From the Guidelines
The estimated blood volume for a patient weighing 79 pounds is approximately 2.8 liters, based on 8% of body weight, as children have a higher blood volume per kilogram than adults, with approximately 8-9% of ideal body weight being the normal blood volume in children 1.
Calculation
To calculate the blood volume, we can use the formula: blood volume = 8% of body weight.
- First, convert the patient's weight from pounds to kilograms: 79 pounds ≈ 35.8 kg.
- Then, calculate the blood volume: 35.8 kg × 8% = 35.8 kg × 0.08 = 2.864 liters or approximately 2.9 liters, but since the question is about the blood volume, we should consider the evidence that children have approximately 8-9% of ideal body weight as normal blood volume 1.
Considerations
- The actual blood volume can vary based on factors such as age, sex, and body composition.
- Children typically have higher blood volume per kilogram than adults.
- This information is clinically relevant for determining appropriate fluid replacement, blood transfusion volumes, and medication dosing that depends on blood volume.
- In cases of significant blood loss, understanding the patient's baseline blood volume helps in assessing the percentage of blood lost and guiding resuscitation efforts.
Evidence
The study from the British Journal of Anaesthesia 1 provides the most relevant information for estimating blood volume in children, stating that normal blood volume is approximately 8-9% of ideal body weight in children.
- Although another study from the American Journal of Kidney Diseases 1 discusses iron deficiency and blood volume in patients with chronic kidney disease, it does not provide the most relevant information for estimating blood volume in a patient weighing 79 pounds.
From the Research
Estimating Blood Volume
To estimate blood volume in a patient weighing 79lb (approximately 35.8 kg), we can consider the following evidence:
- A study published in 2006 2 suggests that the mean value for indexed blood volume in normal weight adults is 70 mL/kg, but this value decreases in a non-linear manner with increasing weight.
- A study published in 2008 3 evaluated a mathematical algorithm for estimating individual blood volume and found a wide range of blood volume estimates (minimum, 35 mL/kg; mean +/- SD, 64 +/- 22 mL/kg; maximum, 129 mL/kg).
- A study published in 2018 4 established a new formula using ideal body weight with adjustment for gender or age to estimate total blood volume and found that the conventional formula (70 mL/kg) was unable to predict the actual haemodilution rate accurately.
Calculation of Blood Volume
Using the information from these studies, we can estimate the blood volume of a patient weighing 79lb (approximately 35.8 kg) as follows:
- Using the mean value of 70 mL/kg, the estimated blood volume would be approximately 2510 mL (35.8 kg x 70 mL/kg).
- However, this value may not be accurate due to the non-linear decrease in blood volume with increasing weight.
- The study published in 2018 4 suggests using ideal body weight with adjustment for age to estimate total blood volume, but does not provide a clear formula for patients weighing less than 40 kg.
Limitations
It is essential to note that estimating blood volume can be complex and may require individualized calculations based on various factors, including age, weight, and medical condition. The studies cited above provide some guidance, but the accuracy of these estimates may vary depending on the specific patient population and clinical context. Therefore, these estimates should be used with caution and in conjunction with other clinical assessments.