x

Body Mass Index: | 22.0 |

Total Body Weight: | x kg |

Ideal Body Weight: | x kg |

Lean Body Weight: | x kg |

Normalized Lean Wt: | x kg |

Adjusted Body Weight: | x kg |

Generalized Wt Scalar: | x kg |

x

Body Mass Index: | 22.0 |

Blood Volume Index: | x cc/kg |

Estimated Blood Volume: | x cc |

Maximum Blood Loss: | x cc |

Fractional Blood Loss: | x% |

x

Snoring:

Loud enough to be heard through closed doors

Tired:

Frequent daytime tiredness or sleepiness

Observed:

Observed to have stopped breathing during sleep

Pressure:

Hypertension or treated hypertension

BMI:

Body mass index more than 35

Age:

Age over 50 years old

Neck:

Neck circumference more than 40 cm

Gender:

Male gender

Sex: | Male |

Weight (kg): | |

Height (cm): | |

Age (yr): | |

Initial Hb: | |

Minimum Hb: |

*BigSleep* calculates several formulas that are
useful in the anesthetic management of patients with obesity.

The formulas provided require interpretation by properly trained and qualified professionals. Patient management must consider individual factors including age, fitness and medical conditions, as well as ongoing additions to the medical literature.

*
BigSleep 7.0
John Friesen MD FRCP(C)
2013, 2020
www.bigsleep.altervista.org
*

Contact: slumbersoftware@gmail.com

You may also be interested in Bedside Stewart, an app for the simplified Stewart approach to blood gas interpretation.

*Friesen JH.* Practical estimation of ideal body weight and normalized lean weight. Obes Surg. 2020.
Online

*Friesen JH.* Propofol induction: normalizing the dose in morbidly obese patients. Can J Anesth. 2017;64(5):456-60.
Full text

*Friesen JH.* Propofol pharmacokinetic model and lean body weight scalar for dose estimation in morbid obesity. Br J Anaesth. 2019.
Online

*Friesen JH.* Estimating the induction dose of propofol in morbid obesity: striking a happy medium. Br J Anaesth. 2016;116(5):730-1.
Full text

*Friesen JH.* Lean-scaled weight can be used to estimate blood volume for obese patients. Can J Anesth. 2014;61:1059-1060.
Full text

**For iPhone or iPad:**

Download *BigSleep* for iPhone and iPad from the App Store.

**For Android:**

Download *BigSleep* for Android from the Google Play Store.

Evidence suggests that lean body weight correlates with safe and effective dosages for several drugs used in anesthesia, including fentanyl, sufentanil, remifentanil, and induction doses of propofol(1). In particular, the front end kinetics of drugs given as intravenous boluses are in large part determined by cardiac output, which itself correlates with lean body weight(2). Nonetheless, lean body weight systematically underestimates drug doses(3) by a factor of 1.53 for women and 1.23 for men(4).

**Clinical Example**

Consider the estimation of propofol induction doses.
A woman weighing 105 kilograms with a height of 170 centimeters and a BMI of 36.3 presents
for surgery. She is obese and her calculated lean body weight(5) is 55.2 kilograms.
An otherwise similar woman of the same height but weighing only 65 kilograms is not obese.
It is not consistent with clinical experience to estimate a smaller dose of propofol
for the larger patient, but this would be the result if lean body weight were used as the weight scalar for the obese patient.

**Normalized Lean Weight**

Normalized lean weight(6) is calculated by normalizing lean body weight to ideal body weight.
It is proportional to lean body weight for all obese and non-obese patients.
The normalization factor (1.53 for females and 1.23 for males) is calculated by substituting a nominal BMI of 22 into the
Janmahasation equation, and then solving for the ratio of total body weight to lean body weight.

The normalized lean weight for the 105 kilogram woman in the example is 84.2 kilograms. This is greater than her ideal body weight in direct proportion to her lean body weight.

Normalized lean weight is easily estimated without a calculator. It is 100% of total body weight for non-obese patients, and about 75% at a BMI of 40, 60% at a BMI of 60, and 50% at a BMI of 80.

1.
*Lemmens HJM, Ingrande J. *
Pharmacology and obesity.
Int Anesthesiol Clin. 2013;5:52-66.

