I wonder if it stands for "mezzo di base" (or "i mezzi di base" in this case); this is an ambulance without advanced life support equipment on board, usually with just two ambulance crew plus driver. So the patient came in with chest pain, but the emergency services deemed him not to be in life-threatening danger, otherwise an ALS ambulance and "automedica" with physician would have been called.

"A Basic Life Support (BLS) ambulance is staffed with 2 Emergency Medical Technicians (EMT) and can provide emergency medical services, including oxygen administration. BLS services are meant for situations where a patient needs to go somewhere by ambulance for medical reasons but is not in imminent or life-threatening danger. This can include appointments for cancer treatment, pain management, dialysis, MRI/CT Scans, as well as trips to the emergency room."
https://www.unitedamb.com/bls-ambulance-services/

“In quest’occasione – spiega il dott. Salvatore Badalamenti, responsabile dell’EAS di Humanitas – la Direzione del 118 ha presentato i risultati del 2004. Da questi è emerso che, oltre alle ambulanze non in rete con il 118, Humanitas ha ricevuto quasi 7.000 missioni del 118 con mezzi di base e 230 missioni di soccorso avanzate (198 automedica e 32 elicotteri).
https://www.humanitasalute.it/prima-pagina-ed-eventi/64194-i...



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Note added at 15 hrs (2019-01-07 12:10:43 GMT)
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You're welcome! Always happy to help!