DESCRIPCION DE UNA ILUSTRACIÓN CIENTÍFICA

The Institute of Liver studies, King’s College Hospital, Denmark Hill, London, UK Introduction: Percutaneous tracheostomy (PT) is recog-

nised as a safe and effective method for prolonged ventila- tory support, avoiding the complications of lengthy endotracheal intubation. Bleeding has been reported as the most common perioperative complication in most series [1]. Patients with severe liver disease frequently have a coagulopathy and thrombocytopaenia. We wished to assess the safety of PT in this group of patients, who are at high risk of haemorrhagic complications.

Methods: A retrospective study of PT’s performed in patients admitted to a dedicated liver intensive care unit in a supraregional liver centre over two years. All procedures followed the Ciaglia technique [2] and were performed by, or under the supervision of, experienced operators. All patients had liver disease, which included decompensated chronic disease, hyperacute and acute liver failure and liver transplantation with severe postoperative complica- tions.

Patients considered to be at particularly high risk for bleeding, based on platelet count (Plts) and international normalised ratio (INR) received transfusions of platelets and/or fresh frozen plasma prior to the procedure.

Results: The median age was 46.5 years with 20 males and 14 females. Five of the thirty-four patients suffered complications, none of which were fatal, and included minor bleeding only (requiring no treatment). The Mann- Whitney U test was used to compare differences in INR and Plts between those with and without complications. The median pre-correction INR for the group without com- plications was 1.1 (0.82–1.79) and for those with compli- cations was 1.34 (0.92–1.76) and this did not reach statistical significance. There was a trend towards lower Plts in those with a complication (median 44; range 21–84) compared to those without (median 76; range 3–510) but again this was not statistically significant (P<0.3).

Conclusions: Although this is a small study, based on the data presented, PT appears to be a safe procedure in this group of patients, at high risk of bleeding, when standard precautions are followed.

References:

1. Petros S: Percutaneous tracheostomy.Crit care1999, 3:R5–R10. 2. Ciaglia P, Firsching R, Syniec C: Elective percutaneous dilatational

P107

In vitro study of a new vaporization humidifying device: DAR HC 2000

1st. Anestesia e Rianimazione, Osp. Maggiore Policlinico-IRCCS, via F. Sforza, 35, 20122, Milan, Italy; *Dip. di Scienze Cliniche e Biologiche, Univ. dell’Insubria, Varese, Italy

In long-term mechanical ventilation, inspired gas should reach the endotracheal tube with a temperature of 30–32°C, and an absolute humidity (H_A) of approximately 28–30 mg H₂O/l.

We tested a humidifier (DAR HC 2000®_{) composed of: 1)} a vaporising cartridge, containing a hydrophobic mem- brane that separates the gas flow from the heated water; 2) a heating wire that runs all along the inspiratory line, maintaining the heat and moisture conditions of the inspired gas from the cartridge to the patient; 3) a double temperature control that permits the setting of the desired heating level both at the cartridge (T°c) and at the patien- t’s mouth (T°p).

At first, we studied the system’s ability to condition inspired gas at different min ventilation (V_E=6, 10, 14 l/min), tidal volume (V_T=250, 500, 750 ml), and inspiratory flow (V_i=0.5, 1 l/sec) with different settings of T°c and T°p (32–32, 34–34, 36–36, 34–32, 36–32, 36–34, 38–34, 39–34°C, respectively). We found that increasing V_Efrom 6 to 10 to 14 l/min led to a progressive decrease of H_A from 33.0±4.4 to 27.5±4.4 to 23.4±3.6 mg/l (P<0.01), while V_Tand V_ihad no effects on humidification.

Secondly, we tested the humidifier at different V_E (from 4 to 16 l/min, 2 l/min per step) with different T°p (32, 34, 36°C; T°c initially set at the same level as T°p, and then increased at 1°C at a time until condensate appeared in the thermoregulated tube). Adequate humidification levels (H_A=30 mg/l) can be obtained for each V_E and T°p by increasing the temperature set at the cartridge enough.

The third part of the study was to verify the decrease of microbiological contamination by simulating a contami- nated patient (using a reservoir containing Micrococcus luteus) and analysing the inspiratory line with the heating wire switched first OFF, then ON: keeping the thermoreg- ulated tube OFF, as supposed, a bacterial growth was seen in the inspiratory line, while switching it ON, no M. luteuscolonies were observable.

In conclusion, DAR HC 2000® _{humidifier allows us to} obtain adequate levels of humidity of inspired gas for each ventilatory set, just by increasing the cartridge tempera- ture. Besides, the hydrophobic membrane and the heating wire avoid, respectively, the gas–water contact and the condensate formation in the inspiratory line resulting in a reduction of microbiological contamination of the circuit.

