Dead Space Ventilation

Dead space is the volume of air which is inhaled but does not take part in gas exchange either because it remains in the conducting airways or in alveoli that are poorly perfused i.e. not all the air in each breath is able to be used for the exchange of oxygen and carbon dioxide.

 The total dead space (also known as physiological dead space) is the sum of the anatomical dead space plus the alveolar dead space.

Anatomic dead space

Approx one-third tidal volume ~150mL in a healthy adult.

Alveolar dead space

• Sum of the volumes of alveoli which have little or no blood flowing through their adjacent pulmonary capillaries i.e., alveoli that are ventilated but not perfused, and where, as a result, no gas exchange can occur.
• Alveolar dead space is negligible in healthy individuals, but can increase dramatically in some lung diseases due to ventilation-perfusion mismatch.

References

• http://en.wikipedia.org/wiki/Dead_space_(physiology)
• http://www.paramedicine.com/pmc/End_Tidal_CO2.html

Blood Gas normal values

 

	Arterial	Venous
pH	7.35-7.45	7.3-7.35
pCO2	35-45	45-46 (good representation of ventilation)
pO2	80-100	20-80 (uninterprable)
SaO2	95-100
HCO3-	22-28
BE	+/- 3

 

Home Oxygen

Criteria to qualify:

• pO2 < 55
• SaO2 < 90% RA
• Non-smoker!
  

Can be regular/PRN.

Asthma Hx

Things you need to ask someone with asthma:

• Sx: wheeze, dyspnoea, cough, disturbed sleep
• Exercise - quantify distance to breathlessness
• Days per week off work or school
  
• Diurnal variation
• Triggers - RTIs, cold, exercise, pets, emotions, drugs
  
• How & when they monitor their asthma e.g. peak flow meter
• Use of preventers and inhalers. Has there been any changes?
• For elderly people, ask if they've had Fluvax and Pneumovax vaccines (major RF for infective exacerbation)
• Any other atopic diseases like eczema, hay fever, allergy
• Previous attacks or hospitalisations
• Family history of asthma

References:

• http://www.aippg.net/forum/viewtopic.php?p=55254
  

Management of adult cardiorespiratory arresst

Relationship between pulmonary embolism (PE) and atrial fibrillation (AF)

If PE and AF occur together it is most likely that the PE is the cause of the AF.

The mechanism of the AF is said to be acute right ventricular dilatation with "strain" due to the embolus in the pulmonary circulation creating backpressure into the right ventricle.

If a patient presents with unexplained AF, look for an accompanying PE as the cause.

It is postulated that AF can cause PE if a clot originates in the right atrium rather than the left, but this is less common than clots originating from the right ventricle and more studies need to be done in this area.

References:

• Flegel K., When atrial fibrillation occurs with pulmonary embolism, is it the chicken or the egg?, CMAJ 1999;160:1181-2