A 28-year-old woman presents with an acute asthma attack. She is using her salbutamol inhaler every 30 minutes, is breathless on talking, her respiratory rate is 28/min, heart rate is 115/min, and PEFR is 60% of her best. There is no cyanosis or altered consciousness. According to BTS/SIGN guidelines, what is the most appropriate initial management in primary care while awaiting ambulance transfer?