The transcutaneous oxygen challenge test: a noninvasive method for detecting low cardiac output in septic patients

Transcutaneous oxygen pressure-related variables as noninvasive indicators of low lactate clearance in sepsis patients after resuscitation

The transcutaneous oxygen challenge test (OCT) is associated with central venous oxygen saturation and cardiac output index, and has predictive value for prognosis. Whether the change of transcutaneous oxygen pressure (P_tcO₂)-related variables can reflect lactate clearance in sepsis patients is worth studying. We conducted a prospective observational study of 79 patients with sepsis or septic shock in the ICU. Immediately after enrollment, P_tcO₂ monitoring was continuously performed for 6 h. The OCT was performed at enrollment (T0) and the sixth hour (T6). The correlation between lactate clearance and P_tcO₂-related variables such as P_tcO₂ at T6, ΔP_tcO₂ (P_tcO₂ at T6 - P_tcO₂ at T0), ΔP_tcO₂ index (P_tcO₂/PaO₂ at T6 - P_tcO₂/PaO₂ at T0), 10 OCT [(P_tcO₂ after 10 min on FiO₂ of 1.0) - (P_tcO₂ at baseline)], Δ10 OCT (10 OCT at T6 - 10 OCT at T0) was analyzed. The difference of P_tcO₂-related variables was compared between the high and low lactate clearance groups. P_tcO₂ at T6 (r = 0.477, p < 0.001), ΔP_tcO₂ (r = 0.592, p < 0.001), ΔP_tcO₂ index (r = 0.553, p < 0.001) and Δ10 OCT (r = 0.379, p = 0.001) were significantly correlated with the lactate clearance. To discriminate low lactate clearance, the area under the ROC curve was largest for ΔP_tcO₂, which was 0.804. P_tcO₂ at T6, P_tcO₂ index, ΔP_tcO₂, ΔP_tcO₂ index and Δ10 OCT were significantly different between the two different lactate clearance groups. Low lactate clearance in the initial 6 h of resuscitation of septic shock was associated to lower improvements in P_tcO₂-related variables.

Perfusion indices revisited

Monitoring of tissue perfusion is an essential step in the management of acute circulatory failure. The presence of cellular dysfunction has been a basic component of shock definition even in the absence of hypotension. Monitoring of tissue perfusion includes biomarkers of global tissue perfusion and measures for assessment of perfusion in non-vital organs. The presence of poor tissue perfusion in a shocked patient is usually associated with worse outcome. Persistently impaired perfusion despite adequate resuscitation is also associated with worse outcome. Thus, normalization of some perfusion indices has become one of the resuscitation targets in patients with septic shock. Although the collective evidence shows the clear relation between impaired peripheral perfusion and mortality, the use of different perfusion indices as a resuscitation target needs more research.

Clinical classification of tissue perfusion based on the central venous oxygen saturation and the peripheral perfusion index

Introduction

We investigated whether combining the peripheral perfusion index (PI) and central venous oxygen saturation(ScvO₂) would identify subsets of patients for assessing the tissue perfusion and predicting outcome during the resuscitation in critically ill patients.

Methods

A total of 202 patients with central venous catheters for resuscitation were enrolled in this prospective observational study. The arterial, central venous blood gas and the PI were measured simultaneously at the enrollment (T0) and 8 h (T8) after early resuscitation. Based on the distribution of the PI in healthy population, a cutoff of PI ≥1.4 was defined as a normal PI. Moreover, the critical value of PI was defined as the best cutoff value related to the mortality in the study population. The PI impairment stratification is defined as follows: a normal PI(≥1.4), mild PI impairment (critical value < PI < 1.4) and critical PI impairment (PI ≤ critical value).

Results

The PI at T8 was with the greatest AUC for prediction the 30-day mortality and PI is an independent risk factor for 30-day mortality. Moreover, a cutoff of PI < 0.6 is related to poor outcomes following resuscitation. So, based on cutoffs of ScvO₂ (70 %) and critical PI (0.6) at T8, we assigned the patients to four categories: group 1 (PI ≤ 0.6 on ScvO₂ < 70 %), group 2 (PI ≤ 0.6 on ScvO₂ ≥ 70 %), group 3 (PI > 0.6 on ScvO₂ < 70 %), and group 4 (PI > 0.6 on ScvO₂ ≥ 70 %). The combination of low ScvO₂(<70 %) and PI(≤0.6) was associated with the lowest survival rates at 30 days [log rank (Mantel–Cox) = 87.518, p < 0.0001]. The sub-group patients who had high ScvO₂(>80 %) at T8 were with low mortality and high PI. Moreover, the normal PI (≥1.4) did not show a better outcome than mild impaired PI (0.6-1.4) patients who had a normalized ScvO₂(>70 %) after resuscitation. The PI was correlated with the lactate, P(v-a)CO₂, and ScvO₂ in all the measurements (n = 404). These relationships are strengthened with abnormal PI (PI < 1.4) but not with normal PI (PI ≥ 1.4).

Conclusion

Complementing ScvO₂ assessment with PI can better identify endpoints of resuscitation and adverse outcomes. Pursuing a normalized PI (≥1.4) may not result in better outcomes for a mild impaired PI after ScvO₂ is normalized.

Electronic supplementary material

The online version of this article (doi:10.1186/s13054-015-1057-8) contains supplementary material, which is available to authorized users.

