Sellery - A Review of the Cause of Postoperative Hypoxia

Question 1

Where does shunts usually come from postoperatively and how can they cause post-op hypoxia?

Answer 1

(As a preface, remember that true shunting occurs when venous blood from pulmonary circuit returns to the left heart without being oxygenated).

Shunting usually occurs under anesthesia due to small areas of atelectasis. Leads to lowered arterial O2 content and subsequently pO2 (this effect cannot be overcome by O2 enrichment due to alveolar collapse –> lack of ventilation)..

Question 2

What is the relationship between cardiac output and the presence of shunts?

Answer 2

(According to Fick’s principle), cardiac output is inversely proportional to the difference between venous and artierial O2 difference.

Significant shunt = less (pulm) venous O2 –> less arterial O2

Significant hypotension due to decrease in CO2 (from the initial venous-arterial O2 difference caused by the shunt(s)) will lower arterial O2 tension.

Question 3

Sigh reflex in post-op patients - why is it important as a normal reflex and how does anesthesia affect it?

Answer 3

A sigh is a spontaneous large breath of greater volume than those preceding or following it. To a certain extent, it prevents atelectasis.

The sigh reflex is ablated in patients post-operatively which may contribute to increased atelectasis.

Question 4

To prevent or decrease atelectasis post op, hyperinflation of pt intra-op have been suggested. What are some findings?

Answer 4

Nunn (1962) found that for this to have any effect, hyperinflation must have been done at a pressure of 40 cm H2O for 40 seconds which resulted in severe cardiac depression due to high pressure in the intrathoracic space (due to decreasing venous return, ventricular filling, etc..).

A study conducted by this paper’s group in 1968 attempted to test this by hyperinflating every 10 minutes intra-op using inflation volumes 3x expected tidal volume. Found no difference in post-op O2 tension between control and experimental pts.

In short, the author suggests that alveolar recruitment intra-op doesn’t really carry over or translate to any post-op effects on atelectasis.

Question 5

What are some other studied (potential) causes of post-op hypoxia?

Answer 5

• Shunts –> atelectasis*
• Decreased CO in the presence of normal venous mixture (remember that in addition to medication, position can affect this)*
• Inadequate humidification and hydration due to ciliary paralysis and secretion retention
• Patients straining against the ETT
• Broncospasm incidences
• Duration of procedure (>68 minutes) and site of surgery
• Diffusion hypoxia

[Note: the studies referenced are old and mostly inconclusive so instead of diving into too much detail, I will just post these considered factor. The main idea in the context of contemporary anesthesia is that most of these considerations are actually things we address in the OR today.]

Question 6

What is the most dangerous time during the patient’s stay at the hospital in which they may develop post-op hypoxia?

Answer 6

Immediately following termination of surgery in the operating room.