Question

please help

why can't let a patient with COPD have a lot amount of oxygen

Answer 1

Excessive oxygen administration can lead to hypercapnic respiratory failure in some COPD patients

COPD patients with more severe hypoxemia are at higher risk of CO2 retention from uncontrolled O2 administration

# MECHANISM :-

The traditional theory is that oxygen administration to CO2 retainers causes loss of hypoxic drive, resulting in hypoventilation and type 2 respiratory failure. This is a myth.

Patients suffering from COPD exacerbation, regardless of whether they have CO2 retention, generally have supra-normal respiratory drive (unless there is impending hypercapnic coma)

The real explanation involves:

increased V/Q mismatch (most important)

the Haldane effect

1. V/Q mismatch

In COPD, patients optimise their gas exchange by hypoxic vasoconstriction leading to altered alveolar ventilation-perfusion (Va/Q) ratios

Excessive oxygen administration overcomes this, leading to increased blood flow to poorly ventilated alveoli, and thus increased Va/Q mismatch and increased physiological deadspace

this increase in Va/Q mismatch occurs in both CO2 retainers and non-retainers, the difference is presumably one of degree

2. The Haldane effect

deoxygenated hemoglobin (Hb) binds CO2 with greater affinity than oxygenated hemoglobin (HbO2)

hence oxygen induces a rightward shift of the CO2 dissociation curve, which is called the Haldane effect

in patients with severe COPD who cannot increase minute ventilation, the Haldane effect accounts for about 25% of the total PaCO2 increase due to O2 administration

# MANAGEMENT

- Target SpO2 88-92% in these patients

- the targeted approach is associated with decreased mortality in COPD patients and less respiratory acidosis

- The oxygen flow rate administered is not important, the (alveolar) PAO2 (and, indirectly, the SaO2) achieved is important.

- Never withhold oxygen from a seriously ill hypoxic patient due to fear of cause hypercapnic respiratory failure