2.
*Krejcie TC, Avram MJ. *
What determines anesthetic induction
dose? It's the front-end kinetics, doctor!
Anesth Analg 1999; 89: 541-4

3.
*Bouillon T, Shafer SL. *
Does size matter?
Anesthesiology. 1998 Sep;89(3):557-60.

4.
*Friesen JH.* Propofol induction: normalizing the dose in morbidly obese patients. Can J Anesth. 2017;64(5):456-60.
Full text

5.
*Janmahasatian S, Duffull SB, Ash S, Ward LC, Byrne NM, Green B. *
Quantification of lean bodyweight.
Clin Pharmacokinet. 2005;44(10):1051-65.

6.
*Friesen JH. *
Lean-scaled weight: a proposed weight scalar to calculate drug doses for obese patients.
Can J Anesth. 2013 Feb;60(2):214-5.
Full text

Propofol (induction) |
LBW |

Propofol (mantenance) |
TBW |

Thiopental | LBW |

Etomidate | LBW |

Fentanyl | LBW |

Sufentanil | LBW |

Alfentanil | LBW |

Remifentanil | LBW |

Succinylcholine | TBW |

Rocuronium | LBW/IBW |

Vecuronium | LBW/IBW |

Cisatracurium | LBW/IBW |

Atracurium | LBW/IBW |

**TBW:** Total body weight

**LBW:** Lean body weight*

**IBW:** Ideal body weight

**Adapted from:** *Lemmens HJM, Ingrande J.* Pharmacology and obesity. Int Anesthesiol Clin. 2013;51(3):52-66.

*Normalized lean weight (NLW) is a weight scalar proportional to LBW:

*Friesen JH.* Lean-scaled weight: a proposed weight scalar to calculate drug doses for obese patients.
Can J Anesth. 2013 Feb;60(2):214-5.

**Calculation:**

BMI = TBW/Height^{2}

(Where TBW is in kg and Height is in meters)

**Use:**

The BMI is widely used to classify obesity and to quantify risk.
However, it does not distinguish between muscle and fat
and is not recommended as a weight scalar for calculating
drug dosage for patients with obesity^{1,2,3}.

**Calculation:**

TBW = body weight in kilograms

**Use:**

The TBW is the appropriate weight scalar for use with
patients of normal weight. In morbidly obese individuals
the increased weight is composed of proportionately greater
adipose tissue than muscle mass.
Drug distribution and clearance are both altered in obesity, and
using a mg per kg dose based on total body weight may result
in overdose^{1,2,3}.

**Calculation:**

IBW = 22×Height^{2}
= TBW×22/BMI

(Where Height is in meters)

**Use:**

The IBW is the weight that predicts the maximum life expectancy
for a given height, but it does not reflect changes in body composition
with obesity.^{3} All patients of the same height have the same calculated IBW,
regardless of how much they weigh. Nonetheless, studies suggest that the IBW
may be useful for dosing some drugs.^{1,2}
The IBW calculation used here
is the weight the patient would have if the BMI
were to be 22, given his or her height^{11}.
This gives a result similar to the commonly used Devine formula^{8,15}.
No calculation is required: it can be read directly from any BMI chart.
Alternatively, the proportionality factor 22/BMI can be estimated.

**Calculation: ^{5}**

LBW = 9270×TBW/(6680+216×BMI) For men

LBW = 9270×TBW/(8780+244×BMI) For women

**Use:**

The LBW is the TBW minus the fat mass, and represents
the fat free mass. It is not the ideal or the normal weight.
The LBW correlates with the cardiac output,
drug clearance, and blood volume^{3,17}. It is arguably the
single best weight scalar for use in morbid obesity^{4},
although the volume of distribution for lipophilic drugs
may be more dependent on the TBW or fat mass^{3}.
The model used here to calculate the LBW is that of
Janmahasatian^{5}, which does better in patients with obesity
than does the James formula seen in older literature.^{3,14}

**Calculation: ^{12}**

NLW = 1.233×LBW

= 11432×TBW/(6680+216×BMI) For men

NLW = 1.526×LBW

= 14148×TBW/(8780+244×BMI) For women

(LBW from Janmahasatian calculation

**Use:**

Since the LBW is what remains after fat mass is subtracted from the TBW,
it is always less than the TBW, including for patients who are not obese.
For this reason the LBW can not be used directly to calculate drug doses on a
mg per kg basis: it will result in systematic underestimation of doses, and must be scaled upwards^{6,7}.
The NLW is calculated by normalizing the LBW functions to a nominal BMI of 22.
The normalization factors are 1.233 for men and 1.526 for women^{12}.
The NLW is proportional to the LBW:
it is intended to be used for those drugs whose doses are
expected to vary in proportion to the lean body weight^{13}.

