Rational manipulation of oxygen delivery

White matter has impaired resting oxygen delivery in sickle cell patients

Although modern medical management has lowered overt stroke occurrence in patients with sickle cell disease (SCD), progressive white matter (WM) damage remains common. It is known that cerebral blood flow (CBF) increases to compensate for anemia, but sufficiency of cerebral oxygen delivery, especially in the WM, has not been systematically investigated. Cerebral perfusion was measured by arterial spin labeling in 32 SCD patients (age range: 10-42 years old, 14 males, 7 with HbSC, 25 HbSS) and 25 age and race-matched healthy controls (age range: 15-45 years old, 10 males, 12 with HbAS, 13 HbAA); 8/24 SCD patients were receiving regular blood transfusions and 14/24 non-transfused SCD patients were taking hydroxyurea. Imaging data from control subjects were used to calculate maps for CBF and oxygen delivery in SCD patients and their T-score maps. Whole brain CBF was increased in SCD patients with a mean T-score of 0.5 and correlated with lactate dehydrogenase (r2  = 0.58, P < 0.0001). When corrected for oxygen content and arterial saturation, whole brain and gray matter (GM) oxygen delivery were normal in SCD, but WM oxygen delivery was 35% lower than in controls. Age and hematocrit were the strongest predictors for WM CBF and oxygen delivery in patients with SCD. There was spatial co-localization between regions of low oxygen delivery and WM hyperintensities on T2 FLAIR imaging. To conclude, oxygen delivery is preserved in the GM of SCD patients, but is decreased throughout the WM, particularly in areas prone to WM silent strokes.

Allometric Prediction of Energy Expenditure in Infants and Children

Predicting energy needs in children is complicated by the wide range of patient sizes, confusing traditional estimation equations, nonobjective stress-activity factors, and so on. These complications promote errors in bedside estimates of nutritional needs by rendering the estimation methods functionally unavailable to bedside clinicians. Here, the authors develop a simple heuristic energy prediction equation that requires only body mass (not height, age, or sex) as input. Expert estimation of energy expenditure suggested a power-law relationship between mass and energy. A similar mass-energy expenditure relationship was derived from published pediatric echocardiographic data using a Monte Carlo model of energy expenditure based on oxygen delivery and consumption. A simplified form of the equation was compared with energy required for normal growth in a cohort of historical patients weighing 2 to 70 kg. All 3 methods demonstrate that variation in energy expenditure in children is dominated by mass and can be estimated by the following equation: Power(kcal/kg/d) = 200 × [Mass(kg)^(−0.4)]. This relationship explains 85% of the variability in energy required to maintain expected growth over a broad range of surgical clinical contexts. A simplified power-law equation predicts real-world energy needs for growth in patients over a wide range of body sizes and clinical contexts, providing a more useful bedside tool than traditional estimators.

Red blood cell transfusion in critically ill children: a narrative review

OBJECTIVE

To review the pathophysiology of anemia, as well as transfusion-related complications and indications for red blood cell (RBC) transfusion, in critically ill children. Although allogeneic blood has become increasingly safer from infectious agents, mounting evidence indicates that RBC transfusions are associated with complications and unfavorable outcomes. As a result, there has been growing interest and efforts to limit RBC transfusion, and indications are being revisited and revamped. Although a so-called restrictive RBC transfusion strategy has been shown to improve morbidity and mortality in critically ill adults, there have been relatively few studies on RBC transfusion performed in critically ill children.

DATA SOURCES

Published literature on transfusion medicine and outcomes of RBC transfusion. STUDY SELECTION, DATA EXTRACTION, AND SYNTHESIS: After a brief overview of physiology of oxygen transportation, anemia compensation, and current transfusion guidelines based on available literature, risks and outcomes of transfusion in general and in critically ill children are summarized in conjunction with studies investigating the safety of restrictive transfusion strategies in this patient population.

