The Distribution of Aeration Loss and Excess Tissue in Patients with ARDS

The diffuse injury of the alveolar-capillary membrane that characterizes the ARDS lung, produces a high-permeability type pulmonary edema. The resulting increase in lung tissue detected on CT [16] is distributed from the apex to the diaphragmatic cupola, predominant in the upper lobes and frequently associated with a massive loss of aeration [20]. In caudal parts of the lung, although the regional loss of aeration is always massive, the excess lungtissue is absent or minimum in one-third of lower lobes [16]. Inversely, although the excess lungtissue is constantly observed in the cephalic parts of the lung, the aeration remains either partially preserved or entirely normal in two-third of upper lobes.

In the supine position at zero end-expiratory pressure (ZEEP), the degree of aeration of the upper lobes determines the lung morphology and the radiological pattern. In a minority of patients with ARDS, the loss of aeration is massive and equally distributed within the lung parenchyma (Fig. 1). In such patients with diffuse and bilateral CT attenuations, arterial hypoxemia is severe and the mortality y

Ards Prone Position

Fig. 1. Six CT sections obtained at zero end-expiratory pressure (ZEEP) in a 40-year old patient with ARDS secondary to posttraumatic peritonitis and characterized by a diffuse loss of aeration. On the right side of each CT section, the corresponding lung aeration is represented using a color code included in the software Lungview. Nonaerated lung regions characterized by CT attenuations ranging between 0 and -100 Hounsfield units (HU) are colored in black. Poorly aerated lung regions characterized by CT attenuations ranging between -100 and -500 HU are colored in light gray. Normally aerated lung regions characterized by CT attenuations ranging between -500 and -900 HU are colored in dark gray. At ZEEP, less than 10% ofthe lung is normally aerated. The loss of aeration is homogeneously distributed between upper and lower lobes and there is a moderate increase in lung tissue: gas volume in upper lobes = 135 ml (normal values = 1636 319 ml), gas volume in lower lobes = 190 ml (normal values = 1391 367 ml), tissue volume in upper lobes = 616 ml (normal values = 461 68 ml), tissue volume in lower lobes = 530 ml (normal values = 482 89 ml).

Fig. 1. Six CT sections obtained at zero end-expiratory pressure (ZEEP) in a 40-year old patient with ARDS secondary to posttraumatic peritonitis and characterized by a diffuse loss of aeration. On the right side of each CT section, the corresponding lung aeration is represented using a color code included in the software Lungview. Nonaerated lung regions characterized by CT attenuations ranging between 0 and -100 Hounsfield units (HU) are colored in black. Poorly aerated lung regions characterized by CT attenuations ranging between -100 and -500 HU are colored in light gray. Normally aerated lung regions characterized by CT attenuations ranging between -500 and -900 HU are colored in dark gray. At ZEEP, less than 10% ofthe lung is normally aerated. The loss of aeration is homogeneously distributed between upper and lower lobes and there is a moderate increase in lung tissue: gas volume in upper lobes = 135 ml (normal values = 1636 319 ml), gas volume in lower lobes = 190 ml (normal values = 1391 367 ml), tissue volume in upper lobes = 616 ml (normal values = 461 68 ml), tissue volume in lower lobes = 530 ml (normal values = 482 89 ml).

rate is above 70%. A primary insult to the lung is the most frequent cause of ARDS. The typical radiological presentation is of bilateral and diffuse hyperdensities resulting in 'white lungs' [16, 21]. In the majority of patients with ARDS, the aeration of upper lobes is either entirely or partially preserved despite a regional excess of lung tissue and the loss of aeration involves predominantly lower lobes. When aeration loss in the lower lobes is caused by alveolar flooding, a marked increase in lung tissue is observed, the overall lobar volume is preserved (Fig. 2) and bilateral radiological densities of the lower quadrants are present erasing the diaphragmatic cupola. When the regional loss of aeration is causedby compression atelectasis, a moderate, or no, increase in lung tissue is observed, the overall lower lobe volume is markedly reduced (Fig. 3), andbilateral radiological densities ofthe lower quadrants are discrete leaving apparent the diaphragmatic cupola. In such patients with 'focal' CT attenuations, arterial oxygenation impairment is severe contrasting with the lack of extensive radiological abnormalities and the mortality rate is around 40% [21].

Images Ards

Fig. 2. Six CT sections obtained at zero end-expiratory pressure (ZEEP) in a 23-year old patient with ARDS secondary to pulmonary contusion and characterized by an unevenly distributed loss of lung aeration (some regions of the upper lobes remain normally aerated). On the right side of each CT section, the corresponding lung aeration is represented using a color code included in the software Lungview. Nonaerated lung regions characterized by CT attenuations ranging between 0 and -100 Hounsfield units (HU) are colored in black. Poorly aerated lung regions characterized by CT attenuations ranging between -100 and -500 HU are colored in light gray. Normally aerated lung regions characterized by CT attenuations ranging between -500 and -900 HU are colored in dark gray. At ZEEP, 30% of the lung is normally aerated. The loss of aeration predominates in lower lobes and there is a marked increase in lung tissue homogeneously distributed between upper and lower lobes: gas volume in upper lobes = 995 ml (Normal values = 1636 319 ml), gas volume inlower lobes = 212 ml (Normal values = 1391 367 ml), tissue volume in upper lobes = 1182 ml (Normal values = 461 68 ml), tissue volume in lower lobes = 1166 ml (Normal values = 482 89 ml).

