Central venous oxygen saturation monitoring in the critically ill patient

Sepsis in the 21st century: recent definitions and therapeutic advances

Sepsis as a disease has received renewed interests since recent publications of a revised clinical definition and crucial clinical trials showing the benefits of early goal-directed resuscitation, recombinant human activated protein C, and low-dose corticosteroids. The epidemiology of sepsis has also been further examined. Management guidelines and international quality improvement efforts have been developed targeting increased disease identification, clinician education, and optimal patient care with the result of decreasing patient mortality. The evidence suggests that early recognition and early intervention are most important in affecting outcome. This article will summarize these developments in the diagnosis and management of sepsis at the turn of this century.

Pharmaceutical metabolic down-regulation by protein synthesis inhibition in a conscious rat model

Pharmaceutically induced metabolic down-regulation may be a useful therapeutic adjunct when tissue oxygen supply is restricted. We hypothesized that protein synthesis inhibition in a non-hibernating species should lower oxygen demand, resulting in aerobic metabolic rate depression at the whole animal level. We compared metabolic responses and measures of systemic oxygenation of conscious catheterized rats given either protein synthesis inhibition (PSI) agents or carrier controls (normal saline and DMSO). Core temperature was measured by implanted transmitters, and VO2 was determined in an open flow-through metabolic chamber at 25 degrees C. Mean arterial pressure MAP and heart rate HR were determined from arterial pressure transducer tracings; arterial blood gases and lactate were sampled every 15 min. PSI rats exhibited an immediate transient decline in VO2, followed by a secondary decline to new resting levels; VO2 for the first hour was significantly lower than that for rats receiving DMSO vehicle. Unlike controls, PSI rats showed an overall 3.5 degrees C decline in core temperature, coupled with increased arterial lactate. There were no differences in MAP and HR of PSI rats compared to controls. Although hypothermic response to toxic agents typical of rodents cannot be ruled out completely, the mild hypothermia and reduced VO2 exhibited by PSI rats may be partially attributed to the action of protein synthesis agents.

Noninvasive cerebral oximeter as a surrogate for mixed venous saturation in children

We evaluated the relationship between regional cerebral oxygen saturation (rSO(2)) measured by near-infrared spectroscopy (NIRS) cerebral oximeter with superior vena cava (SVC), inferior vena cava (IVC), right atrium (RA), and pulmonary artery (PA) saturation measured on room air and 100% inspired oxygen administered via a non-rebreather mask (NRB) in children. Twenty nine pediatric post-orthotopic heart transplant patients undergoing an annual myocardial biopsy were studied. We found a statistically significant correlation between rSO(2) and SVC saturations at room air and 100% inspired oxygen concentration via NRB (r = 0.67, p = 0.0002 on room air; r = 0.44, p = 0.02 on NRB), RA saturation (r = 0.56, p = 0.002; r = 0.56, p = 0.002), and PA saturation (r = 0.67, p < 0.001; r = 0.4, p = 0.03). A significant correlation also existed between rSO(2) and measured cardiac index (r = 0.45, p = 0.01) and hemoglobin levels (r = 0.41, p = 0.02). The concordance correlations were fair to moderate. Bias and precision of rSO(2) compared to PA saturations on room air were -0.8 and 13.9%, and they were 2.1 and 15.6% on NRB. A stepwise linear regression analysis showed that rSO(2) saturations were the best predictor of PA saturations on both room air (p = 0.0001) and NRB (p = 0.012). In children with biventricular anatomy, rSO(2) readings do correlate with mixed venous saturation.

Critical care in the emergency department: monitoring the critically ill patient

The aim of monitoring patients is to detect organ dysfunction and guide the restoration and maintenance of tissue oxygen delivery. Monitoring is a crucial part of the care of the critically ill patient in the emergency department as the physiological response to critical illness is linked strongly to outcome. As it is important to appreciate the limitations of monitoring systems and monitored data, and to understand that invasive monitoring may be hazardous, this review concentrates on the techniques used to monitor critically ill patients in the emergency department. End tidal carbon dioxide monitoring, pulse oximetry, arterial blood pressure monitoring, central venous pressure monitoring, continuous central venous oxygenation saturation monitoring, temperature monitoring, and urine output are discussed. Practitioners should be familiar with the physiology and technology underlying these monitoring techniques and be aware of the pitfalls in interpretation of monitored data.