P108

Non-invasive two-point estimation of arterial PCO

and alveolar deadspace

*Emma Children’s Hospital, PICU, and †_{Department of Anaesthesiology, Academic Medical Centre, University of Amsterdam,}

Meibergdreef 9, NL-1105 AZ Amsterdam, The Netherlands Introduction: Often end-tidal PCO₂(etPCO₂) gives a good estimate of arterial PCO₂(PaCO₂). However, there are con- ditions, namely diseases with pulmonary ventilation/perfu- sion mismatch, where PaCO₂is severely underestimated.

Objective: We aimed to develop a method for the accu- rate non-invasive estimation of PaCO₂from etPCO₂.

Method: The method is based on the model assumption that etPCO₂is a mixture of PCO₂from both the ventilated and perfused alveoli, and the ventilated but non-perfused ones (‘alveolar deadspace’). Knowing the etPCO₂ under conditions of inspiratory gases with different CO₂content (PCO₂_a; PCO₂_b), it is possible to calculate the PCO₂ of the ventilated and perfused alveoli (PCO₂alv). Knowl- edge of PCO₂alv allows to calculate the fraction of alveo- lar deadspace ventilation. As long as para-alveolar (i.e. cardiac) shunting is less than 20%, PCO₂alv accurately reflects PaCO₂.

(etPCO₂_a • PCO₂_b)–(etPCO₂_b • PCO₂_a) PCO₂alv =

PCO2_b–etPCO2_b–PCO2_a + etPCO2_a

The model was applied on ventilated adult patients while inpiring different levels of PCO₂. Estimates of PaCO₂and alveolar deadspace were compared to the results of inva- sive determination.

Results: Preliminary results indicate a good correlation between invasively and non-invasively determined PaCO₂ and alveolar deadspace (Table 1).

Conclusions: Using gas void of CO₂ as one of the two inspiratory gases further simplifies the formula. Our prelim- inary findings show that a difference in the levels of inspired PCO₂ of at least 3 kPa gives an estimate of PaCO₂with an accuracy of 10% or better. We provide a non-invasive method for the accurate estimation of PaCO₂

and alveolar deadspace ventilation. We suggest its imple- mentation in ventilators for the close monitoring of pul- monary treatment response.

Table 1

PaCO₂(kPa) Vd/Vt

PCO2_a etPCO2_a PCO2_b etPCO2_b

No. (kPa) (kPa) (kPa) (kPa) calc. measured difference (%) calc. measured

1 0 5.3 2.9 6.0 6.97 7.10 -1.8 0.32 0.34 2 0 5.3 5.0 6.6 7.16 7.10 +0.9 0.35 0.34 3 0 5.5 4.0 5.2 5.12 5.61 -8.8 -0.07 0.02 4 0 5.5 4.4 5.4 5.38 5.61 -4.1 -0.02 0.02 5 0 5.2 2.0 5.6 6.50 6.89 -5.7 0.25 0.33 6 0 4.1 1.3 4.4 5.36 5.98 -10.4 0.31 0.46 7 0 4.1 3.2 4.9 5.51 5.98 -7.8 0.34 0.46 8 0 4.1 4.4 5.6 6.22 5.98 +4.0 0.52 0.46

P109

Hemodynamic effects of the inspiratory flow rate in patients with septic shock

ICU, Nikaia-Piraeus General State Hospital, Fanarioton Street, Nikaia, Greece Purpose: The purpose of the study was to assess the

effects of changes in inspiratory flow (IF) on hemodynamic parameters of mechanically-ventilated patients with septic shock.

Method: Eight patients with septic shock (pneumonia n=2 and abdominal sepsis n=6) were included in the study. A pulmonary artery catheter with continuous SvO₂ and CO display had been inserted to titrate fluid and vasopressor therapy. The patients were on volume control ventilation with a square flow waveform. Three different levels of IF were applied, each for a period of thirty min (IF₁=40l/min, IF₂=50l/min, IF₃=60l/min). At the end of each period the following parameters were recorded: CVP, PCWP, MAP, MPAP, CI, SVRI, PVRI,

LVSWI, RVSWI, SaO₂, SvO_2, DO₂. The paired t-test method was used to compare the results at the three dif- ferent levels of IF.

Results: Two patients were withdrawn from the study, due to necessary manipulation in the infusion rate of fluids and/or vasopressors. In six patients, a significant increase in LVSWI from IF₁to IF₃(5.22±1.38 g._m/m2_{[mean ±SE],} P=0.013) was observed. No further significant differences were recognized.

Conclusion: The change in the IF from 40 to 60l/min did not have any negative effect in the hemodynamic profile of patients with septic shock. On the contrary, it led to an increase of the LVSWI.

P110

Automatic tube compensation combined with pressure support ventilation – improved work of