Department of Anaesthesiology, Johannes Gutenberg-University, Mainz, Germany
* Corresponding author. E-mail: david{at}mail.uni-mainz.de
Accepted for publication May 27, 2004.
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Abstract |
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Keywords: complications, acute respiratory distress syndrome ; lung, extracorporeal assist ; ventilation, high-frequency oscillatory
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Introduction |
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A new supportive therapy for use in ARDS patients is the Interventional Lung Assist device (ILA, NovaLung®, Hechingen, Germany), which removes carbon dioxide from the blood. This system contains a specially designed low resistance lung membrane, which uses the pressure difference between the arterial and venous circulation as the driving force for blood flow. The extracorporeal blood flow is approximately 25% of the cardiac output. This system enables the use of high airway pressures for oxygenation in combination with very low tidal volumes to avoid ventilator-induced lung injury, and this gains time for lung recovery.
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Case report |
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The patient accidentally aspirated paraffin oil (40 ml) during a performance as a fire-eater. Immediately after aspiration, he experienced slight dyspnoea and chest pain during inspiration. By 8 h, the patient had severe dyspnoea and the chest X-ray demonstrated pulmonary infiltration in both lower lobes and the right middle lobe. On examination, chest auscultation revealed bilateral dry rales over the lower chest, core body temperature was 39.3°C, invasive arterial pressure was 93/30 mm Hg, heart rate 136 min1 in sinus rhythm, and central venous pressure 16 mm Hg. Computed tomography of the lung at that time showed bilateral lower lobe consolidation, atelectasis, and infiltration in the right middle lobe. The was 6.9 kPa. The trachea was intubated and the lungs were ventilated mechanically using a PCV mode. Bronchoscopy revealed solitary blood patches in the central bronchial system and brownish secretion in both lower lobes. Diagnostic bronchoalveolar lavage (40 ml of normal saline) produced dark debris, which smelled of paraffin oil. Despite increased positive end-expiratory pressure (PEEP; 20 cm H2O), mean airway pressure (Pmean; 27 cm H2O), and repeated recruiting manoeuvres (continuous positive airway pressure level of 45 cm H2O for at least 30 s), oxygenation did not improve over 2 h. The
(kPa)/
ratio was 10.7 kPa, oxygenation index (Pmeanx
x100/
(mm Hg)) 32, and hypercapnia (
12 kPa, arterial pH 7.20, arterial base excess 4.0 mmol litre1) was evident.
HFOV (SensorMedics 3100B, Yorba Linda, CA, USA) was started, which included an initial lung recruitment strategy. The first setting was an increase of the mean airway pressure of 5 cm H2O compared with the last measured mean airway pressure during PCV, an oscillatory frequency of 5 Hz, an inspiratory time of 33% of the respiratory cycle, a flow bias of 30 litre min1, and a of 1.0. The arterial oxygen tension and the arterial carbon dioxide tension were monitored continuously with an in-line arterial multiparameter sensor (Partrend 7, Diametrics Medical Ltd, England). The oscillating pressure amplitude was scaled to the online measurement of
. With HFOV, lung volume was then recruited by a stepwise increase (2 cm H2O) of the adjusted mean airway pressure up to a maximum of 40 cm H2O. After each step,
and its trend were analyzed, and mean airway pressure was increased as long as
increased. If
decreased when increasing the mean airway pressure, we also analyzed the trend in
. Any decrease in
without an initial positive trend was interpreted as no further lung recruitment and over-distension of open lung units. Mean airway pressure was then reduced. After achieving maximum recruitment using this strategy, the lowest possible mean airway pressure was selected that would keep the lung open. This was determined by stepwise reductions in mean airway pressure to the point where collapse of alveolar units became evident from a decrease in
. Mean airway pressure was then set at 23 cm H2O above this pressure. Oxygenation improved during HFOV within 1 h (
ratio 22.9 kPa; oxygenation index 17) and remained stable thereafter (Fig. 1).