**Calculation:**

ABW = IBW+0.4×(TBW-IBW)

**Use:**

The ABW adds a percentage of the excess weight to the IBW in an attempt
to adjust drug doses for patients with obesity.
The factor of 40% used by Servin^{9}
is employed here, resulting in a close linear approximation to the NLW.

**Calculation:**

GWS = TBW×(22/BMI)^{S}

If *S* is set to 0.5:

GWS = TBW×√ 22/BMI

**Use:**

This app was originally developed to help clinicians calculate useful
weight scalars.
The scalars recommended in the literature included total body weight,
ideal body weight, lean body weight, and adjusted body weight.
In the process of coding these formulas it became apparent that
the lean body weight requires a normalization factor:
as a result the normalized lean weight formula was developed^{12}.

Further work^{18,19} has demonstrated that a generalized weight scalar (GWS)
can be derived from the power function
(*y* = *a*×TBW^{b}).
Like the lean body weight, this function needs to be normalized to the ideal body weight.
This is accomplished by setting the parameter *a*
equal to IBW^{(1−b)}. Substituting IBW = TBW×22/BMI results in the
following formula:
GWS = TBW×(22/BMI)^{S}, where the
exponent *S* = 1−*b*. For *S* = 0.5 this generalized weight scalar
is equal for all practical purposes to the normalized lean weight scalar,
and it is easy to calculate. For values of
*S* from 0 to 1 it varies smoothly between the total body weight and the ideal
body weight. This function is mathematically tractable, and may be useful when modelling
experimentally determined weight scalars.

The NLW, ABW and GWS are equivalent for all practical purposes.
The NLW is mechanistic and based directly on the Janmahasatian^{5} LBW equation.
The ABW (with an adjustment factor of 0.4) is a linear approximation to the NLW. It can be varied from IBW to TBW
by setting the adjusment factor to some value between 0 and 1.
The GWS (with an exponent of 0.5) is clinically equivalent to the NLW and the ABW.
It is a generalized scalar calculated as a power function, and it can be varied from TBW to IBW
by setting the exponent to some value between 0 and 1.

The above weight scalars are useful in the management of patients with obesity when estimating
safe and effective doses for different classes of drugs.
The choice of which scalar is most appropriate for a given drug is controversial
and will continue to evolve as more evidence becomes available.^{1,2,14,16}
The actual dose chosen must be adjusted for individual patient factors such as age, sex, and medical conditions.

1.
*Ingrande J, Lemmens HJM. *
Dose adjustment of anaesthetics in the morbidly obese.
Br J Anaesth.
2010 Dec;105 Suppl 1:i16-23.

2.
*Lemmens HJM. *
Perioperative pharmacology in morbid obesity.
Curr Opin Anaesthesiol.
2010 Aug;23(4):485-91.

3.
*Morrish GA, Pai MP, Green B. *
The effects of obesity on drug pharmacokinetics in humans.
Expert Opin Drug Metab Toxicol.
2011 Jun;7(6):697-706.

4.
*Han PY, Duffull SB, Kirkpatrick CMJ, Green B. *
Dosing in obesity: a simple solution to a big problem.
Clin. Pharmacol. Ther.
2007 Nov;82(5):505-8.

5.
*Janmahasatian S, Duffull SB, Ash S, Ward LC, Byrne NM, Green B. *
Quantification of lean bodyweight.
Clin Pharmacokinet.
2005;44(10):1051-65.

6.
*Bouillon T, Shafer SL. *
Does size matter?
Anesthesiology.
1998 Sep;89(3):557-60.

