Electronic Repository to Monitoring the microcirculation in the critically ill patient: reflectance spectroscopy
Assumed that arterial oxygen saturation (SaO2) is available and that venous oxygen saturation equals µHbO2), regional oxygen consumption (rVO2) may be estimated by the following equation:
1)rVO2 = (SaO2- µHbO2) * Flow * const[$]
Considering the example given in figure 1 of De Backer`s review[1], only the flow was considered, but not the oxygen extraction rate, which of course will change with different flow levels (i.e. the lower the flow, the higher the oxygen extraction rate). Let us assume that area 1, 2 and 3 show the following values: Flow = 100 ml/min and µHbO2 = 60% in healthy tissue. In sepsis an increase in heterogeneity will occur: Area 1, hyperperfusion with Flow = 200 ml/min and µHbO2 = 80%; Area 2, hypoperfusion with Flow = 50 ml/min and µHbO2 = 20%; and Area 3, hypoxia with Flow = 10 ml/min and µHbO2 = 10%. Calculating regional oxygen consumption (rVO2) in the tissue gives the following results:
- for healthy areas: rVO2 = (100-60) * 100 = 4000
- for sepsis area 1 (hyperperfusion): rVO2 Area1 = (100-80) * 200 = 4000
- for sepsis area 2 (hypoperfusion): rVO2 Area2 = (100-20) * 50 = 4000 (i.e. the lower flow is compensated by a higher oxygen extraction)
- for sepsis area 3 (hypoxia): rVO2 Area3 = (100-10) * 10 = 900 (i.e. a very low flow cannot be compensated anymore so that rVO2 decreases)
If we compare rVO2 in area 1 and 2, we see that it is still the same as in healthy tissue. This shows that heterogeneity itself (without an absolute measure of flow) might not be the ideal indicator for sepsis within the microcirculation. If we then integrate areas 1, 2 and 3, we see that the total oxygen consumption of that region is in fact reduced, indicating a limitation in the oxygen supply of the tissue. It further illustrates that an integrated value (such as rVO2) must not be considered a draw back (as suggested in De Backer`s review[1]), rather than an advantage over microvideoscopic techniques, because an integrated value derived from a bigger catchment volume will be as sensitive to flow and metabolic heterogeneity (i.e. the presence of hypoxic areas) as an image but will have a higher relevance to the entire organ function due to the measured volume.
The distance between the illuminated glass fibre and the detecting glass fibre (so called separation) determines the possible measurement depth of the probes. The current variety of probes already offers detection depths in the range of approximately 150 μm (mucosa) up to 15 mm (transcutaneous skeletal muscle), for pre-, intra-, and post-operative use as well as for use in intensive care and experimental settings. The spatial resolution of the probe is the higher the smaller the separation of the probe is. For instance, with a probe of a small separation, very small areas can be measured and the heterogeneity within the tissue can be determined by slight movement of the probe (e.g. through peristaltic movements). The O2C provides therefore both probes with high spatial resolution and probes for measuring mean values in a rather big catchment volume (representative for a whole organ). In addition, the O2C technology provides further small flat probes allowing good probe application – continuously for various tissue types and detection depths (skin, muscle, liver, kidney, bone, tendon). The method has been used clinically to assess hepatic microcirculation[2], the microvascular response in patients with septic shock[3], during cardiopulmonary bypass in adults[4]and children[5], during gastric tube reconstruction[6], during extreme anaemia[7] as well as for flap viability monitoring[8] and to predict early transplant function[9] and wound healing in burn injury[10].
At present, three types of application the O2C have been described for intensive care medicine:
The easiest way to use the probe is to place it in the mouth between the dental arch and the cheek (buccal), e.g. by fixing it with an anatomically shaped probe holder with the surface of the probe pointing towards the cheek. Measurements in the mouth are minimally invasive and have the advantage of not being influenced by temperature, sympathetic or parasympathetic nervous regulation. The probe fixation is easy and almost free of pressure artefacts and can be performed continuously, also in patients having mild sedation only. Other applications include installing the probe into the stomach via a nasogastric tube or a suction catheter or placing the probe on the rectal or intestinal mucosa (e.g. via artificial stomata). The probes in stomach/gut can be placed without sight, only by the help of the control windows on the O2C screen. The probe shows haemoglobin spectra only if it is in contact with the mucosa.
References describing the first application of reflectance spectroscopy (EMPHO): it has been used in numerous experimental studies by our group looking at the effects of positive airway pressure [11, 12], hypercapnia [13, 14], vasoactive drugs[15-18] and anaesthetics[19, 20] as well as in clinical studies investigating the effects of intraabdominal pressure [21] and cardiopulmonary bypass[22] on the splanchnic microcirculation.
