The influence of intra-aortic balloon counter pulsation on central venous blood oxygen saturation

OBJECTIVES

We aim at identifying the changes in venous blood saturation values that associates intra-aortic balloon pump (IABP) in cardiac surgery patients with reduced left ventricular function (LVF).

METHODS

A retrospective observational study was conducted in a cardiothoracic intensive care unit (CTICU) in a tertiary cardiac center over 5 years in Qatar. A total of 114 patients with at least moderate impairment of LVF with ejection fraction (EF) less than 40% were enrolled. According to the association of IABP, patients were segregated into two groups with and without IABP (groups 1, 40 patients and group 2, 74 patients). Sequential arterial and venous blood gases were analyzed. The primary outcome was to analyze the changes in the central venous saturation (ScvO₂) in both groups and the secondary outcome was to analyze whether these changes affect the overall outcome in terms of intensive care unit (ICU) length of stay.

RESULTS

There was no significant difference between both groups with regard to age, preoperative EF, hemoglobin, and arterial oxygen saturation (SaO₂) in blood gases. Patients with IABP have a higher cScvO₂ when compared to the other group (71.5 ± 12.5 vs 63.5 ± 9.3, 68.3 ± 12.6 vs 60.1 ± 9.5, 62.7 ± 10.8 vs 55.63 ± 8.1, and 60.6 ± 7.6 vs 54.9 ± 8.1; p = 0.04, 0.05, 0.03, and 0.5, respectively). However, generalized estimating equations (GEE) analysis showed that compared with the participants showing that there is a decreasing trend in mean levels within the groups during follow-ups, overall difference between both groups' mean levels was not statistically significant.

CONCLUSIONS

In this study, we observed that after cardiac surgeries, patients with IABP had non-significant higher ScvO₂ when compared with a corresponding group with moderate impairment of LVF. Further prospective studies are required to validate these findings.

Cerebral metabolic rate of oxygen during transition from wakefulness to sleep measured with high temporal resolution OxFlow MRI with concurrent EEG

During slow-wave sleep, synaptic transmissions are reduced with a concomitant reduction in brain energy consumption. We used 3 Tesla MRI to noninvasively quantify changes in the cerebral metabolic rate of O2 (CMRO2) during wakefulness and sleep, leveraging the 'OxFlow' method, which provides venous O2 saturation (SvO2) along with cerebral blood flow (CBF). Twelve healthy subjects (31.3 ± 5.6 years, eight males) underwent 45-60 min of continuous scanning during wakefulness and sleep, yielding one image set every 3.4 s. Concurrent electroencephalography (EEG) data were available in eight subjects. Mean values of the metabolic parameters measured during wakefulness were stable, with coefficients of variation below 7% (average values: CMRO2 = 118 ± 12 µmol O2/min/100 g, SvO2 = 67.0 ± 3.7% HbO2, CBF = 50.6 ±4.3 ml/min/100 g). During sleep, on average, CMRO2 decreased 21% (range: 14%-32%; average nadir = 98 ± 16 µmol O2/min/100 g), while EEG slow-wave activity, expressed in terms of δ -power, increased commensurately. Following sleep onset, CMRO2 was found to correlate negatively with relative δ -power (r = -0.6 to -0.8, P < 0.005), and positively with heart rate (r = 0.5 to 0.8, P < 0.0005). The data demonstrate that OxFlow MRI can noninvasively measure dynamic changes in cerebral metabolism associated with sleep, which should open new opportunities to study sleep physiology in health and disease.

Optimization of quantitative susceptibility mapping for regional estimation of oxygen extraction fraction in the brain

PURPOSE

We sought to determine the degree to which oxygen extraction fraction (OEF) estimated using quantitative susceptibility mapping (QSM) depends on two critical acquisition parameters that have a significant impact on acquisition time: voxel size and final echo time.

METHODS

Four healthy volunteers were imaged using a range of isotropic voxel sizes and final echo times. The 0.7 mm data were downsampled at different stages of QSM processing by a factor of 2 (to 1.4 mm), 3 (2.1 mm), or 4 (2.8 mm) to determine the impact of voxel size on each analysis step. OEF was estimated from 11 veins of varying diameter. Inter- and intra-session repeatability were estimated for the optimal protocol by repeat scanning in 10 participants.

