IBW and the Relationship to Tidal Volume
Even with the determination of using the ideal body weight instead of actual body weight, historically, tidal volumes of 10 ml for every kilogram of IBW were often used. This approach still resulted in volutrauma and VILI. In the early 2000s, many studies were done to find the best tidal volume range based on ideal body weight to decrease the risk of VILI and ARDS. Researchers also studied patients diagnosed with ARDS to see if the amount of tidal volume impacted morbidity and mortality. The results were conclusive that using tidal volumes of 6-8 ml/kg of ideal body weight directly improved morbidity and mortality. This discovery revolutionized tidal volume strategies and has become the gold standard of adult tidal volume ranges to decrease the risk of VILI from volutrauma/barotrauma. Today, 8 ml/kg is the hard limit for tidal volumes to be set by clinicians in control modes. Once the IBW is calculated, it is multiplied by 6 and then 8 to get the range of safe tidal volumes for a patient on a mechanical ventilator.
Calculating Tidal Volume Range
Let’s review the steps to determine a safe tidal volume for your patients.
- Determine your patient’s height and sex.
- Use appropriate IBW formula (male versus female) to calculate your IBW.
- Multiply your 6, 7 and 8 ml/kg and write these in your chart, so you have a handy reference point for the tidal volumes you can use now and in the future. Note: you will always round your result up or down to match the settings on the ventilator (usually, they use whole numbers only and count by 5s).
We have already learned how to calculate IBW (steps 1 and 2), so let’s try step 3 using the IBWs that were calculated earlier in this chapter. Remember, the safe range for tidal volumes is 6-8 ml/kg.
Patient A | IBW 82.2 kg
Tidal volume of 6ml/kg = 82.2 kg x 6 ml/kg
= 493.2 ml
= approx. 490ml
Tidal volume of 7ml/kg = 82.2 kg x 7 ml/kg
= 575.4 ml
= approx. 575 ml
Tidal volume of 8ml/kg = 82.2 kg x 8 ml/kg
= 657.6 ml
= approx. 660ml
Therefore, the safe range of ventilation is 490-660ml (6-8ml/kg) and the respective tidal volumes for 6,7,8 ml/kg are 490, 575 and 660 ml respectively.
Patient B | IBW 52.4 kg
Tidal volumes 52.4 x 6 = approx. 315 ml
52.4 x 7 = approx. 365 ml
52.4 x 8 = approx. 420 ml
The safe range of ventilation for this patient is 315-420ml and the respective tidal volumes for 6, 7, and 8 ml/kg are 315, 365 and 420 ml respectively.
Key Concept
When patients have very fragile and damaged lungs, some ventilation strategies will go as low as 4 ml/kg to decrease the risk of barotrauma/volutrauma. Refer to your health care facility and physician orders for your low limit based on patient pathophysiology, but 8 ml/kg is the absolute maximum you will every use for a tidal volume. You will not go above this number unless specifically ordered by a doctor. The 8 ml/kg number is a hard limit, and exceeding this limit is never accepted in critical care medicine since the studies came out proving how harmful it is to the patient.
Take this information with you, as we now split to both volume control and pressure control for their individual settings.
