residual volume
The volume of air left in the lungs after a maximal forced expiration
Define functional residual capacity
Functional residual capacity is the volume of air that remains in the lungs after a normal tidal exhalation and is comprised of the residual volume (RV) and the expiratory reserve volume (ERV)
List the lungs volumes and capacities and there approximate normal values in a 70 kg adult.
Draw the spirometry trace with the lung volumes included
VT - 500 mL
IRV - 2500 mL
ERV - 1500
RV - 1500
IC = VT + IRV = 3000 mL VC = IRV + VT + ERV = 4500 mL FRC = RV + ERV = 3000 mL TLC = VT + IRV + ERV + RV = 6000 mL
Trace axes: Lung volume versus time
Why is FRC so important in anaesthesia
Pre-oxygenation denitrogenates the FRC which replaces the nitrogen with 80 - 100% O2. Theoretically this provides a ± 3 L O2 reservoir which can prevent desaturation in apnoea for up to 10 minutes ( depending on VO2 of patient which would be increased in kids/pregnancy/sepsis/pain/SIRS etc)
List factors that reduce the FRC
Position: Supine, Lithotomy, Trendelenburg
Muscle relaxation: (NDMR/Volatiles)
Intra-abdo pain/pressure: Preg/obese/ascites/lapscope
Decreasing age
Lung disease (fibrosis/oedema/atelectasis/ARDS)
List factors that increase FRC
PEEP
Emphysema
Asthma
Increasing age
What is the difference between static and dynamic compliance
The measurement technique
Static compliance
- simultaneous pressure and volume measurements are taken at a steady state (when there is no gas flow) and plotted)
- the patient inspires in 500 ml increments and then pauses and the pressure and volume are measured.
Dynamic compliance
- Continuous measurements of P and V are taken during the respiratory cycle. There are two points where flow ceases: end expiration and end inspiration. These points are plotted and a straight line connects them = the gradient of this line = dynamic compliance
Why is dynamic compliance always less than static compliance?
This is due to airway resistance only incurred during the measurement of dynamic compliance.
Draw the pressure volume loops to illustrate static compliance during a vital capacity breath and a tidal volume breath.
Chambers - page 84
Define Oxygen Consumption, Oxygen Delivery and the Oxygen Extraction Ratio
VO2 - the volume of O2 consumed by the body per minute
DO2- the volume of O2 delivered to the tissues from the lungs per minute
OER - the ratio of oxygen consumption to oxygen delivery
What is VO2 max and how is it calculated
As exercise intensity increases the oxygen consumption exceeds oxygen delivery. At this point, anaerobic metabolism ensues. This point is called VO2 max i.e the point where the tissues switch over to anaerobic metabolism.
Which principle is used to calculate oxygen consumption and what is the formula for this calculation
Reverse fick principle
VO2 = CO (CaO2- CvO2) x 10
CaO2 = (1.34 x Hb x SaO2/100%) + (0.003 x PaO2)
CaO2 = (O2 on Hb) + (O2 in plasma)
What is Hufner’s constant
Each Hb molecule can bind 1.34 mL of O2. 1.34 is Hufner’s constant.
This is used in the Content of blood oxygen equation to calculate how much O2 is bound to Hb.
What is the formula for Oxygen delivery DO2
DO2 = CO x CaO2
Dimensional analysis
CO units: L blood/min
CaO2 units: mL O2/ L blood
DO2 units: ml O2 / minute
What is the formula for OER and how does this differ in different tissues
OER = VO2/DO2
Heart = 0.6 (higher O2 consumption –> less buffer for reduced delivery)
Other tissues = 0.25
Draw the oxygen HB dissociation curve
Chambers page 30
How does stored blood bank blood affect the OHDC
Depletion of 2.3 DPG –> Leftward shift P50 OHDC –> reduced ability of Hb to release O2 to tissues.
After a few hours the 2.3 DPG starts to recover and takes 1 - 3 days to fully recover.
Define Hypoxia
An intracellular (mitochondrial) O2 tension below the critical level to sustain oxidative phosphorylation.
This is between 0.15 kPa and 0.3 kPa and is known as Pasteur point and is the point at which anaerobic metabolism begins.
Define hypoxaemia
PaO2 < 8 kPa (60 mmHg)
Classify and define the different causes of hypoxia
Hypoxaemic - Low PaO2
Anaemic - Low Hb or incapacitated Hb (anaemia | CO )
Stagnant - Low DO2 (circulatory failure)
Histotoxic - Mitochondria fail to use O2 (sepsis | Cyanide)
List the causes of hypoxaemic hypoxia
- Low FiO2
- Low V
- V/Q mismatch
- Pathological shunt (refractory to FiO2)
- Diffusion problem
Describe the causes of increased and decreased SvO2
Increased SvO2
- Increased O2 delivery: High FiO2
- Decreased O2 demand (hypothermia | anaesthesia | NMB)
- High flow states: sepsis | liver dx | hyperthyroid
Decreased SvO2
- Decreased DO2 (anaemia/shock/hypoxia)
- Increased VO2 (hyperthermia/shiver/pain/seizure)
High SvO2 despite evidence of end-organ hypoxia
- microvasc shunting (sepsis)
- histotoxic hypoxia (cyanide)
- abN distribution of blood flow
