The use of the word 'the' in English indicates a singularity of the noun. This sentence indicates that 'there is only one ambulance (in the whole world?) and I need it'. 'An' ambulance does not have this wierdness
Well, maybe there was a discussion earlier about an ambulance and the fact that it may be needed, so 'the' would be kind of understandable... but I definitely agree that 'an' should be the default translation.