Oxygen consumption and evaporative water loss in infants with congenital heart disease

The inaccuracy of LaFarge equations estimating the oxygen consumption in children under 3 years age

Accurate oxygen consumption measurement (VO₂), is essential to obtain a reliable hemodynamic assessment, particularly in patients with congenital heart diseases undergoing cardiac catheterization. LaFarge equations can be unreliable in predicting VO₂, particularly in the pediatric population. In a clinical setting, these inaccurate estimates can lead to important hemodynamic parameter miscalculations, with possible therapeutic or diagnostic consequences. Our aim is to validate LaFarge equations (the most widely used for estimating VO₂) and compare them with direct measurement in children during cardiac catheterization in the cath lab. We performed a prospective observational study in 21 patients (0-3 years of age) with different congenital cardiac diseases, scheduled for diagnostic cardiac catheterization. Under general anesthesia and mechanical ventilation, VO₂ was measured directly with a metabolic module in our cath lab, and also estimated using LaFarge equations. Statistical analysis included Bland-Altman plots, Pearson coefficient and percentage error, among others. LaFarge equations overestimated VO₂ values in all study patients. Therefore, in pediatric patients under 3 years of age, the use of direct VO₂ measurement methods are more accurate and acceptable than LaFarge equations.

Validity of the LaFarge equation for estimation of oxygen consumption in ventilated children with congenital heart disease younger than 3 years--a revisit

Background

The LaFarge equation is the most commonly used equation to estimate oxygen consumption (Vo₂) in patients of all ages with congenital heart disease, although it was generated in patients older than 3 years. We sought to determine the validity of the LaFarge equation in estimating Vo₂ in children younger than 3 years undergoing cardiac catheterization with general anesthesia.

Methods

Vo₂ was measured directly using respiratory mass spectrometry in 75 sedated, paralyzed, and mechanically ventilated children in the pediatric cardiac catheterization laboratory. Age ranged from 0.13 to 24 years; 40 children being younger than 3 years. Estimated values for Vo₂ were calculated using the LaFarge equation for all patients. The agreement between measured and estimated Vo₂ was evaluated by the bias and limits of agreement in the 2 age groups. Regression analysis was used to analyze the influence of age on the agreement.

Results

A failure of agreement between measured and estimated Vo₂ was noted in both groups of children. As compared to the older group of patients, the agreement was significantly poorer in children younger than 3 years, with a significantly greater overestimation introduced by the LaFarge equation (11% ± 21% vs 53% ± 52%, P < .0001).

Conclusion

The LaFarge equation introduces significant error in the estimation of Vo₂ in ventilated patients with congenital heart disease of all ages, particularly in children younger than 3 years.

Iatrogenic hyperthermia during cardiac magnetic resonance imaging

We report the occurrence of accidental hyperthermia in a young child undergoing anesthesia for cardiac magnetic resonance imaging. Although the tendency during anesthesia is to develop hypothermia, the absorbed radiofrequency energy from magnetic resonance scanning is added to metabolic energy and must be balanced by appropriate heat loss to maintain normothermia. In addition to stressing the clinical importance of temperature monitoring, this report suggests that the recommended specific absorption rates to prevent excessive patient heating may need to be revised for infants and young children.

Total but not resting energy expenditure is increased in infants with ventricular septal defects

OBJECTIVE

The purpose of this study was to determine the effect of left-to-right shunting on the resting energy expenditure (REE), total energy expenditure (TEE), and energy intake in a group of 3- to 5-month-old infants with moderate to large unrepaired ventricular septal defects (VSDs) compared with age-matched, healthy infants.

METHODS

Eight infants with VSDs and 10 healthy controls between 3 to 5 months of age participated in the study. Indirect calorimetry was used to measure REE and the doubly-labeled water method was used to measure TEE and energy intake. An echocardiogram and anthropometric measurements were performed on all study participants. Daily urine samples were collected at home for 7 days. Samples were analyzed by isotope ratio mass spectrometry. Data were compared using analysis of variance.

RESULTS

No significant differences were found in REE (VSD, 42.2 +/- 8.7 kcal/kg/d; control, 43.9 +/- 14.1 kcal/kg/d) or energy intake (VSD, 90.8 +/- 19.9 kcal/kg/d; control, 87.1 +/- 11.7 kcal/kg/d) between the groups. The percent total body water was significantly higher in the VSD infants and the percent fat mass was significantly lower. TEE was 40% higher in the VSD group (VSD, 87.6 +/- 10.8 kcal/kg/d; control, 61.9 +/- 10.3 kcal/kg/d). The difference between TEE and REE, reflecting the energy of activity, was 2.5 times greater in the VSD group.

CONCLUSIONS

REE and energy intake are virtually identical between the two groups. Despite this, infants with VSDs have substantially higher TEE than age-matched healthy infants. The large difference between TEE and REE in VSD infants suggests a substantially elevated energy cost of physical activity in these infants. These results demonstrate that, although infants with VSDs may match the energy intake of healthy infants, they are unable to meet their increased energy demands, resulting in growth retardation.

Energy expenditure in congenital heart disease

Growth failure is a well recognised consequence of severe congenital heart disease. Total daily energy expenditure (TDEE) was investigated in eight infants with severe congenital heart disease to determine whether an increase in this parameter is an important factor in their failure to thrive, and to estimate the energy intake that would be required to allow normal growth. The infants were studied over a seven day period before surgery using the doubly labelled water method. Growth failure was evident; their mean age standardised body mass index was 80% of the expected value. Mean TDEE was 425 kJ/kg, significantly greater than in healthy infants (mean TDEE/kg SD score = +1.4; 95% confidence interval +0.27 to +2.57). In contrast, their energy intake was only 82% of the estimated average requirements. It was estimated that in early infancy a gross energy intake of 600 kJ/kg/day is required for normal growth in patients with congenital heart disease. This is unlikely to be achieved by energy supplements alone and early recourse to nasogastric feeding should be considered.

The pathophysiology of disorders of oxygen transport in the infant

The major attempt in this monograph has been to provide a systematic approach to OT in the infant under normal and abnormal conditions based on an appreciation of pathophysiologic mechanisms. This has included a consideration of oxygen uptake, gas transfer from the lungs to the circulation, oxygen delivery to the tissues, and various adaptive responses. We have discussed disorders involving the control of respiration, the upper and lower airways, lung parenchyma, pulmonary circulation, heart, and the oxygen carrier, hemoglobin, that may impair OT to tissues, and how compensation for these conditions can be achieved. The clinical presentation in each of these pathologic processes represents the net effect of the lesion on the multiple steps involved in the transport of oxygen to the tissues and the adaptation provided by the lungs, heart, circulation, and hematologic system. These must be taken into account in developing a plan for medical and/or surgical treatment that is focused on improving the supply of oxygen to the various organ systems of the body.

Variations in the resting oxygen consumption of small babies

Over 200 measurements of the resting rate of oxygen consumption using an open-circuit method were made on 15 small babies nursed in their usual clinical setting during the first month of life. There were striking and persistent variations between babies that could not be explained by postnatal age, relationship to feed, sleep, or time of day. It was not possible from clinical examination to predict which babies had the higher or lower metabolic rates, except that babies who were light-for-dates generally had higher values. Because of these variations the appropriate thermal temperature for small babies cannot be predicted from average values adjusted for body weight and postnatal age alone.