RESULTS

Final echo time was found to have no significant effect on OEF. The effect of voxel size was significant, with larger voxel sizes underestimating OEF, depending on the proximity of the vein to the superficial surface of the brain and on vein diameter. The last analysis step of estimating vein OEF values from susceptibility images had the largest dependency on voxel size. Inter-session coefficients of variation on OEF estimates of between 5.2% and 8.7% are reported, depending on the vein.

CONCLUSION

QSM acquisition times can be minimized by reducing the final echo time but an isotropic voxel size no larger than 1 mm is needed to accurately estimate OEF in most medium/large veins in the brain. Such acquisitions can be achieved in under 4 min.

Calibration of T2 oximetry MRI for subjects with sickle cell disease

PURPOSE

Cerebral T₂ oximetry is a non-invasive imaging method to measure blood T₂ and cerebral venous oxygenation. Measured T₂ values are converted to oximetry estimates using carefully validated and potentially disease-specific calibrations. In sickle cell disease, red blood cells have abnormal cell shape and membrane properties that alter T₂ oximetry calibration relationships in clinically meaningful ways. Previous in vitro works by two independent groups established potentially competing calibration models.

METHODS

This study analyzed pooled datasets from these two studies to establish a unified and more robust sickle-specific calibration to serve as a reference standard in the field.

RESULTS

Even though the combined calibration did not demonstrate statistical superiority compared to previous models, the calibration was unbiased compared to blood-gas co-oximetry and yielded limits of agreement of (-10.1%, 11.6%) in non-transfused subjects with sickle cell disease. In transfused patients, this study proposed a simple correction method based on individual hemoglobin S percentage that demonstrated reduced bias in saturation measurement compared to previous uncorrected sickle calibrations.

CONCLUSION

The combined calibration is based on a larger range of hematocrit, providing greater confidence in the hematocrit-dependent model parameters, and yielded unbiased estimates to blood-gas co-oximetry measurements from both sites. Additionally, this work also demonstrated the need to correct for transfusion in T₂ oximetry measurements for hyper-transfused sickle cell disease patients and proposes a correction method based on patient-specific hemoglobin S concentration.

Venous-arterial CO2 difference in children with sepsis and its correlation with myocardial dysfunction

Objective: This study aimed to determine the association between venous–arterial CO₂ difference (Pv-aCO₂) and clinical outcomes of interest in children with severe sepsis and septic shock. Design: An analytical observational study of a prospective cohort was conducted. Setting: The study was carried out from January 2015 to January 2018 in the pediatric intensive care unit of a referral hospital. Materials and methods: Of a total of 1159 patients who were admitted to pediatric critical care, 375 had severe sepsis and septic shock, of which 67 fulfilled the inclusion criteria. Arterial and venous gases were drawn simultaneously with a transthoracic echocardiogram, Pv-aCO₂, and other measures of tissue perfusion such as arterial lactate, venous, and evolution to multiple organ failure. Measurements and main results: Half (53.7%) of the patients were under 24 months old, with a slight predominance of male patients. The main site of infection was the lungs in 56% of the cases, with a 91.2% survival rate. Patients who died had a higher venous lactate level (interquartile range 16.2–33.6, p = 0.02). However, there was no correlation between myocardial dysfunction seen on echocardiogram and a Pv-aCO₂ greater than 6 mm Hg in children with severe sepsis and septic shock (r = 0.13). Pv-aCO₂ and central venous saturation had low sensitivity to detect multiple organ failure and poor correlation with the number of compromised systems (r = 0.8). Conclusion: Pv-aCO₂ was not associated with myocardial dysfunction, measured by echocardiogram, in children with severe sepsis and septic shock. It also did not correlate with the number of organs involved or mortality.

Optical oximetry of volume-oscillating vascular compartments: contributions from oscillatory blood flow

