Pulmonary problems during anesthesia
Once the airway is secured, many things can still go wrong with the pulmonary system. Often – but not invariably – pulse oximetry gives the first signal of trouble:
(i) We have to provide oxygen into the airway. Problems arise when (inadver-tently) another gas is substituted for oxygen, or when a mechanical problem affects the delivery mechanism (like failure to turn on the ventilator or a disconnection of the ventilator from the breathing circuit).
(ii) We need to have adequate alveolar ventilation, i.e., tidal volumes in excess of deadspace. Problems include a kinked or plugged ETT, a leak somewhere (allowing the gas to vent to the atmosphere), bronchospasm, pneumothorax, a plug (mucus, blood, tissue, foreign body) in a bronchus, decreased lung compliance, increased intrathoracic pressure (as with insufflation of carbon dioxide into the abdomen for laparoscopy), inadequate fresh gas flow rate, or increased apparatus deadspace as from machine valve failure.
(iii) When the oxygen arrives at the alveolus, it has to be able to get into the blood stream. Problems here include a diffusion block in the alveolus (pulmonary edema fluid), lack of blood flow to the alveolus (pulmonary embolism), or inability of the blood to pick up oxygen, e.g., carbon monoxide poisoning – though this would fool the SpO2 into reporting normal saturation; see Mon-itoring chapter).
(iv) Finally, the oxygenated blood has to make it to the location of the pulse oxi-meter for analysis. Problems here would include dilution of the oxygenated blood with venous blood (shunt), flow blockade to the location of the pulse oximeter (distal to an inflated blood pressure cuff or tourniquet), presence of dyes that can alter the color of the blood (methylene blue), inaccurate probe placement (only partially on the finger), or failure of the oximeter probe itself.
So, in addition to calling for help . . .
(i) Check FiO2 (if unexpectedly low, disconnect from wall oxygen source and use oxygen from a cylinder or room air).
(ii) Increase FiO2, e.g., turn off nitrous oxide, increase fresh gas flow with oxygen.
(iii) Check capnogram shape of ETCO2 waveform – in short, confirm adequate gas exchange.
(iv) Check pulse oximeter waveform and probe (reposition as needed).
(v) Listen to breath sounds bilaterally – mainstem intubation? Pneumothorax? Inadvertent extubation?
(vi) Check peak inspiratory pressure – if low, there may be a leak; if high, an obstruction.
(A) Give several manual breaths – while it turns out that even “educated hands” cannot gauge compliance and resistance well, a few slow, deep manual breaths allow control over the pattern of inspiration, which may improve the situation. However, we must be careful not to get stuck just squeezing the bag to feel as if we are doing something, tying up our hands when we could be handling other needs. Also, anesthesia machines and ventilators will generate peak inspiratory pressure and tidal vol-ume data (rather than impressions). Observing these two parameters during anesthesia helps us detect trends that might herald problems before they become emergencies.
(B ) Suction ETT – confirms patency and removes secretions.
(vii) Check exhaled tidal volume (to ensure there is no leak).
(viii) Consider obtaining arterial blood gases and chest radiograph.
(ix) PEEP – administering PEEP may improve the saturation, since often the cause is decreased FRC. Inspiratory pressures and/or venous return can constrain the level of PEEP.