Final answer:
The concentration of oxygen used for positive-pressure ventilation in a newborn of 34 weeks gestation should be between 21% and 30% to maintain oxygen saturation within the target range.
Step-by-step explanation:
When a newborn of 34 weeks gestation is not breathing at birth and requires positive-pressure ventilation, the concentration of oxygen used should be between 21% and 30%. The goal is to deliver enough oxygen to support the baby's breathing, but not to exceed the baby's physiological requirements to avoid potential harm caused by excessive oxygen exposure. This concentration range is based on the principle of titrating oxygen delivery to maintain oxygen saturation within the target range for the baby's age and clinical condition.