Pharmacologic Support of the Failing Heart

Cardiovascular failure in critically ill patients carries a high mortality. Identification and treatment of the underlying etiology simultaneously with prompt therapy are indicated to avoid the consequences of prolonged shock. Physicians should assess patients using all available clinical, radiologic, and laboratory data to avoid the pitfalls associated with use of single measures of regional or global perfusion. Continued evidence of inadequate perfusion despite fluid resuscitation warrants consideration of placement of a pulmonary artery catheter or pharmacologic support of the cardiovascular system. Finally, the dynamic nature of physiology in critically ill patients requires constant patient reassessment and flexibility in treatment to tailor therapy individually as the pathologic state evolves.

Continuous monitoring of central venous oxygen saturation in neonates and small infants: in vitro evaluation of two different oximetry catheters

BACKGROUND

Accurate assessment and monitoring of the cardiocirculatory function are essential during major pediatric and pediatric cardiac surgery. Monitoring of the central venous oxygen saturation (ScvO(2)) may be a better indicator of tissue oxygenation and derangement of cellular oxygen utilization than the more commonly used vital parameters. Therefore, we compared oxygen saturation measurements with thin fiberoptic oximetry catheters and standard blood gas oximetry in an in vitro setting.

METHODS

Two different size continuous fiberoptic oximetry catheters (2-4-F) were inserted in an extracorporeal circuit filled with human red blood cells in normal saline (haematocrit 30%, flow 600 ml.min(-1)). The results of fiberoptic oximetry were then compared with standard blood gas oximetry for a wide range of different oxygen saturations using linear regression.

RESULTS

The oxygen saturations found ranged from 9% to 100%. The results of the two different fiberoptic oximetry catheters correlated significantly (r = 0.99, P < 0.0001) with standard blood gas oximetry.

CONCLUSION

The results of fiberoptic oximetry are nearly identical with standard blood gas oximetry for a wide range of different oxygen saturations. Thin oximetry catheters can be inserted percutaneously even in neonates and small infants. The continuous monitoring of ScvO(2) may be beneficial, especially in patients who are in danger of developing low cardiac output or sudden cardiovascular collapse.

Venous oximetry

PURPOSE OF REVIEW

Mixed venous and central venous oxygen saturations reflect the balance between oxygen requirement and oxygen delivery, and thus may be used to assess the adequacy of tissue oxygenation. This review discusses recent data on the impact of using venous oximetry by obtaining mixed venous oxygen saturation or central venous oxygen saturation as useful monitoring parameters in critically ill patients.

RECENT FINDINGS

Monitoring of changes in the central venous oxygen saturation after major surgery has been shown to be associated with outcome. Furthermore, early goal-directed therapy for patients with severe sepsis or septic shock, which includes treatment goals for mean arterial pressure, central venous pressure, and central venous oxygen saturation, was able to increase survival in these patients. In contrast, pulmonary artery catheter usage does not affect outcome in critically ill patients.

SUMMARY

Early goal-directed therapy should be implemented in the initial resuscitation of septic patients. Central venous saturation may have prognostic significance following major surgery. Further evaluation of peri-operative trends in central venous saturation is required. Measurement of central venous oxygen saturation can easily be applied in intensive care unit patients and offers a useful, indirect indicator for the adequacy of tissue oxygenation.

Interrater reliability of cardiac output measurements by transcutaneous Doppler ultrasound: implications for noninvasive hemodynamic monitoring in the ED

INTRODUCTION

Hemodynamic monitoring is an important aspect of caring for the critically ill patients boarding in the emergency department (ED). The purpose of this study is to investigate the interrater agreement of noninvasive cardiac output measurements using transcutaneous Doppler ultrasound technique.

METHODS

This is a prospective observational cohort study performed in a 32-bed adult ED of an academic tertiary center with approximately 65000 annual patient visits. Patients were enrolled after verbal consent over a 7-month period. The raters were ED personnel involved in patient care. Paired measurements of cardiac index (CI) and stroke volume index (SVI) were obtained from a transcutaneous Doppler ultrasound cardiac output monitor.

