Large Tidal Volume Ventilation Does Not Improve Oxygenation in Morbidly Obese Patients During Anesthesia

Abstract

Eight morbidly obese patients (body mass index [BMI] = 46) were studied during general anesthesia and controlled mechanical ventilation. To evaluate the effect of large tidal volume ventilation on oxygenation and ventilation, the baseline 13 mL/kg tidal volume (VT) (calculated by the ideal body weight) was increased in 3 mL/kg volume increments to 22 mL/kg, while ventilatory rate (RR) and inspiratory time (TI) were kept constant. Each volume increment was maintained for 15 min. Gas exchange was assessed by measuring the arterial blood oxygen tensions, and calculating the indices of alveolar-arterial oxygen tension difference [P(A-a)O2] and arterial/alveolar oxygen tension ratio (a/A). Peak inspiratory airway pressure (Ppeak), end-inspiratory airway pressure (Pplateau), and compliance of the respiratory system (CRS) were recorded using the Capnomac Ultima (Datex, Helsinki, Finland) on-line respiratory monitor. Increasing tidal volumes to 22 mL/kg increased the recorded Ppeak (26.3 +/- 4.1 vs 37.9 +/- 3.2 cm H2O, P < 0.008), Pplateau (21.5 +/- 3.6 vs 27.7 +/- 4.3 cm H2O, P < 0.01), and CRS (39.8 +/- 7.7 vs 48.5 +/- 8.3 mL/cm H2O) significantly without improving arterial oxygen tension and resulted in severe hypocapnia. Since changes in arterial oxygenation were small and not statistically significant, mechanical ventilation of morbidly obese patients with large VTS seems to offer no advantage to smaller (13 mL/kg ideal body weight) VTS.

Desafios no manuseio peri-operatório de pacientes obesos mórbidos: como prevenir complicações

“…As compared to studies recommending TV levels above 15 ml.kg -1 of optimal weight, TV values used in our study were close to 0.62 l (calculated by optimal weight, TV was approximately 11 ml.kg . Their conclusion was that the increase in tidal volume was not followed by a major increase in oxygenation and has caused severe hypocapnia 40 . PEEP may improve oxygenation although this finding is not a consensus in the literature According to those authors, decreased oxygenation was associated to decreased pulmonary volume, suggesting that a significant pulmonary collapse is probably present in obese patients 41 .…”

“…A pressão intra-abdominal pode desempenhar um papel importante na diminuição da complacência e aumento da resistência pulmonar 35,36 . Outro ponto importante é a falta de consenso na literatura sobre o valor ideal de volume corrente (VC) a ser estabelecido durante a anestesia, fato este já debatido anteriormente em artigos 38,39 , bem como o uso e indicação do PEEP (pressão expiratória final positiva) 40,41 . Como se sabe, os pacientes obesos, devido aos volumes pulmonares reduzidos, à diminuição da capacidade residual funcional e à alta capacidade de oclusão, têm tendência a desenvolver hipoxemia devido a distúrbios de ventilação/perfusão e aumento no shunt intrapulmonar 2,42 .…”

“…Intra-abdominal pressure may play an important role in decreasing compliance and increasing pulmonary resistance 35,36 . Another important point is the lack of consensus in the literature about the optimal tidal volume (TV) to be established during anesthesia, and which has already been discussed in previous studies 38,39 , as well as the use and indication of PEEP (positive end expiratory pressure) 40,41 . As it is known, obese patients, due to decreased pulmonary volumes, decreased functional residual capacity and high occlusion capacity, tend to develop hypoxemia due to ventilation/perfusion changes and increased intrapulmonary shunt 2,42 .…”

Obstáculos diagnósticos e desafios terapêuticos no paciente obeso

“…Uma eventual anestesia agrava ainda mais essas alterações, já que leva a uma redução adicional de 50% da CRF em obesos, comparado a uma queda de 20% em indivíduos não obesos anestesiados (39). A CRF pode ser aumentada pela ventilação com grande volume corrente (exemplo, 15-20ml.kg -1 ), embora a melhora na pressão arterial de O 2 (p a O 2 ) seja pequena (40). A adição de pressão positiva expiratória final (PEEP) leva a melhora tanto da CRF como da p a O 2 , porém às custas de redução do débito cardíaco (41,42).…”

A comparative study of volume control ventilation versus pressure control ventilation in patients of trauma

“…2 The traditional approach of using large tidal volume in volume control ventilation in the trauma patients with central nervous system trauma injury or abdominal trauma injury causes cardiovascular embarrassment, risk of peak inspiratory pressure and plateau pressure without significant improvement in the arterial oxygenation. 3 More over high tidal volume causes excessive stretch of non dependant lung region and promotes alveolar rupture, leading to the volume trauma of the lungs. 4 On the contrary, the decelerating inspiratory flow used during pressure control ventilation generates high flow rate causing more rapid alveolar inflation.…”