Role of the Heart in the Loss of Aeration Characterizing Lower Lobes in Acute Respiratory Distress Syndrome

Abstract

In the acute respiratory distress syndrome (ARDS), lower lobes appear essentially non-aerated in contrast to upper lobes whose aeration can be preserved in some patients. The aim of this study was to assess the mechanical compression exerted by the heart on lower lobes in patients with ARDS. Fourteen healthy volunteers and 38 patients with ARDS free of left ventricular failure were studied. Cardiorespiratory parameters were recorded and the cardiac dimensions, the pressure exerted by the heart on subjacent lower lobes, and the gas tissue ratio of lower lobes in the supine position were measured using computer tomography and Lungview, a specifically designed software. In patients with ARDS, the heart was larger and heavier than in healthy volunteers. The enlargement of the heart was mainly related to a left cardiac protrusion and the pressure exerted by the left heart on the lower lobes was higher in patients with ARDS than in healthy volunteers (8 +/- 3 g. cm(-)(2) versus 6 +/- 1 g. cm(-)(2), p < 0.01). As a consequence, the faction of gas represented 62% of the left lower lobes in healthy volunteers and 12% only in patients with ARDS. The present study demonstrates that apart from the already known anteroposterior and cephalocaudal gradients of pressure depending on the lung weight and abdominal pressure, the heart plays an important role in the dramatic loss of aeration characterizing lower lobes of patients with ARDS lying in the supine position.

Comment on “Relationship between gas exchange response to prone position and lung recruitability during acute respiratory failure”

“…Although Malbouisson described a significant increase in cardiac dimensions in ARDS patients compared to healthy controls on CT imaging [3], it would be interesting to know if the heart size at baseline between high percentage ''recruiters'' and those showing a reduction in PaCO 2 during prone ventilation was significantly different to the rest of the study population in the present study. If this were to be the case, we wonder whether in addition to recruitability during CT scanning, cardiac chamber size on CT imaging may be predictive of PaCO 2 response and recruitment during prone position.…”

“…Secondly, as previously described by J.J. Rouby and co-workers [3], the weight of the heart plays an important role in loss of aeration, especially of the lower lobes, which will be exacerbated in the context of heart failure and cardiomegaly [4]. In prone position the weight of the heart is supported by the sternum, reducing the anteroposterior pressure gradient on the dependent lung areas.…”

Atelectasias durante anestesia: fisiopatologia e tratamento

“…Pacientes obesos, submetidos a procedimento cirúrgico abdominal alto ou laparoscópico podem ter aumentada a participação da elevação diafragmática no colapso das regiões dorsais do pulmão em virtude do peso do panículo adiposo abdominal 24 . Outro fator que pode contribuir para a diminuição na pressão transpulmonar nas regiões dependentes e caudais do pulmão é a compressão direta dos lobos inferiores induzida pelo coração e estruturas do mediastino [25][26][27] . Em posição supina, o coração e as estruturas do mediastino repousam sobre as partes mediais dos lobos inferiores exercendo pressão em torno de 5 cmH 2 O.cm -2 em pacientes adultos sem doença cardíaca, e em torno de 7 a 8 cmH 2 O.cm -2 nos pacientes com lesão pulmonar aguda 27 .…”

“…Outro fator que pode contribuir para a diminuição na pressão transpulmonar nas regiões dependentes e caudais do pulmão é a compressão direta dos lobos inferiores induzida pelo coração e estruturas do mediastino [25][26][27] . Em posição supina, o coração e as estruturas do mediastino repousam sobre as partes mediais dos lobos inferiores exercendo pressão em torno de 5 cmH 2 O.cm -2 em pacientes adultos sem doença cardíaca, e em torno de 7 a 8 cmH 2 O.cm -2 nos pacientes com lesão pulmonar aguda 27 . Nos pacientes submetidos à anestesia e ventilação mecâ-nica, é provável que a compressão mecânica das regiões pulmonares dependentes e caudais seja o fator mais importante para o colapso pulmonar observado no intra-operató-rio 26,28,29 .…”

“…In obese patients undergoing upper abdominal surgery or laparoscopy, the elevation of the diaphragm can be even more pronounced, worsening pulmonary collapse in the dorsal areas of the lungs secondary to the weight of the abdominal adipose tissue 24 . The direct compression of the inferior lobes by the heart and mediastinal structures is another factor that can contribute to reduce transpulmonary pressure in dependent and caudal regions of the lungs [25][26][27] . In the supine position, the heart and other mediastinal structures rest on the medial regions of the inferior lobes, exerting a 5 cmH 2 O.cm -2 pressure in adult patients without cardiac diseases, and approximately 7 to 8 cmH 2 O.cm -2 in patients with acute pulmonary lesion 27 .…”