References:
1.De Backer D, Ospina-Tascon G, Salgado D, Favory R, Creteur J, Vincent JL, (2010) Monitoring the microcirculation in the critically ill patient: current methods and future approaches. Intensive Care Med 36:1813-1825
2.Ladurner R, Feilitzsch M, Steurer W, Coerper S, Konigsrainer A, Beckert S, (2009) The impact of a micro-lightguide spectrophotometer on the intraoperative assessment of hepatic microcirculation: a pilot study. Microvasc Res 77:387-388
3.Sakr Y, Gath V, Oishi J, Klinzing S, Simon TP, Reinhart K, Marx G, (2010) Characterization of buccal microvascular response in patients with septic shock. Eur J Anaesthesiol 27:388-394
4.Maier S, Hasibeder WR, Hengl C, Pajk W, Schwarz B, Margreiter J, Ulmer H, Engl J, Knotzer H, (2009) Effects of phenylephrine on the sublingual microcirculation during cardiopulmonary bypass. Br J Anaesth 102:485-491
5.Schindler E, Photiadis J, Lagudka S, Fink C, Hraska V, Asfour B, (2010) Influence of two perfusion strategies on oxygen metabolism in paediatric cardiac surgery. Evaluation of the high-flow, low-resistance technique. Eur J Cardiothorac Surg 37:651-657
6.Buise M, van Bommel J, Jahn A, Tran K, Tilanus H, Gommers D, (2006) Intravenous nitroglycerin does not preserve gastric microcirculation during gastric tube reconstruction: a randomized controlled trial. Crit Care 10:R131
7.Menzebach A, Mutz C, Scheeren TW, (2008) Microcirculatory monitoring of a Jehovah's Witness suffering from haemorrhagic shock. Eur J Anaesthesiol 25:81-83
8.Hölzle F, Rau A, Löffelbein DJ, Mücke T, Kesting MR, Wolff KD, (2010) Results of monitoring fasciocutaneous, myocutaneous, osteocutaneous and perforator flaps: 4-year experience with 166 cases. Int J Oral Maxillofac Surg 39:21-28
9.Fechner G, von Pezold J, Luzar O, Hauser S, Tolba RH, Muller SC, (2009) Modified spectrometry (O2C device) of intraoperative microperfusion predicts organ function after kidney transplantation: a pilot study. Transplant Proc 41:3575-3579
10.Merz KM, Pfau M, Blumenstock G, Tenenhaus M, Schaller HE, Rennekampff HO, (2010) Cutaneous microcirculatory assessment of the burn wound is associated with depth of injury and predicts healing time. Burns 36:477-482
11.Fournell A, Schwarte LA, Kindgen-Milles D, Müller E, Scheeren TWL, (2003) Assessment of microvascular oxygen saturation in gastric mucosa in volunteers breathing continuous positive airway pressure. Crit Care Med 31:1705-1710
12.Fournell A, Scheeren TWL, Schwarte LA, (1997) Oxygenation of the intestinal mucosa in anaesthetized dogs is attenuated by intermittent positive pressure ventilation (IPPV) with positive end-expiratory pressure (PEEP). Adv Exp Med Biol 428:385-389
13.Schwartges I, Picker O, Beck C, Scheeren TW, Schwarte LA, (2010) Hypercapnic Acidosis Preserves Gastric Mucosal Microvascular Oxygen Saturation in a Canine Model of Hemorrhage. Shock 34:636-642
14.Schwartges I, Schwarte LA, Fournell A, Scheeren TW, Picker O, (2008) Hypercapnia induces a concentration-dependent increase in gastric mucosal oxygenation in dogs. Intensive Care Med 34:1898-1906
15.Scheeren TWL, Schwarte LA, Loer SA, Picker O, Fournell A, (2002) Dopexamine but not dopamine increases gastric mucosal oxygenation during mechanical ventilation in dogs. Crit Care Med 30:881-887
16.Schwarte LA, Picker O, Schindler AW, Fournell A, Scheeren TWL, (2003) Fenoldopam-but not dopamine-selectively increases gastric mucosal oxygenation in dogs. Crit Care Med 31:1999-2005
17.Schwarte LA, Picker O, Schindler AW, Fournell A, Scheeren TWL, (2004) Dopamine under alpha1-blockade, but not dopamine alone or fenoldopam, increases depressed gastric mucosal oxygenation. Crit Care Med 32:150-156
18.Schwarte LA, Picker O, Bornstein SR, Fournell A, Scheeren TW, (2005) Levosimendan is superior to milrinone and dobutamine in selectively increasing microvascular gastric mucosal oxygenation in dogs. Crit Care Med 33:135-142
19.Schwarte LA, Picker O, Höhne C, Fournell A, Scheeren TWL, (2004) Effects of thoracic epidural anaesthesia on microvascular gastric mucosal oxygenation in physiological and compromised circulatory conditions in dogs. Br J Anaesth 93:552-559
20.Schwarte LA, Schwartges I, Schober P, Scheeren TW, Fournell A, Picker O, (2010) Sevoflurane and propofol anaesthesia differentially modulate the effects of epinephrine and norepinephrine on microcirculatory gastric mucosal oxygenation. Br J Anaesth 105:421-428
21.Schwarte LA, Scheeren TWL, Lorenz C, De Bruyne F, Fournell A, (2004) Moderate Increase in Intraabdominal Pressure Attenuates Gastric Mucosal Oxygen Saturation in Patients Undergoing Laparoscopy. Anesthesiology 100:1081-1087
22.Fournell A, Schwarte LA, Scheeren TWL, Kindgen-Milles D, Feindt P, Loer SA, (2002) Clinical evaluation of reflectance spectrophotometry for the measurement of gastric microvascular oxygen saturation in patients undergoing cardiopulmonary bypass. J Cardiothor Vasc Anesth 16:576-581
[$] the constant contains Hüfner's constant describing the theoretical oxygen carrying capacity of haemoglobin
for simplification const has been omitted in all calculations
for simplification units have been omitted in all calculations