07 / Anaesthesia · Physiology
The numbers, for this patient
The textbook prints one MAC and one blood volume. The patient on the table has their own. Set the age, the agent, the weight and the haematocrit, and watch the numbers you would actually use.
Minimum alveolar concentration
MAC for this patient, not the textbook one
MAC awake
0.61%
The concentration at which half of patients open their eyes to command on emergence. Roughly a third of MAC.
MAC
1.80%
The end-tidal concentration at which half of patients do not move to a standard surgical incision. A population statistic, not a target for one patient.
MAC BAR
2.34%
The concentration that blocks the adrenergic response, the tachycardia and hypertension, to incision in half of patients. Opioids bring it down sharply.
Sevoflurane: MAC 1.8% at 40, blood-gas coefficient 0.65. Sweet-smelling and non-irritant, so it is the agent for gas inductions in children and the everyday maintenance agent in most theatres. Fast on, fast off.
Age adjustment after Mapleson: MAC falls about six percent per decade from forty, and the same curve runs the other way into childhood.
What moves it
Raise, lower, or nothing?
Nitrous oxide alongside
Fluids and blood
The sums, for the patient you set
Maintenance, 4-2-1
110 mL/h
4 mL/kg for the first 10 kg, 2 for the next 10, 1 for the rest.
Fasting deficit
880 mL
Maintenance times hours fasted; half replaced in the first hour. Clear fluids until two hours before make it small.
Estimated blood volume
4900 mL
Weight times the group’s mL/kg. Neonates and infants carry proportionally more.
Allowable blood loss
1983 mL
Blood volume times the fall in haematocrit over the starting haematocrit. Beyond this, red cells.
- One unit of red cells raises the haemoglobin by about 10 g/L in an adult. Which is why the target is a number, not a bag count.
- Restrictive threshold: transfuse below 70 g/L in most; below 80 g/L with cardiac disease. Liberal transfusion does not improve outcomes and carries its own harms.
- Massive haemorrhage: red cells, plasma and platelets in a ratio approaching 1:1:1, and tranexamic acid 1 g within three hours of trauma. Replacing red cells alone dilutes the clotting factors that stop the bleeding.
- Keep the patient warm, the calcium up and the fibrinogen above 1.5 g/L. Cold, acidosis and hypocalcaemia each stop the clotting cascade; citrate in every bag binds calcium.