CONCLUSIONS

The available evidence does not support the extensive use of RBC transfusions in general or critically ill patients. Transfusions are still associated with risks, and although their benefits are established in limited situations, the associated negative outcomes in many more patients must be closely addressed. Given the frequency of anemia and its proven negative outcomes, transfusion decisions in the critically ill children should be based on individual patient's characteristics rather than generalized triggers, with consideration of potential risks and benefits, and available blood conservation strategies that can reduce transfusion needs.

Vasopressor Therapy Using Vasopressin Prior to Crystalloid Resuscitation in Irreversible Hemorrhagic Shock under Isoflurane Anesthesia in Dogs

In the present study, we tested the hypothesis that vasopressin administration prior to crystalloid resuscitation can be used to improve hemodynamic and oxygen delivery functions. Hemorrhagic shock was experimentally induced by maintaining mean arterial pressure at 60 mmHg for 30 min in sixteen healthy dogs weighing from 8 to 10.6 kg. Vasopressin was administered and then volume resuscitation was performed for the 6 dogs of V-C group, while vasopressin was administered at the end of volume resuscitation in the 5 dogs of C-V group. The control group (n=5) was administered 0.4 IU/kg of vasopressin after induction of shock without fluid resuscitation. In all groups, hemodynamic parameters were measured pre- and post-hemorrhage and for 60 min after fluid resuscitation. The dogs in V-C group had substantially increased systolic arterial pressure (SAP) for 60 min and improved pulmonary capillary wedge pressure (PCWP), cardiac output (CO), oxygen delivery, and oxygen consumption indexes compared with C-V and control groups. Diastolic pressure and systemic vascular resistance was significantly lower in the V-C group than those in the C-V and control groups (P<0.05). In the V-C group, there was effective and rapid restoration of the SAP, CO, PCWP, and oxygen delivery parameters after treatment. This study indicates that vasopressin administration before crystalloid resuscitation is a more efficient way of improving hemodynamic and oxygen delivery functions in hemorrhagic shock in dogs.

Hemodynamic Characteristics of Vasopressin in Dogs with Severe Hemorrhagic Shock

The effect of vasopressin was compared with that of the established vasopressor epinephrine in experimentally induced hemorrhagic shock. After rapid crystalloid resuscitation in a ratio of three volumes of 0.9% saline to one volume of blood (3:1 crystalloid resuscitation), six dogs were given 0.4 IU/kg vasopressin and another six dogs were given 0.1 mg/kg epinephrine. Five dogs in the control group were given fluid resuscitation in the same manner as above without administration of any drugs. Administration of vasopressin increased diastolic arterial pressure (DAP) from 45.0 +/- 4.9 to 91.2 +/- 9.6 mmHg within 5 min, compared with epinephrine from 46 +/- 4.0 to 51.8 +/- 7.7, and control from 47.3 +/- 7.5 to 46.3 +/- 7.3. Systolic arterial pressure (SAP) did not increase significantly following vasopressin compared with epinephrine and control group. Results of DAP and systemic vascular resistance index (SVRI) suggested that vasopressin administration was vasoconstrictive after fluid resuscitation in decompensatory hemorrhagic shock in dogs, whereas epinephrine did not compared with control. In addition, epinephrine did not affect the cardiac index (CI) and SVRI, while a significant decrease in CI and increase in SVRI were observed in vasopressin group. The pressor effect of epinephrine in the vascular system was abrupt and only lasted a short period of time (within 5 min), while that of vasopressin was steady and lasted for more than 1 hr, especially regard to in DAP. When compared with epinephrine, vasopressin can be a more effective and safer choice in patients with severe hemorrhagic shock.

The Impact of Heparin-coated Circuits on Hemodynamics During and After Cardiopulmonary Bypass

This study was performed to investigate if heparin-coated extracorporeal circuits can reduce the systemic inflammatory reaction with the subsequent release of vasoactive substances during and after cardiopulmonary bypass. Fifty-one patients scheduled for coronary artery bypass grafting were perfused with either a heparin-coated or an uncoated circuit. During bypass the mean arterial pressure was maintained as near as possible to 60 mm Hg. Mediators for inflammation, hemodynamic, and oxygen parameters were determined during and after bypass. To reach the target mean arterial pressure in the first hour of bypass the pump flow in the uncoated group had to be increased (P<0.05), consequently the systemic vascular resistance index decreased (P<0.05). After bypass more inotropic support was necessary in this group to reach this pressure. In the coated group less bradykinin, complement activation, and elastase was generated during bypass (P<0.05). The results of this study suggest that heparin coating not only improves biocompatibility, but also ameliorates the hemodynamic instability during and after bypass.

