UA-PTC SUMMER
MATH UNIT: IDEAL BODY WEIGHT CALCULATIONS
FOR MEN:
FOR WOMEN
Historically, mechanical ventilation was initiated with a tidal volume based on
a patient’s actual body weight, which was believed to reflect metabolic need.
This remains common practice for pediatric ventilation, and for much of adult
ventilation. However, a ‘lung-protective strategy’ is increasingly the standard
of care for acute ventilation based on data showing that this approach to
treating acute respiratory distress syndrome (ARDS) in adults was associated
with reduced mortality. Lung-protective ventilation has also been shown to
improve outcomes in patients ventilated in the operating room and in the
intensive care unit (ICU). Recent consensus guidance recommends the lungprotective strategy also be applied in pediatric acute lung injury.
Key elements of a lung-protective strategy are the application of positive endexpiratory pressure (PEEP), limitation of plateau pressure, and a minimal
tidal volume scaled to a ‘predicted’, rather than actual, body weight. The use of
predicted weight is based on the assumption that volutrauma might be
minimized by delivering a volume appropriate to the patient’s lung capacity.
Lung capacity and respiratory system compliance relate more closely to height
than to weight, at least in normal subjects. Therefore, by calculating initial
tidal volume based on predicted (or lean) body weight rather than actual
weight, configuration of the ventilator retains some connection to metabolic
need (weight), while also reducing potential for volutrauma (height). The tidal
volume scaling factor is 5–8 mL/kg of predicted body weight (PBW) (or less at
elevated plateau pressure).
Men: IBW (lbs) 106 + 6(Ht-60)
Women: IBW (lbs) 105 + 5(Ht-60)
Practice:
1. Male, 69 in tall
IBW = ____________ lbs
IBW = ____________ kg
2. Female, 64 in tall
IBW = ___________ lbs
IBW = ___________ kg
3. Male, 71 in tall
IBW = ___________ lbs
IBW = ___________ kg
4. Female, 68 in tall
IBW = ___________ lbs
IBW = ___________ kg
5. Male, 70 in tall
IBW = ___________ lbs
IBW = ___________ kg
6. Female, 60 in tall
IBW = ___________ lbs
IBW = ___________ kg
Once a patient is placed on a ventilator, the Vt must be slightly increased to
compensate for increased dead space (volume of gas that does not make it
down to alveolar-capillary membrane for gas exchange; may be anatomical,
alveolar, or mechanical). This is because we have changed how normal
ventilation occurs. Dead space is approximately 1/3 of the total Vt of the
patient.
Ex: If a patient’s Vt is 550 mls, the Vd = 182 mls.
Ex: If a patient’s Vt is 425 mls, the Vd = 140 mls.
For the mechanically ventilated patient, the normal Vt is on the higher end of
the range (6-8 ml/kg). We often just use 8ml/kg when calculating Vt for a
patient on a mechanical ventilator.
If a patient needs to be mechanically ventilated, we calculate by multiplying
the patient’s weight in kg by 8 ml/kg.
Ex: patient weighs 59.5 kg. Calculate Vt before placing patient on ventilator.
59.5 kg x 8 ml/kg = 476 ml
Practice:
1. Patient weighs 62.5 kg. Calculate Vt for a MV patient.
2. Patient weighs 71.3 kg. Calculate Vt for a MV patient.
3. Patient weighs 86.4 kg. Calculate Vt for a MV patient.