hypoxia

introduction

hypoxic hypoxia can be arbitrarily defined as arterial oxygen partial pressure of less than 80mmHg in room air
an arterial PO2 < 60mmHg indicates respiratory failure as, at oxygen levels below this, the Hb-oxygen dissociation curve is steep which means that the ability for Hb to carry much oxygen to the tissues is markedly diminished, as it evolved to be off-loading oxygen to the tissues at these pressures.
- pO2 of 60mmHg equates to an oxygen saturation of less than 90% (see Hb-oxygen dissociation curve - see wikipedia)
other causes of “hypoxia” or insufficient cellular oxygen usage include:
- anaemic hypoxia - insufficient haemoglobin to carry the oxygen in the blood
- impaired oxygen delivery due to carbon monoxide poisoning or methaemoglobinaemia
- ischaemic hypoxia due to impaired blood flow such as from either blockages, vasoconstriction or venous stagnation
- histotoxic hypoxia due to impaired cellular utilisation of oxygen such as with cyanide poisoning
the following assumes we are discussing hypoxic hypoxia

an understanding of this equation if critical in emergency medicine
each minute, a human at rest consumes ~4ml oxygen per kg body weight and produces ~3ml carbon dioxide per kg body weight
the ratio of CO2 produced to oxygen used is the “respiratory quotient”
normal alveolar ventilation = 5L/min in adults with a tidal volume approx. 500ml (7ml/kg)
FiO2 = fraction of oxygen in inspired gas
- for room air, this is 0.21, for 100% oxygen, this is 1.00
Patm = atmospheric pressure = 760mmHg at sea level (approx) = 101kPa = 12.8 psi
Pwater = saturated water vapour pressure at body temperature and at prevailing atmospheric pressure = 47mmHg
PaCO2 = arterial carbon dioxide partial pressure in mmHg (normal ventilation = 40mmHg)
RQ = respiratory quotient = 0.8 for most people
if FiO2 is low, the equation can be simplified by approximating FiO2 x (1-RQ) = 0

alveolar PO2 = FiO2 x (Patm - Pwater) - (PaCO2 x (1-FiO2[1-RQ]))/RQ

another essential concept to understand in emergency medicine
an increased A-a gradient suggests their is a pulmonary oxygen transfer aetiology to hypoxia such as impaired diffusion, V/Q mismatch, or right-to-left shunt
normal range is 5-20mmHg, increasing by 1mmHg for every decade in age
see medcalc3000 A-a gradient calculator

A-a gradient = alveolar PO2 as determined from the alveolar gas equation - actual measured arterial PaO2

the body normal maintains alveolar ventilation to give a relatively constant PaCO₂ of 40mmHg by adjusting respiratory rate and tidal volume
- this level will be lowered in hyperventilation which will result in respiratory alkalosis
- this level will rise in hypoventilation which will result in respiratory acidosis

alveolar ventilation = (tidal volume - physiologic dead space) x respiratory rate

NB. physiologic dead space is usually ~150mL

PaCO₂ = (rate of CO₂ production (usually 200mL/min) x conversion constant (usually 0.863) / alveolar ventilation) + PiCO₂

inspired CO₂ in mmHg = PiCO₂ = FiCO₂ x (Patm - water vapour pressure)

where FiCO₂ = fractional amount of CO₂ in inspired air, Patm is usually ~760mmHg, and the water vapour pressure is usually ~47mmHg

NB. 1% = 0.01 fractional, to get fractional from PPM, divide PPM by 1 million;

the industrial standard for max safe level of FiCO₂ = 5000ppm or 0.5% while hypercapnia level is 3% and severe toxicity level 5%

in normal circumstances, ETCO₂ of exhaled air = 5% or 50,000 ppm at a usual minute volume of 5L/min

minutely rate of rise in FiCO₂ = (CO₂ production in L/minute) / (Volume of the space in litres)
this is why people suffocate when placing their head inside a sealed plastic bag - it doesn't take long for CO₂ levels to rise to dangerous levels
- eg. 5L bag will rise in 0.2L/min / 5L = 0.04 = a rise of 40,000ppm within 1 minute which would cause CO₂ narcosis, severe hypoxia and death
- eg. sleeping in a sealed 2P tent of volume 2700L would result in a rise of FiCO₂ = 0.2L/min / 2700 = 74ppm per minute and time to 3% would be 400 minutes (ie. 6.75hrs)
  - fortunately inner tents for sleeping are made of breathable fabrics (ie not with PU waterproofing) and these will not allow CO₂ levels to rise significantly as they have higher gas diffusion rates through the weaves

ensure adequate airway and ventilation
- may require positive pressure ventilation such as BiPAP or via intubation
increase FiO2 as needed
improve lung function
- Rx the cause where possible
in suitable patients, when severe hypoxic hypoxia persists despite the above:
- consider ECMO
- in the future, whilst awaiting ECMO, we may be able to give an immediate iv infusion of John Kheir's idea of oxygen filled nanoparticles which seem to work in preliminary testing