Clinical Effects of Anaesthetic Techniques on the Respiratory System Flashcards
Which region of the lung is more compliant during upright spontaneous ventilation?
The base
Which region of the lung is more compliant during upright IPPV?
The apex
Which region of the lung is more compliant during supine IPPV?
The uppermost region
Explain how IPPV causes increased V/Q mismatch
IPPV therefore alters the compliance curve of the lung and causes non-dependent regions to become relatively more compliant than dependent regions. Since dependent regions are better perfused, this causes increased V/Q mismatch.
Considering the increased V/Q mismatch in IPPV - what are the reasons why SaO2 and ETCO2 rarely fall during IPPV in the supine patient
- Hypoxic Pulmonary Vasoconstriction (HPV)
HPV protects against the detrimental effects of V/Q mismatch by diverting blood away from poorly ventilated areas. HPV is thought to be diminished to a degree by volatile anaesthetic agents, although this is difficult to quantify. - High reserve in most patients with healthy lungs
List four common causes for extreme V/Q mismatch in anaesthesia
- Endobronchial intubation
- Mucus plugging
- Iatrogenic pneumothorax/haemothorax
- Extreme hypotension
How does the ETT affect respiratory physiology
- Obliterated Intrinsic PEEP (vocal cords bypassed)
2. Pressure necrosis of tracheal mucosa
Explain the pathophysiology of V:Q mismatch in the supine patient
Decreased FRC and increased closing volume –> Airway collapse –> V/Q mismatch –> Ineffective gas exchange –> Fall in PaO2 and rise in PaCO2
What is the overall affect of volatile agents on the respiratory system
Increase RR
Decrease VT
Apnoea at high doses
What is the overall affect of induction agents on the respiratory system
Reduce response to hypoxia
exception: ketamine –> respiratory stimulant and maintains airway tone
What is the overall affect of opioid agents on the respiratory system
Decrease sensitivity to CO2
Increase Vt
Decrease RR
Describe the effects of the induction agents on airway reflexes
Propofol - decreases airway reactivity - used with LMA !
Thiopental - Airways remain relatively reactive (LMA not advised)
Ketamine - Airway reflexes intact - used in developing world in facilities
Which volatile anaesthetic agents can be used for inhalational induction and why
Sevoflurane
Halothane
Least irritant to airways
What is diffusion hypoxia
N2O is more soluble than N2.
N2 is the main constituent of alveolar gas when breathing air.
At the end of anaesthesia –> N2O in the blood enters the alveolus faster than N2 leaves it. This reduces the concentration gradient for diffusion of O2 into the pulmonary capillaries.
What is absorption atelectasis
N2 splints alveoli open as O2 is absorbed.
Lack of Nitrogen leads to alveolar collapse = absorption atelectasis