Induction and detection of disturbed homeostasis in cardiopulmonary bypass

During cardiopulmonary bypass (CPB) haemodynamic alterations, haemostasis and the inflammatory response are the main causes of homeostatic disruption. Even with CPB procedures of short duration, the homeostasis of a patient is disrupted and, in many cases, requires intensive postoperative treatment to re-establish the physiological state of the patient. Although mortality is low, disruption of homeostasis may contribute to increased morbidity, particularly in high-risk patients. Over the past decades, considerable technical improvements in CPB equipment have been made to prevent the development of the systemic inflammatory response syndrome (SIRS). Despite all these improvements, only the inflammatory response, to some extent, has been reduced. The microcirculation is still impaired, as measured by tissue degradation products of various organs, indicating that CPB may still be considered as an unphysiological procedure. The question is, therefore, whether we can detect the pathophysiological consequences of CPB in each individual patient with valid bedside markers, and whether we can relate this to determinant factors in the CPB procedure in order to assist the perfusionist in improving the adequacy of CPB. The use of these markers could play a pivotal role in decision making by providing an immediate feedback on the determinant quality of perfusion. Therefore, we suggest validating the proposed markers in a nomogram to optimize not only the CPB procedure, but also the patient's safety.

The Impact of Heparin Coated Circuits Upon Metabolism in Vital Organs: Effect Upon Cerebral and Renal Function During and After Cardiopulmonary Bypass

During cardiopulmonary bypass (CPB), the brain and the kidneys may be damaged because of microemboli, ischemia, and inflammation. The latter has been reduced by the use of heparin coated circuits. We questioned whether heparin coated circuits could also reduce cerebral and renal damage and whether inflammatory markers correlate with damage to the brain and the kidneys. Fifty-one patients scheduled for coronary artery bypass grafting were perfused with either a heparin coated or an uncoated circuit. To compare the effect of a heparin coated circuit with an uncoated circuit upon cerebral and renal function in relation to inflammation, we assessed markers of cerebral (S100beta) and renal (N-acetyl-beta-D-glucosaminidase [NAG], creatinine, and urea) function, inflammation, and oxygen metabolism. S100beta levels and NAG levels increased during CPB in both groups as compared with baseline levels (p < 0.01), without differences between the groups. After 15 minutes on CPB, C4b/c levels were significantly higher in the coated group compared with the uncoated group (p < 0.02). C4b/c correlated with S100beta (p < 0.01). Total body oxygen delivery (DO2) and consumption (VO2) decreased significantly in both groups during CPB (p < 0.01), but recovery was better in the coated group. After protamine infusion, total body oxygen delivery and consumption correlated negatively with S100beta levels (both p < 0.05) and with NAG levels (both p < 0.01). This study suggests that, if adequate tissue perfusion is not maintained, the use of a heparin coated circuit gives no additional benefit beyond that of the uncoated circuit. The inverse relationship of both cerebral and renal markers with DO2 and VO2 suggests that increased levels of S100beta and NAG during CPB may primarily be caused by an oxygen deficit and secondary to the inflammatory response.

Skeletal muscle acidosis correlates with the severity of blood volume loss during shock and resuscitation

BACKGROUND

Continuous assessment of tissue perfusion and oxygen utilization may allow for early recognition and correction of hemorrhagic shock. We hypothesized that continuously monitoring skeletal muscle (SM) PO2, PCO2, and pH during shock would provide an easily accessible method for assessing the severity of blood loss and the efficacy of resuscitation.

