Why do patients need extra oxygen during a general anaesthetic?

Written by anaesthesianews

Oxygenation is impaired during GA by four mechanisms:

  1. Hypercapnia reducing alveolar PO2,
  2. Abnormal respiratory muscle activity, which leads to shunting of venous blood through areas of atelectasis, and
  3. Mismatch of ventilation to perfusion leading to areas of lung with low V˙/Q˙ ratios.
  4. Abnormal muscle activity and atelectasis cannot be rectified by increasing FIO2, but the other two are easily corrected with quite small increases to around 30–40% inspired oxygen, which normally results in acceptable arterial saturation for most patients.

(i) Abnormal respiratory muscle activity:

The spinal muscles relax, increasing thoracic spinal curvature; relaxation of intercostal muscles leads to a reduction in the cross-sectional area of the ribcage; and the diaphragm is displaced in a cephalad direction, particularly in dependent regions because of the weight of the abdominal contents. Together, these three changes to the chest cavity shape cause an immediate reduction in functional residual capacity (FRC) of 15–20% compared to the value when awake and supine.

(ii) Formation of atelectasis:

Even if FRC remains above the closing capacity, the changes in chest wall and diaphragm shape commonly result in direct compression of lung tissue in the caudal and dependent regions behind the diaphragm. This leads to atelectasis in 75–90% of patients. The usage of oxygen 100% at various stages of GA may exacerbate the formation of atelectasis. (During maintenance of anaesthesia the use of PEEP helps to limit the amount of atelectasis that forms, but once formed, a recruitment manoeuvre with high airway pressures (30–40 cm H2O) is required to re-expand the collapsed areas.)

(iii) Abnormal regional ventilation and perfusion matching in the lungs:

During GA, with IPPV when ventilation becomes distributed much more to the ventral regions while perfusion distribution is unchanged. This leads to increased scattering of V˙/Q˙ ratios (i.e. an increase in lung areas with both high and low V˙/Q˙ ratios). In dependent regions, V˙/Q˙ ratios are usually less than one because of poor regional ventilation and maintained perfusion. This explains impaired oxygenation during GA even in the absence of atelectasis (where V˙/Q˙=0).

(iv) Development of hypercapnia:

Minute ventilation is significantly reduced during anaesthesia if artificial ventilation is not used. The normal reflex ventilatory responses to both hypoxia and hypercapnia are also attenuated. This hypoventilation, along with the increased alveolar dead space during GA, commonly results in hypercapnia in a spontaneously breathing patient. Hypercapnia is often more marked in laparoscopic or thoracoscopic surgery. Any increase in alveolar PCO2 will be accompanied by an almost identical decrease in alveolar PO2:

PAO2 = PiO2 – PACO2/RQ

Ref: Lumb AB. Why do patients need extra oxygen during a general anaesthetic? BJA Educ. 2019 Feb;19(2):37-39. doi: 10.1016/j.bjae.2018.11.005. Epub 2018 Dec 17. PMID: 33456867; PMCID: PMC7807960.

What lung do we deliver to the post-operative ward?

The following may be proposed for better post operative outcome:

1. Pre-oxygenation with 100% O2 should be followed by a recruitment maneuver to reopen collapsed alveoli, or induction of anesthesia can be done with CPAP/PEEP to maintain FRC (that would allow 100% O2 with no atelectasis formation), alternatively. Pre-oxygenation with 80% O2 may be possible in the lung-healthy, nonobese patient with no anticipated difficulty in airway management, followed by a gentle infla[1]tion of the lung (shorter apnea tolerance time but easier to open closed airways than collapsed alveoli).

2. Recruitment by inflation of the lung to an airway pressure of 40 cm H2O for 10 s in lung-healthy, normal-weight patients and to higher airway pressures in patients with reduced abdominal compliance (obese and patients with abdominal disorders) after pre-oxygenation with 100% O2 and every 30 min, or a continuous PEEP of 7–10 cm H2O.

3. Low inspired oxygen concentration, 30–40% or even less, if no need of higher concentration.

4. High inspired oxygen concentration shall be given only together with PEEP.

5. Post-oxygenation with or without a recruitment maneuver and with or without airway suctioning should not be done routinely but on indication.

6. Deliver a patient with no atelectasis to the post[1]operative ward and keep the lung open.

Ref: HEDENSTIERNA, G. (2012), Oxygen and anesthesia. Acta Anaesthesiol Scand, 56: 675-685.

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