Clinician Estimates of Ideal Body Weight Are Inaccurate

Abstract & Commentary

By David J. Pierson, MD, Professor, Pulmonary and Critical Care Medicine, Harborview Medical Center, University of Washington, Seattle. Dr. Pierson reports no financial relationship relevant to this field of study.
This article originally appeared in the November 2006 issue of Critical Care Alert. It was peer reviewed by William Thompson, MD. Dr. Thompson is Staff Pulmonologist, VA Medical Center; Associate Professor of Medicine, University of Washington. He reports no financial relationship relevant to this field of study.

Synopsis: When ICU clinicians estimated rather than calculated ideal body weight in mechanically ventilated patients in order to set tidal volume, the majority of such estimations were sufficiently inaccurate that the resulting tidal volume was outside the unit's target range.

Source: Diacon AH, et al. Challenges in the estimation of tidal volume settings in critical care units. Intensive Care Med. 2006;32:1670-1671.

Over a 3-month period, all patients who required mechanical ventilation in the ICU of Tygerberg Academic Hospital in Cape Town, South Africa, were enrolled in this study. To test the authors' hypothesis that the ideal body weight (IBW) estimated by experienced clinicians would be significantly different from calculated IBW, the former was recorded for each patient in a concealed fashion by 21 ICU nurse practitioners, 2 ICU technologists, 9 registrars (residents), and 4 senior intensivists. These estimated IBWs were then compared to the value calculated for the patient using gender-specific formulas similar to those used in the ARDS Net low-tidal-volume study.1 The authors then classified the resulting tidal volume (VT) settings as appropriate (6-8 mL/kg), too low (< 6 mL/kg), or too high (> 8 mL/kg).

The clinicians generated 545 estimates of IBW from the 18 men and 22 women who required mechanical ventilation during the study period. Mean estimated and calculated IBW values were 68.3 ± 19.1 and 57.2 ± 11.2 kg, respectively—a difference that averaged 19% too high. ICU nurses' estimated IBW values were significantly higher than those of the other clinicians (69.7 ± 20.1 vs 65.9 ± 16.9 kg; P = 0.02). The median range of IBW for each patient among the 36 staff members was 31 kg. Only 44% of the estimated IBW values (240/545) resulted in VTs in the target range, with 25.5% of estimates too low and 30.5% too high. More men than women had estimations that led to too-small VT (101 vs 38; P < 0.01), and more women than men had estimations that led to VT settings that were too large (> 8 mL/kg; 143 vs 23; P < 0.01).

The authors conclude that the practice of estimating IBW informally at the bedside results in markedly inaccurate weights and thus VT settings that are outside the intended range in the majority of patients. They recommend that patient height be measured routinely when patients are placed on mechanical ventilation, and that VT be based on calculated rather than estimated IBW.

Commentary

Lung-protective ventilation, with low VT based on IBW and maintenance of end-inspiratory static pressures less than about 30 cm H2O, has become the standard of care for patients with acute lung injury (ALI) or the acute respiratory distress syndrome (ARDS). The ARDS Network demonstrated that ventilating ALI-ARDS patients with VT of 6 mL/kg IBW or less, as opposed to 12 mL/kg IBW, saved 1 life for every 11 patients enrolled.1 As a result of that study and several others, clinicians everywhere are using lower VT than they used to use—not only for patients with ALI-ARDS but also in other patients as well.

Although it has been shown convincingly in animals with initially normal lungs that large-volume ventilation can induce an ALI-ARDS-like condition, and although there is some evidence suggesting that critically ill patients without ALI-ARDS are more likely to get it if large tidal volumes are used, the benefit of low-VT ventilation has not been directly demonstrated in such patients. There is also some evidence suggesting that the trend toward using lower VT in routine ventilator management in recent years has been accompanied by an increase in the incidence of atelectasis.2 Thus, while low-VT ventilation is evidence-based in ALI-ARDS and has rightly become the standard of care, it remains to be seen whether this is the best approach for patients without either ALI-ARDS or severe obstructive lung disease.

In the present study, we are not told whether any of the patients had ALI-ARDS. The study thus demonstrates that clinicians are poor at estimating IBW at the bedside, but it is not entirely certain that the target VTs used were optimal. I still recommend VT 10-12 mL/kg for the average ventilated patient without ALI or obstructive lung disease, keeping plateau pressure < 30 cm H2O. This places me in an ever-dwindling minority, however, and I have to admit that the evidence for harm from low-VT ventilation in patients without ALI-ARDS is weak. An initial VT of 8 mL/kg has become the standard for virtually all ventilated patients at my institution. Fortunately, calculation of IBW has also become part of the routine ICU admission procedure for ventilated patients. As shown by this study, actually calculating IBW rather than estimating it at the bedside is the only way to be sure that an appropriate VT will be employed, whatever the target value may be.

References

1. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med. 2000;342:1301-1308.

2. Wongsurakiat P, et al. Changing pattern of ventilator settings in patients without acute lung injury: Changes over 11 years in a single institution. Chest. 2004;126:1281-1291.