7.
*McLeay SC, Morrish GA, Kirkpatrick CM, Green B. *
Encouraging the move towards predictive population models for the obese using propofol as a motivating example.
Pharm. Res.
2009 Jul;26(7):1626-34.

8.
*Devine BJ. *
Gentamicin therapy.
Drug Intell Clin Pharm.
1974; (8):650-655

9.
*Servin F, Farinotti R, Haberer JP, Desmonts JM. *
Propofol infusion for maintenance of anesthesia in morbidly obese patients receiving nitrous oxide. A clinical and pharmacokinetic study.
Anesthesiology.
1993; 78:657-665.

10.
*Duffull SB, Dooley MJ, Green B, Poole SG, Kirkpatrick CMJ. *
A standard weight descriptor for dose adjustment in the obese patient.
Clin Pharmacokinet.
2004;43(15):1167-78.

11.
*Lemmens HJM, Brodsky JB, Bernstein DP. *
Estimating ideal body weight--a new formula.
Obes Surg.
2005 Aug;15(7):1082-3.

12.
*Friesen JH. *
Lean-scaled weight: a proposed weight scalar to calculate drug doses for obese patients.
Can J Anesth.
2013 Feb;60(2):214-5.
Full text

13.
*Friesen JH.* Propofol induction: normalizing the dose in morbidly obese patients. Can J Anesth. 2017;64(5):456-60.
Full text

14.
*Cullen A, Ferguson A. *
Perioperative management of the severely obese patient: a selective pathophysiological review.
Can J Anesth.
2012 Oct;59(10):974-96.

15.
*Pai MP, Paloucek FP. *
The origin of the "ideal" body weight equations.
Ann Pharmacother.
2000 Sep;34(9):1066-9.

16.
*Schumann R. *
Anaesthesia for bariatric surgery.
Best Pract Res Clin Anaesthesiol.
2011 Mar;25(1):83-93.

17.
*Friesen JH.* Lean-scaled weight can be used to estimate blood volume for obese patients. Can J Anesth. 2014;61:1059-1060.
Full text

18.
*Friesen JH.* Practical estimation of ideal body weight and normalized lean weight.
Obes Surg.
2020 Jun;30(6):2437–8.
Full text

Preoperative estimation of a patient's blood volume is helpful in order to guide the administration of crystalloids, colloids, red blood cells and coagulation factors. The increase in total blood volume with obesity is non-linear, which can result in its overestimation for patients with morbid obesity.

**Normal Weight Patients:**

The blood volume index is usually estimated as 70 cc/kg for men,
and 65 cc/kg for women.

**Patients with Morbid Obesity:**

Lemmens et al have derived an empirical equation that corrects
the blood volume index for obesity (where BMI/22 estimates the
deviation from ideal weight):^{1}

Blood Volume Index (cc/kg) = 70÷sqrt(BMI/22)

*BigSleep* applies this formula to the calculation of the
blood volume index for any patient whose BMI is greater than 22.0

**Influence of Age:**

In normal, but sedentary, patients the blood volume index
decreases with advancing age.^{1} This decrease does not occur
in those who remain aerobically fit.

BV_{index} = 90−0.4×age For men

BV_{index} = 85−0.4×age For women

*BigSleep* will apply this regression for patients more than fifty years old.
Entering an age of 50 years or less will
remove the effect of age from the calculation of blood volumes.

**Hypertension and other corrections:**

Blood volume is contracted in lean hypertensive patients. This effect
diminishes as weight increases and does not apply to patients with obesity.
It is necessary to allow for this or any
other factors that may apply to the individual patient.

**Maximum Untransfused Blood Loss:**

It is useful to estimate the maximum allowable blood loss that a patient
can tolerate without transfusion of red cells. The formula used is:^{3}

Max bood loss = EBV×ln(H_{init}/H_{min})

where H_{init} is the initial Hb or Hct, H_{min}
is the minimum (target, not trigger) thought to be safe
for the individual patient, and ln is the natural logarithm. The
frequently used linear approximation^{4}
EBV×(H_{init}−H_{min})÷
½(H_{init}+H_{min})
slightly underestimates the MBL.