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We decided to establish the ILA for carbon dioxide removal to offer less aggressive ventilation with HFOV (high mean airway pressures with lower oscillatory pressure amplitudes and high oscillatory frequencies). We used the left femoral artery and the right femoral vein (Seldinger technique) for insertion of a 17 French gauge arterial and 19 French gauge venous cannula. After insertion, the pre-filled (isotonic saline) ILA was connected; initial passive blood flow was 2.1 litre min1, and gas flow (oxygen) in the membrane lung was 12 litre min1. Because of the heparin bonded system, systemic anticoagulation with heparin was targeted only to an activated clotting time of 120140 s. The patient tested negative for heparin-induced thrombocytopenia (HIT II). After an additional recruitment, HFOV was maintained with a mean airway pressure of 32 cm H2O, whereas oscillatory frequency was increased (from 3.5 to 9 Hz). Oscillatory amplitude was decreased from 95 to 25 cm H2O to reduce tidal volumes. Immediately after ILA initiation, fell from 13.3 (arterial pH 7.14, arterial base excess 5 mmol litre1) to 9.3 kPa (arterial pH 7.22, arterial base excess 2 mmol litre1) and returned to normal within 4 h. The
ratio was unchanged after ILA started and remained higher than 20 kPa (Fig. 1) with the HFOV setting used. Blood flow through the extracorporeal system was measured by ultrasound (Blood Flow Monitoring System, NovaLung®, Hechingen, Germany). Mean arteriovenous shunting with the ILA during treatment was 24 (4)% of the cardiac output, measured by a thermodilution technique. The targeted mean arterial pressure was 70 mm Hg and the minimal cardiac index to ensure sufficient blood flow through the membrane lung was 2.5 litre min1 m2. Norepinephrine was only used for 13 h (maximum dosage: 1.5 µg kg1 min1) after starting ILA to reach this index. Figure 2 shows the measured blood flow and adjusted gas flow (oxygen) during ILA treatment.
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We observed no ILA related adverse events during treatment despite reduced systemic anticoagulation for percutaneous tracheostomy on day 12. Weaning from ventilation was interrupted by nosocomial pneumonia (Staphylococcus aureus and Stenotrophomonas maltophilia) on day 20, which resolved. After 43 days, the tracheostomy was removed. The patient suffered from acute renal failure on admission and required renal replacement therapy for 30 days. The patient was discharged from hospital on day 52 for rehabilitation.
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Discussion |
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The effect of HFOV on oxygenation and its safety in patients with ARDS has been documented in several studies.3 4 The HFOV allowed application of a constant high mean airway pressure, and avoided high cyclic inspiratory pressures as well as end-expiratory lung collapse with reopening on inspiration. If hypercapnia is evident with HFOV, reduced oscillatory frequencies, increased oscillatory amplitudes, and increased bias flow are used to decrease it. However, permanent administration of oscillatory frequencies below 4 Hz in combination with high oscillating amplitudes results in higher tidal volumes, which cancels the benefit of the HFOV treatment. So called permissive hypercapnia is one element of accepted protective lung ventilatory strategies, but only if the target primarily is to avoid end-expiratory lung collapse and to reduce alveolar stretch with low tidal volumes. No clear evidence demonstrates that moderate respiratory acidosis worsens the condition of critical ill patients, and the safe level of hypercapnia and respiratory acidosis remains unknown. We used the new pumpless ILA for carbon dioxide removal in a clinical situation where oxygenation improved only with high mean airway pressures, whereas adjusted ventilator settings were insufficient to control the resulting hypercapnic acidosis. The ILA was only supportive therapy to enable less aggressive ventilator settings and not a ventilator replacement. The application of high airway pressures to maintain oxygenation during ILA is possible with either conventional PCV modes or with HFOV. We continued HFOV during ILA therapy because of the improvement in the ratio after starting it, but used high oscillatory frequencies and low oscillatory amplitudes to achieve very low tidal volumes.
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References |
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