Airway obstruction can be described by the level at which it affects e.g. …
oral, supraglottic, laryngeal, mid tracheal, lower tracheal or bronchial obstruction
may be multilevel in e.g. oedema
main causes of airway obstruction?
- reduced GCS
- vomit/blood contaminating the airways from regurgitation of stomach contents or trauma
- foreign body
- oedema due to burns
- inflammation or anaphylaxis and laryngeal spasm
- Lower airway obstruction may also be caused by excessive bronchial secretions, bronchospasm and pulmonary oedema.
Signs of partial airway obstruction?
- inspiratory stridor
- expiratory wheeze
- gurgling
- snoring
- crowing
signs of complete airway obstruction?
- paradoxical chest and abdominal movements
- see-saw breathing - no sound from mouth/upper airway
- use of accessory muscles – neck and shoulder muscles, recession of intercostal and subcostal muscles; tracheal tug
The most basic respiratory measurement is minute volume (VM) - what is this?
= tidal volume (VT) x respiratory rate (RR)
what is the alveolar ventilation?
the portion of the minute volume that takes part in gas exchange.
Each tidal volume contains a proportion of gas which is wasted, either because it remains in the large airways or goes to parts of the lung where there is ineffective gas exchange (dead space)
V/Q
In the healthy lung: V/Q = ?
In the diseased lung, 2 types of V/Q mismatching may occur
- Shunt: Q ? V
- Dead space: V ? Q
1 (ventilation and perfusion are well matched and respiratory gas exchange is efficient)
- Shunt: Q > V wasted perfusion
- Dead space: V > Q wasted ventilation
Elimination of CO2 from the blood via the lungs basically depends on ____ ____, which removes CO2 from the alveoli and maintains a concentration gradient for more CO2 to move from the blood to the alveoli.
alveolar ventilation
Thus, if effective alveolar ventilation falls for any reason, the level of CO2 in the blood will rise.
- anatomical vs
- physiological dead space?
- tidal volume that remains in the large airways, doesn’t participate in gas exchange
- In disease e.g. emphysema, areas of the lung may be ventilated, but not perfused
As total dead space (wasted ventilation) increases, there is a relative reduction in effective ____ ____, thus impeding ___ elimination. This increase in wasted ventilation is initially compensated for by an increase in overall minute volume, particularly by ____.
- alveolar ventilation
- CO2
- tachypnoea
why does shunt have much less impact on CO2 elimination than a reduction in overall alveolar ventilation?
lungs compensate better for shunt
- in the diseased lung, if some blood flow to the lungs bypasses ventilated regions (shunt) more CO2 will be removed in the remaining ventilated part, again due to the concentration gradient from blood to alveoli.
- This compensates for the poorly ventilated region.
- Therefore, arterial concentration of CO2 is largely determined by effective alveolar ventilation over the whole of the lungs.
T/F: unlike CO2 elimination, arterial oxygenation is very sensitive to shunt and patients with significant shunt will become hypoxic.
This is because
- Oxygen is carried in blood almost exclusively bound to Hb, so there’s a limit (set by the Hb concentration) on how much oxygen can be carried in each ml
- Therefore, increasing alveolar ventilation cannot compensate for an area of shunt with poor oxygenation since the blood leaving the well ventilated area cannot have greater than 100% Hb O2 saturation.
If total dead space increases sufficiently to cause effective alveolar hypoventilation, then hypoxia will result, largely due to an increase in alveolar ____ levels.
CO2
a patient apparently moving large volumes of air may still have effective alveolar hypoventilation if they have a large dead space.
name some causes of respiratory failure with CO2 retention which occur with
- normal / increased
- reduced
respiratory drive
- Acute pathologies: pulmonary oedema, contusion, pneumonia, lung collapse, pneumothorax, (airflow resistance and mechanical failure) (eg, due to rib fractures) can increase dead space (wasted ventilation). The onset of hypercapnia (paCO2) may signify the patient is becoming exhausted/ unable to compensate further
- Opioid analgesics (commonest cause), Type II respiratory failure with chronic CO2 retention (uncommon, predisposed individuals e.g. COPD: ‘blue bloaters’
what is ‘the oxygen cascade’?
- describes what happens to the partial pressure of oxygen as it moves from the atmosphere to mitochondria in cells.
- Decreases during each step
What is hypoxia?
The causes of hypoxia can be divided into which 4 groups?
- arterial pO2 < 12kPa
- normal arterial pO2 but the amount of available Hb for O2 carriage is low e.g. anaemia, carbon monoxide poisoning
- normal arterial pO2 and Hb but reduced blood flow to the tissues – may be due to a reduction in cardiac output or a interruption to the blood flow to the tissues e.g embolus
- normal arterial pO2, normal Hb level and normal blood flow to the tissues but the tissues are unable to utilise the oxygen e.g. cyanide poisoning, carbon monoxide poisoning
Hypoxia = reduced amount of oxygen available for tissue respiration
- hypoxic hypoxia (hypoxaemia)
- anaemic hypoxia
- stagnant (ischaemic) hypoxia
- histotoxic (cytotoxic) hypoxia
The hypoxic hypoxia division can be further subdivided into which 4 main causes?
- hypoventilation
- diffusion limitation
- shunt
- ventilation – perfusion inequality
causes of hypoventilation?
- central respiratory depression (drugs, intracranial pathology, hypocapnia, metabolic disturbance (metabolic aklalosis), hypothermia, sleep apnoea
- impaired peripheral mechanism of breathing (respriatory disease, airway obstruction, restricted chest wall movement, muscular weakness, nerve lesions, NM junction impairent
- increased dead space (embolism or anaesthetic apparatus)
what is diffusion limitation/impairment?
occurs when the membrane over which oxygen is required to diffuse is altered, increasing the diffusion time e.g. pulmonary fibrosis, connective tissue disease.
In the healthy lung only a very small proportion of blood (1-2%) bypasses the alveoli.
Large increases in this shunt can occur with any acute pulmonary pathology e.g.
pulmonary oedema, contusion, pneumonia, pneumothorax and large (lobar or segmental) airway collapse
- T/F: Ventilation – perfusion mismatch is normal in the average individual
- However, if for some reason this mismatch should increase then hypoxia is more likely to ensue. There are a wide variety of reasons that mismatch can occur including …
- True - with alveolar ventilation being slightly less than perfusion.
- PE, pulmonary oedema, pneumonia, decreased cardiac output.
Failure of the circulation and pulmonary perfusion can also cause gas exchange problems by interfering with ventilation / perfusion matching.
Examples of when this can occur?
- disturbance of the pulmonary circulation (pulmonary emboli - thrombus, fat, amniotic fluid)
- systemic hypotension secondary to hypovolaemia, sepsis, vasodilation or pump failure which is generally accompanied by pulmonary hypotension.
- These all cause increased dead space, shunt and hypoxia
‘shunt’ = ‘wasted perfusion’
‘dead space’ = ‘wasted ventilation’
ok
what is meant by the ‘triple airway manoeuvre’?
head tilt, chin lift and jaw thrust
(perform if there is evidence of airway obstruction)