**Coagulopathy:**

The total blood volume is also useful for
estimating how much blood can be lost before unacceptible decreases in
fibrinogen or platelets are likely to occur.^{2}

**Weight Scalar:**

The Lemmens blood volume index correction^{1} is mathematically
identical to scaling the patient weight in proportion to
TBW×√ 22/BMI .
This simple scalar is practical for clinical use^{5}, and also may have theoretical
implications for the effective two dimensionality of the allometric scaling of
metabolism with weight gain in obesity^{6,7}.

**Clinical Management:**

Estimates of blood volumes, bleeding, and fluid replacement require ongoing reassessment
based on clinical response, hemodynamics, measured hemoglobin concentrations, and evidence of coagulopathy.

1.
*Lemmens HJM, Bernstein DP, Brodsky JB. *
Estimating blood volume in obese and morbidly obese patients.
Obes Surg.
2006 Jun;16(6):773-6.

2.
*Singbartl K, Innerhofer P, Radvan J, Westphalen B, Fries D, Stogbauer R, et al. *
Hemostasis and hemodilution: a quantitative mathematical guide for clinical practice.
Anesth. Analg.
2003 Apr;96(4):929-935.

3.
*Bourke DL, Smith TC. *
Estimating allowable hemodilution.
Anesthesiology.
1974 Dec;41(6):609-12.

4.
*Gross JB. *
Estimating allowable blood loss: corrected for dilution.
Anesthesiology.
1983 Mar;58(3):277-80.

5.
*Friesen JH.*
Lean-scaled weight can be used to estimate blood volume for obese patients.
Can J Anesth.
2014;61:1059-1060.
Full text

6.
*Friesen JH.*
Practical Estimation of Ideal Body Weight and Normalized Lean Weight.
Obes Surg.
2020 Jun;30(6):2437–8.
Full text

Obstructive sleep apnea (OSA) is common in the pre-operative patient
population, and is associated with increased perioperative risk^{1}.
The Stop-Bang questionnaire is a screening tool widely used to identify
OSA risk: the score is calculated by adding up the positive responses
to eight yes/no questions.

It is simple to use, and has been validated
in the perioperative patient population^{2,3,4}. Its predictive performance
has been confirmed for surgical patients with obesity and morbid obesity^{5}.

In surgical patients, a score of 3 or greater has a high sensitivity for
the presence of OSA and a score of 5 or more has a good specificity for
moderate to severe OSA^{6,7}.
*BigSleep* assigns a rating of "Low Risk" to Stop-Bang scores
less than 3 and "High Risk" to scores of 5 or more.

1.
*Chung F, Elsaid H. *
Screening for obstructive sleep apnea before surgery: why is it important?
Curr Opin Anaesthesiol.
2009 Jun;22(3):405-11.

2.
*Auckley D, Bolden N. *
Preoperative screening and perioperative care of the patient with sleep-disordered breathing.
Curr Opin Pulm Med.
2012 Nov;18(6):588-95.

3.
*Abrishami A, Khajehdehi A, Chung F. *
A systematic review of screening questionnaires for obstructive sleep apnea.
Can J Anaesth.
2010 May;57(5):423-38.

4.
*Chung F, Yegneswaran B, Liao P, Chung SA, Vairavanathan S, Islam S, et al. *
STOP questionnaire: a tool to screen patients for obstructive sleep apnea.
Anesthesiology.
2008 May;108(5):812-21.

5.
*Chung F, Yang Y, Liao P. *
Predictive Performance of the STOP-Bang Score for Identifying Obstructive Sleep Apnea in Obese Patients.
Obes Surg.
2013 Dec;23(12):2050-7.

6.
*Chung F, Subramanyam R, Liao P, Sasaki E, Shapiro C, Sun Y. *
High STOP-Bang score indicates a high probability of obstructive sleep apnoea.
Br J Anaesth.
2012 May;108(5):768-75.

7.
*Nagappa M, Liao P, Wong J, Auckley D, Ramachandran SK, Memtsoudis S, et al. *
Validation of the STOP-Bang Questionnaire as a Screening Tool for Obstructive Sleep Apnea among Different Populations: A Systematic Review and Meta-Analysis.
PLoS ONE.
2015;10(12):e0143697.