We present a quantitative analysis of dynamic diffuse optical measurements to obtain oxygen saturation of hemoglobin in volume oscillating compartments. We used a phasor representation of oscillatory hemodynamics at the heart rate and respiration frequency to separate the oscillations of tissue concentrations of oxyhemoglobin (O) and deoxyhemoglobin (D) into components due to blood volume (subscript V V ) and blood flow (subscript F F ): O=O V +O F O=OV+OF , D=D V +D F D=DV+DF . This is achieved by setting the phase angle Arg(O F )−Arg(O) Arg(OF)−Arg(O) , which can be estimated by a hemodynamic model that we recently developed. We found this angle to be −72  deg −72  deg for the cardiac pulsation at 1 Hz, and −7  deg −7  deg for paced breathing at 0.1 Hz. Setting this angle, we can obtain the oxygen saturation of hemoglobin of the volume-oscillating vascular compartment, S V =|O V |/(|O V |+|D V |) SV=|OV|/(|OV|+|DV|) . We demonstrate this approach with cerebral near-infrared spectroscopy measurements on healthy volunteers at rest (n=4 n=4 ) and during 0.1 Hz paced breathing (n=3 n=3 ) with a 24-channel system. Rest data at the cardiac frequency were used to calculate the arterial saturation, S (a) S(a) ; over all subjects and channels, we found ⟨S V ⟩=⟨S (a) ⟩=0.96±0.02 ⟨SV⟩=⟨S(a)⟩=0.96±0.02 . In the case of paced breathing, we found ⟨S V ⟩=0.66±0.14 ⟨SV⟩=0.66±0.14 , which reflects venous-dominated hemodynamics at the respiratory frequency.

Use of the CDI blood parameter monitoring system 500 for continuous blood gas measurement during extracorporeal membrane oxygenation simulation

The Oximetrix III Opticath (Abbott Critical Care Systems) is used for continuous measurement of venous saturation in a variety of applications, including extracorporeal membrane oxygenation (ECMO), despite clinical reports that have presented data showing poor accuracy of these devices. The CDI Blood Parameter Monitoring System 500 (Terumo) is an inline blood gas monitoring tool commonly used during cardiopulmonary bypass procedures to continuously assess oxygen saturation, blood gases, potassium, and bicarbonate. The purpose of this experiment was to compare the Opticath and the CDI 500 in trending venous blood saturation during a simulation of ECMO. An ECMO simulation circuit consisting of a silicone rubber membrane oxygenator and a stainless steel heat exchanger was constructed, and a standard venous reservoir bag was used to represent the patient. The CDI and the Opticath were incorporated side by side into a shunt that originated just before the oxygenator and returned flow to the venous line. The circuit was primed with fresh porcine blood and conditioned with the addition of CO2 to simulate typical venous blood under ECMO conditions. After an initial calibration procedure, samples were drawn and analyzed by an AVL Opti CCA (Roche/Osmetech) every 4-8 hours for a period of 7 days, with calibration of each device at sample intervals. The data were plotted, and a least squares regression line was calculated. The average error for venous saturation of the CDI and Opticath after 72 hours was 3.86 and 9.51 respectively. At 168 hours, error for the CDI was 8.37, and the Opticath had an error of 14.78. A correlation analysis of the CDI and AVL CCA analyzer yielded a correlation coefficient of r = .88 at 72 hours and r = .84 at 168 hours. Correlation between the Opticath and the AVL CCA yielded a correlation coefficient of r = .77 at 72 hours and r = .55 at 168 hours. Based on these findings, the CDI 500 is an effective tool for monitoring venous blood saturation under simulated conditions of ECMO.

KEYWORDS

CDI 500, Opticath, extracorporeal membrane oxygenation, venous saturation.

Venous oxygen measurements in the inferior vena cava in neonates with respiratory failure

BACKGROUND: The present study was undertaken to examine the feasibility of venous oxygen measurements in the inferior vena cava (IVC) via a catheter through the umbilical vein. This may serve as a proxy for mixed venous oxygenation and the complications of right atrial cannulation can be avoided at the same time. It has the added advantage of not being affected by atrial right-left shunting. RESULTS: The study included 22 neonates requiring mechanical ventilation for respiratory insufficiency. The success rate of catheterization of the IVC via the umbilical vein was 81% and there was no catheter-related complications. Fifty paired blood samples were obtained and analyzed while the patients were hemodynamically stable. Linear regression analysis showed a poor correlation between arterial oxygen tension (PaO2) and the arterial-venous oxygen content difference [C(a-v)O2], r = -0.005, and between PaO2 and the fractional oxygen extraction (FOE), r = -0.114. There was also a poor correlation between arterial oxygen saturation (SaO2) and C(a-v)O2, r = -0.057, and between SaO2 and FOE, r =-0.139. The correlations between venous oxygen tension (PvO2) and C(a-v)O2 and between PvO2 and FOE were r = -0.528 and r = 0.592, respectively. There were good correlations between various oxygen saturation (SvO2) and C(a-v)O2, r = -0.634, and between SvO2 FOE, r = -0.712. CONCLUSION: Venous oxygen measurement in the IVC via an umbilical vein catheter is a simple and safe procedure and provides information about the tissue oxygenation status of critically ill neonates.