07 / Anaesthesia · Ventilation
The capnograph and the ventilator
The capnograph is the one monitor that proves the tube is in the trachea, that blood is reaching the lungs, and that the circuit is whole. 10 shapes, each a diagnosis, sweeping across a monitor that runs from the patient’s numbers.
Read the trace
What is the capnograph telling you?
Ventilated patient · 30 minutes into the case
- HR
- 68
- SpO2
- 99%
- NIBP
- 118/74
- EtCO2
- 5.0 kPa
- RR
- 8
- Temp
- 36.6°
The library
Every shape, running
Normal trace
A square wave: a flat zero baseline, a sharp upstroke, a nearly flat alveolar plateau with a slight upward slope, and a sharp fall as inspiration begins. End-tidal 4.5 to 6 kPa.
Causes: Ventilation matching perfusion, a patent tube, a working circuit.
Do: Nothing. Note the number and the shape, because the next change will be measured against them.
Set the ventilator
Tidal volume from the height, not the scales
Ideal body weight
61.5 kg
Devine: 45.5 kg + 0.91 kg for every cm over 152.4
Tidal volume, 6 to 8 mL/kg
369 to 492 mL
Start at 7, then let the capnograph set the rate.
Set by actual weight, it would be
760 mL
Too big for these lungs: this is how ventilator injury happens.
- Tidal volume6 to 8 mL/kg of ideal body weightLungs scale with height, not with fat. Setting volume by actual weight in an obese patient is how a 150 kg patient gets a 1.2 litre breath and a ventilator-induced injury.
- Respiratory rate12 to 16 breaths a minute, then titrated to the capnographMinute ventilation is rate times volume; the end-tidal carbon dioxide tells you whether the product is right for this patient. Aim 4.5 to 6 kPa.
- PEEP5 cmH2O; 8 to 10 in the obese and for laparoscopyKeeps the small airways open at end-expiration. Atelectasis begins within minutes of induction in every supine anaesthetised patient.
- Inspired oxygenThe lowest fraction that keeps saturation at 94 percent or above, commonly 0.3 to 0.5One hundred percent oxygen causes absorption atelectasis. High fractions are for preoxygenation, emergence, and problems, not for maintenance.
- Inspiratory to expiratory ratio1 to 2; 1 to 3 or longer in obstructive diseaseAn asthmatic or COPD patient needs time to breathe out. Too short an expiration stacks breaths, raises intrathoracic pressure and drops the blood pressure.
- Plateau pressureBelow 30 cmH2O; driving pressure (plateau minus PEEP) below 15The pressure the alveoli actually see. A rising plateau pressure at the same volume means the lungs or the chest wall have stiffened, and the question is why.
- ModeVolume control for most; pressure control for leaks, supraglottic airways and small childrenVolume control guarantees the volume and lets pressure vary; pressure control guarantees the pressure and lets volume vary. Know which one the machine is guaranteeing, because the other one is what will drift.
The ventilated patient who deteriorates
Airway pressure rising, saturation falling. In order.
DHidden until you commit
OHidden until you commit
PHidden until you commit
EHidden until you commit
SHidden until you commit
Before any of it: off the ventilator, onto the bag, 100 percent oxygen. The feel of the bag is the first test.
The nerve stimulator
Train-of-four, and what to do about it
Moderate block: two twitches, about 80 percent of receptors occupied.
Reverse with sugammadex 2 mg/kg
A count of two or three is the threshold for the standard sugammadex dose. Neostigmine at a count of two is slow and unreliable; if it is all you have, give it and wait longer.