METHODS

Thirteen anesthetized pigs (25-35 kg) underwent laparotomy and femoral vessel cannulation. Multiparameter fiberoptic sensors were placed in the deltoid (SM) and femoral artery. Ventilation was maintained at a PaCO2 of 40-45 mm Hg. Total blood volume (TBV) was measured using an Evans blue dye technique. Animals were bled for 15 minutes, maintained at a mean arterial pressure (MAP) of 40 mm Hg for 1 hour, resuscitated (shed blood + 2 times shed volume in normal saline) and observed for 1 hour. Four animals served as controls (sham hemorrhage). Blood and tissue samples were taken at each time point.

RESULTS

Blood loss ranged from 28.5-56% of TBV. SM pH and SM PO2 levels fell rapidly with shock. SM PO2 returned to normal with resuscitation; however, SM pH did not return to baseline. SM PCO2 significantly rose with shock, but returned to baseline promptly with resuscitation. There was a significant correlation between SM pH and blood volume loss at end shock (r2 = 0.73, p < 0.001) and recovery (r2 = 0.84, p < 0.001). Animals (n = 2) whose SM pH did not recover to 7.2 were found to have ongoing blood loss from biopsy sites and persistent tissue hypercarbia despite normal MAP.

CONCLUSION

Continuous multiparameter monitoring of SM provides a minimally invasive method for assessing severity of shock and efficacy of resuscitation. Both PCO2 and PO2 levels change rapidly with shock and resuscitation. SM pH is directly proportional to lost blood volume. Persistent SM acidosis (pH < 7.2) and elevated PCO2 levels suggest incomplete resuscitation despite normalized hemodynamics.

The massively bleeding patient

The resuscitation of the massively bleeding patient may seem superficially to be successful once the patient's vital signs have stabilized. The restoration of stable vital signs, however, does not confirm two critical elements of a thorough physiologic resuscitation: that there is truly adequate delivery of oxygen to all tissue beds and that physiologic disturbances that may have occurred because of massive transfusion during the resuscitation process have resolved. With respect to the adequacy of oxygen delivery, the current clinical endpoints, including mixed venous oxygen saturation, cardiac output, and serum lactate, reflect global perfusion and not regional oxygenation. Of these global measures, serum lactate is currently the best indicator as to whether some circulatory beds remain inadequately perfused. Serum lactate should be followed, and, in the event that elevated levels persist, measures to augment oxygen delivery (e.g., increasing cardiac output, hemoglobin concentration, oxygen saturation) should be undertaken. Gastric tonometry provides a method for specific examination of the splanchnic circulation. The current measurement techniques, however, require steady-state conditions and make it impractical in many physiologically dynamic situations. The physiologic disturbances associated with massive resuscitation (e.g., hyperkalemia, hypocalcemia, hypomagnesemia, hypothermia) should be anticipated. Coagulation disturbances occur, especially when massive transfusion is accompanied by hypotension, hypothermia, or acidosis. Coagulation parameters should be measured with the loss of each one half of blood volume or after each 30-minute interval, whichever occurs first. Evaluation at blood volume intervals is relevant to the development of a strictly dilutional coagulopathy. The development of DIC, occurring because of tissue factor exposure or acidosis, however, is related more to the time lapsed than to the absolute volume lost or replaced.

Cardiac output, wedge pressure, and oxygen delivery

The primary goal of the intensive care clinician can be said to be to optimize global DO2. This approach is the primary means by which the greatest killers of the critically ill patient (sepsis, SIRS, multiple organ dysfunction syndrome) may be addressed at present. Optimizing DO2 means delivering just enough to meet the patient's needs, because therapeutic measures taken to increase DO2 are all associated with some degree of risk. When used correctly, the PAC can allow the clinician to determine if DO2 is optimal and, if not, what steps might be best suited to improve on it. Newer generations of PACs are becoming available and can provide valuable additional insights into a patient's cardiovascular status. Nearly all attempts to increase DO2 address one or more of a relatively short list of variables. The specific endpoints of therapy need to be tailored to the individual patient but include clinical, metabolic, organ function, and hemodynamic markers. As clinicians expand our understanding of the key elements found in survivors of critical illness, it is hoped that this knowledge translates into better outcomes.