Regarding the quantification of peripheral microcirculation--Comparing responses evoked in the in vivo human lower limb by postural changes, suprasystolic occlusion and oxygen breathing

The human skin is an interesting model to explore microcirculation, particularly if using noninvasive technologies such as LDF (Laser Doppler Flowmetry) and tc (transcutaneous) gasimetry and methods as near as possible from the normal physiological state. In this study, we combined those technologies with three classical approaches--leg raising from supine, suprasystolic occlusion (in the ankle), and normobaric oxygen breathing to explore distal peripheral circulation in the foot. These methods are often cited, but a comparative assessment has not been done. The goal of this study was to identify relevant flow related descriptors, method-related advantages and pitfalls, and eventually, to find the best experimental approach. Volunteers (both genders, 22.1 ± 3.7 years old) were subjected to these methods and variables registered during basal, challenge and stabilization phases. Descriptive and comparative statistics were obtained, adopting a 95% confidence level. All flow-related quantitative descriptors potentially useful for the analysis were identified and compared. As expected, male patients consistently showed higher LDF levels and transepidermal water loss (TEWL) and lower tcpO2 values. However, lower results were recorded in the supine position, suggesting a postural dependence. Both leg raising and suprasystolic occlusion produced a hyperemic response after provocation, although different in magnitude, significantly reducing LDF and tcpO2 during provocation. The oxygen breathing method provided the most patient-friendly protocol, consistently reducing LDF (potentially by the inhibition of production of local vasodilators). TEWL increased during the provocation phase in all protocols, although not significantly. Baseline tcpO2 was found to correlate positively with the peak tcpO2 during oxygen breathing and basal LDF with peak flow during leg raising and suprasystolic occlusion. No statistical correlation between TEWL and LDF could be demonstrated under the current experimental conditions. We conclude that although equally useful considering the purpose, these methods involve very different practicalities and do not provide the same information. Also noteworthy, LDF is a highly sensitive indicator that could be further explored to look deeper into blood flow regulating mechanisms.

Influence of arterial dissolved oxygen level on venous oxygen saturation: don't forget the PaO2!

Dissolved oxygen (i.e., unbound to hemoglobin) is often neglected as a determinant of central venous oxygen saturation (ScvO2) in review articles and textbooks. These statements may lead to potential misinterpretation of SCvO2 value across FiO2 changes. In this study, we aimed to explore the influence of PaO2 and FiO2 on ScvO2 in ventilated critically ill patients. This was a prospective observational study in two surgical intensive care units. Mechanically ventilated and sedated patients with cardiac output and ScvO2 monitoring and PaO2/FiO2 > 200 with inspiratory oxygen (FiO2) ≤ 0.5 were enrolled (cohort [ScvO2]). A second cohort of brain-injured patients with jugular venous oxygen saturation monitoring was studied to assess the application of the results to regional circulation (cohort [SjvO2]). Central venous oxygen saturation was measured at baseline FiO2 and at FiO2 = 1. We finally estimated the participation of the dissolved oxygen (PadissolvO2) to the ScvO2 variations. Twenty patients formed the cohort ScvO2 and eight formed the cohort SjvO2. Central venous oxygen saturation rose from 71% (69%-76%) to 84% (78%-88%) after increasing FiO2, whereas PaO2 rose from 100 (85-124) mmHg to 387 (360-449) mmHg. The rise of ScvO2 was mostly ascribable to the dissolved oxygen. The increase of ScvO2 was not explained by changes in cardiac output or hemoglobin levels. Jugular venous oxygen saturation rose from 71% (58%-78%) to 83% (78%-89%) after increasing FiO2. Arterial dissolved oxygen level can significantly influence the ScvO2 value. Therefore, PaO2 should not be overlooked while considering the ScvO2 value as a therapeutic goal. Interpretation of ScvO2 variations in response to a therapeutic challenge (i.e., fluid challenge, inotropic drug initiation) should be performed at constant FiO2.

The peripheral perfusion index and transcutaneous oxygen challenge test are predictive of mortality in septic patients after resuscitation

Introduction

The peripheral perfusion index (PI) is a noninvasive numerical value of peripheral perfusion, and the transcutaneous oxygen challenge test (OCT) is defined as the degree of transcutaneous partial pressure of oxygen (PtcO₂) response to 1.0 FiO_2. The value of noninvasive monitoring peripheral perfusion to predict outcome remains to be established in septic patients after resuscitation. Moreover, the prognostic value of PI has not been investigated in septic patients.

Methods

Forty-six septic patients, who were receiving PiCCO-Plus cardiac output monitoring, were included in the study group. Twenty stable postoperative patients were studied as a control group. All the patients inspired 1.0 of FiO₂ for 10 minutes during the OCT. Global hemodynamic variables, traditional metabolic variables, PI and OCT related-variables were measured simultaneously at 24 hours after PiCCO catheter insertion. We obtained the 10min-OCT ((PtcO₂ after 10 minutes on inspired 1.0 oxygen) - (baseline PtcO₂)), and the oxygen challenge index ((10min-OCT)/(PaO₂ on inspired 1.0 oxygen - baseline PaO₂)) during the OCT.

Results

The PI was significantly correlated with baseline PtcO₂, 10min-OCT and oxygen challenge index (OCI) in all the patients. The control group had a higher baseline PtcO₂, 10min-OCT and PI than the septic shock group. In the sepsis group, the macro hemodynamic parameters and ScvO₂ showed no differences between survivors and nonsurvivors. The nonsurvivors had a significantly lower PI, 10min-OCT and OCI, and higher arterial lactate level. The PI, 10min-OCT and OCI predicted the ICU mortality with an accuracy that was similar to arterial lactate level. A PI <0.2 and a 10min-OCT <66mmHg were related to poor outcome after resuscitation.

Conclusions

The PI and OCT are predictive of mortality for septic patients after resuscitation. Further investigations are required to determine whether the correction of an impaired level of peripheral perfusion may improve the outcome of septic shock patients.