ScientificWorldJournal. 2013 Sep 16;2013:426097. doi: 10.1155/2013/426097
An Obesity Paradox of Asian Body Mass Index after Cardiac Surgery: Arterial Oxygenations in Duration of Mechanic Ventilation
Chiu-Hsia Chang,1 Fan-Yen Lee,2 Chin-ChouWang,1,3 Ying-Ni Chen,1 Hsin-Chu Chen,1 Huei-Ling Hung,1 Meng-Chih Lin,1,3 and Shih-Feng Liu1,3
1 Department of Respiratory Therapy; 2Department of Cardiothoracic and Vascular Surgery; 3Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Kaohsiung Chang Gung Memorial Hospital and Chang Gung University College of Medicine, Kaohsiung 833, Taiwan
Numerous studies have documented an obesity paradox that overweight of Caucasian patients has better prognosis after cardiac surgery. This study is to examine Asian patients’ BMI to see whether an obesity paradox exists in DMV after cardiac surgery.
A retrospective study consisted of 428 patients after cardiac surgery from January 2006 to December 2010 in the medical center of Taiwan. The Asian BMI was divided into 3 groups: under-normal weight patients (BMI < 24; n = 165), overweight patients (BMI 24 to <27; n = 130), and obese patients (BMI ≥ 27; n = 133). Multivariable analysis and paired t were used to compare all variables.
Overweight patients were significantly associated with the shortest DMV. Under-normal weight patients had significantly better oxygenations of AaDO₂ and P/F ratio in the DMV; however, they correlated with the longest DMV, older age, more female, lower LVSV, higher BUN, more dialysis-dependent, and poorer outcomes, namely, 1-year mortality, HAP, reintubation, tracheotomy, and LOS.
Asian overweight patients after cardiac surgery have better prognosis. Under-normal weight patients have higher risk factors, longer DMV, and poorer outcomes; even though they have better arterial oxygenations, they seem to need better arterial oxygenations for successful weaning ventilator.
The weaning procedure data were obtained from the medical chart database. Before surgery of invasive or noninvasive ventilation were excluded calculations of the duration of mechanic ventilation (DMV). The spontaneous breathing trial (SBT) phase was discontinuation of the ventilator for the patients’ extubation. The definition of successful weaning was that nonmechanically assisted at least 5 days in the SBT phase. Thus fewer 5 days in the SBT phase as the weaning failure can be accumulated as the DMV. In addition, the short DMV was also considered when patients died or critically were ill following discharge.
Then, the arterial blood gases (ABGs) were performed by comparing arterial blood from 2 periods: the beginning of mechanically assisted and the finally of successful SBT. Therefore, the ABGs provide valuable information about oxygenation, gas exchange, and lung ventilation. It is a useful method to evaluate pulmonary function and acid-base status dysfunction.
In the present study, it is amazing that the under-normal weight patients are significantly better associated with the oxygenation levels, but the obese patients produce poor results. It is possible that the large amount of fat in the chest wall and abdomen of the obese patients led to influences of the pulmonary mechanics, leading to longer DMV. In addition, the expanded fat tissue mass could have contributed to decreased blood flow to the cells, resulting in relative hypoperfusion with reduced tissue oxygenation and poorer oxygen parameters (see Figure 1).
Consequently, cardiac surgery patients undergoing CPB could experience increases in the body fluid content, causing an increase in lung fluid and dead space along with hypoxemia. Our observations also underscore the key role of oxygenation levels in the ABGs, and in that the SBT produces significantly better results in the under-normal weight patients in whom a longer DMV is indicated, providing them sufficient time to improve their physiologic or respiratory condition before being weaned off the ventilator. In contrast, the overweight patients differ in terms of having lower DMV than the other patients (see Figure 2).
Figure 1: Distribution of the arterial oxygenation levels in the MV and the SBT phases of the BMI groups. AaDO2 = (713 × FiO2)−(pCO2/0.8)−(paO2), (PAO2-paO2, a higher ratio indicates hypoxemia); P/F = PaO2/FiO2 (acute lung injury (ALI) <300, acute respiratory distress syndrome (ARDS) <200).