Fig. 2. Six CT sections obtained at zero end-expiratory pressure (ZEEP) in a 23-year old patient with ARDS secondary to pulmonary contusion and characterized by an unevenly distributed loss of lung aeration (some regions of the upper lobes remain normally aerated). On the right side of each CT section, the corresponding lung aeration is represented using a color code included in the software Lungview. Nonaerated lung regions characterized by CT attenuations ranging between 0 and -100 Hounsfield units (HU) are colored in black. Poorly aerated lung regions characterized by CT attenuations ranging between -100 and -500 HU are colored in light gray. Normally aerated lung regions characterized by CT attenuations ranging between -500 and -900 HU are colored in dark gray. At ZEEP, 30% of the lung is normally aerated. The loss of aeration predominates in lower lobes and there is a marked increase in lung tissue homogeneously distributed between upper and lower lobes: gas volume in upper lobes = 995 ml (Normal values = 1636 319 ml), gas volume inlower lobes = 212 ml (Normal values = 1391 367 ml), tissue volume in upper lobes = 1182 ml (Normal values = 461 68 ml), tissue volume in lower lobes = 1166 ml (Normal values = 482 89 ml).

Interestingly, in lung regions located caudally to the diaphragmatic cupola, compression atelectasis becomes predominant [20]. In deeply sedated patients lying supine, the diaphragm behaves as a passive structure that moves upward in the rib cage [25] and transmits to lower lobes the increased abdominal pressure resulting from abdominal surgery and/or abdominal trauma. It has to be outlined that the lung injury itself can increase abdominal pressure. It is well known, since the mid 1940s, that abdominal pain and distension can be the revealing signs of an acute lung injury [29]. In fact, caudal and dependent parts of the lungs are not only compressed by the abdominal content [25] but also by the heart [30] and the possible pleural effusion. In lung regions located beneath the heart, the loss of aeration is massive and significantly greater than in lung regions located outside the ventricles' limits [30]. Despite the lack of left ventricular failure, the heart is enlarged and heavier in ARDS patients compared to healthy volunteers [30]. Myocardial edema, hyperdynamic profile and pulmonary hypertension-induced

Lungs Ards Patients

Fig. 3. Six CT sections obtained at zero end-expiratory pressure (ZEEP) in a 73-year old patient with ARDS caused by a massive bronchopneumonia and characterized by an unevenly distributed loss of lung aeration. On the right side of each CT section, the corresponding lung aeration is represented using a color code included in the software Lungview. Nonaerated lung regions characterized by CT attenuations ranging between 0 and -100 Hounsfield units (HU) are colored in black. Poorly aerated lung regions characterized by CT attenuations ranging between -100 and -500 HU are colored in light gray. Normally aerated lung regions characterized by CT attenuations ranging between -500 and -900 HU are colored in dark gray. At ZEEP, the loss of aeration concerns exclusively the lower lobes where the increase in lung tissue is moderate: gas volume in lower lobes = 14 ml (Normal values = 1391 367 ml) and tissue volume in lower lobes = 634 ml (Normal values = 482 89 ml). In contrast, upper lobes remain normally aerated despite a marked increase in lung tissue: gas volume in upper lobes = 1629 ml (Normal values = 1636 319 ml) and tissue volume in upper lobes = 1053 ml (Normal values = 461 68 ml). The regional distribution of lung volumes clearly shows that the visual impression of'normal' upper lobes is misleading. The lack of apparent sternovertebral gradient of aeration in upper lobes despite a 128% increase in regional lung tissue does not fit the 'sponge' theory.

right ventricular dilation are potential mechanisms that may contribute to the increased cardiac mass and dimensions in ARDS patients. Finally, in the supine position, there is a nondependent to dependent decrease in regional aeration that is maximum in the juxta-diaphragmatic parts of the rib cage [20, 25]. Lung tissue structures including pulmonary vessels are squeezed by the different forces acting on caudal parts of the ribcage, a compression that likely limits the plasma leakage through the injured alveolar-capillary barrier and explains why compression atelectasis becomes predominant beyond the diaphragmatic cupola.

It was initially believed that the overall volume of the ARDS lung was preserved because the loss of gas was exactly compensated by the excess lung tissue [1]. This hypothesis, based on CT data obtained from single CT sections, has not been verified in the whole lung: multiple CT sections clearly demonstrate that the cephalocaudal dimensions of the ARDS lung are markedly reduced essentially at the expense of the lower lobes [25]. In fact, the ARDS lung is made of a combination of alveolar flooding, interstitial inflammation and atelectasis. In cephalic parts of the lung where external compressive forces are absent, alveolar flooding, when present, induces a massive loss of aeration with a conservation of the overall lung volume [16]. In contrast, in caudal parts of the lungs where external compressive forces abdominal content, cardiac mass, and pleural fluid effusion - are maximum in the supine position, a reduction of the overall lung volume of the lower lobes is frequently observed depending on the relative importance of alveolar flooding and compression atelectasis. This view of the ARDS lung has therapeutic consequences: re-establishment of lung aeration providing adequate gas exchange can be achieved not only by increasing intrathoracic pressure but also by relieving the external forces compressing lower lobes by adequate body positioning. At a given PEEP level, prone and semi-recumbent positions allow the recruitment of caudal lung regions by partially relieving heart and abdominal compressions [31, 32].

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