Question: How are plethysmography and spirometry similar and different? Both measure a patient's heart rate, but only spirometry takes place in a soundproofed box or room. Both measure a patient's heart rate, but only plethysmography takes place in a soundproofed box or room. Both measure a patient's breathing, but only plethysmography takes place in an airtight box or room. Both measure a patient's breathing, but only spirometry takes place in an airtight box or room.