RESULTS

A convenience sample of 107 (50 women and 57 men) patients with a median age of 49 (32, 62) years was enrolled. One hundred two paired measurements were performed in 91 patients in whom adequate Doppler ultrasound signals were obtainable. The raters included 35 emergency medicine attending physicians, 31 emergency medicine residents, 80 medical students, 47 nurses, and 11 emergency medical technicians. Cardiac index range was 0.6 to 5.3 L/min per square meter, and SVI range was 7.7 to 63.0 mL/m(2). The correlation of CI measurements between 2 raters was good (r(2) = 0.87; 95% confidence interval, 0.86-1.00; P < .001). Likewise, SVI measurements between 2 raters also showed acceptable correlation (r(2) = 0.84; 95% confidence interval, 0.81-0.96; P < .001). Interrater reliability was strong for CI (kappa = 0.83 with 92.2% agreement) and SVI measurements (kappa = 0.72 with 88.2% agreement). Most patients had an interrater difference below 10% in CI and SVI measurements.

CONCLUSIONS

Emergency department personnel, regardless of their role in patient care, are able to obtain reliable cardiac output measurements in ED patients over a wide range of CI and SVI. Transcutaneous Doppler ultrasound technique may be an alternative to traditional invasive hemodynamic monitoring of critically ill patients presenting to the ED.

A preliminary study on the monitoring of mixed venous oxygen saturation through the left main bronchus

Introduction

The study sought to assess the feasibility and accuracy of measuring mixed venous oxygen saturation (SvO₂) through the left main bronchus (SpO_2trachea)

Methods

Twenty hybrid pigs of each sex were studied. After anesthesia, a Robertshaw double-lumen tracheal tube with a single-use pediatric pulse oximeter attached to the left lateral surface was introduced toward the left main bronchus of the pig by means of a fibrobronchoscope. Measurements of SpO_2trachea and oxygen saturation from pulmonary artery samples (SvO_2blood) were performed with an intracuff pressure of 0 to 60 cmH₂O. After equilibration, hemorrhagic shock was induced in these pigs by bleeding to a mean arterial blood pressure of 40 mmHg. With the intracuff pressure maintained at 60 cmH₂O, SpO_2trachea and SvO_2blood were obtained respectively during the pre-shock period, immediately after the onset of shock, 15 and 30 minutes after shock, and 15, 30, and 60 minutes after resuscitation.

Results

SpO_2trachea was the same as SvO_2blood at an intracuff pressure of 10, 20, 40, and 60 cmH₂O, but was reduced when the intracuff pressure was zero (p < 0.001 compared with SvO_2blood) in hemodynamically stable states. Changes of SpO_2trachea and SvO_2blood corresponded with varieties of cardiac output during the hemorrhagic shock period. There was a significant correlation between the two methods at different time points.

Conclusion

Measurement of the left main bronchus SpO₂ is feasible and provides similar readings to SvO_2blood in hemodynamically stable or in low saturation states. Tracheal oximetry readings are not primarily derived from the tracheal mucosa. The technique merits further evaluation.

Effects of dobutamine, norepinephrine, and vasopressin on cardiovascular function in anesthetized neonatal foals with induced hypotension

OBJECTIVE

To determine the effects of dobutamine, norepinephrine, and vasopressin on cardiovascular function and gastric mucosal perfusion in anesthetized foals during isoflurane-induced hypotension.

ANIMALS

6 foals that were 1 to 5 days of age.

PROCEDURES

6 foals received 3 vasoactive drugs with at least 24 hours between treatments. Treatments consisted of dobutamine (4 and 8 Sang/kg/min), norepinephrine (0.3 and 1.0 Sang/kg/min), and vasopressin (0.3 and 1.0 mU/kg/min) administered IV. Foals were maintained at a steady hypotensive state induced by a deep level of isoflurane anesthesia for 30 minutes, and baseline cardiorespiratory variables were recorded. Vasoactive drugs were administered at the low infusion rate for 15 minutes, and cardiorespiratory variables were recorded. Drugs were then administered at the high infusion rate for 15 minutes, and cardiorespiratory variables were recorded a third time. Gastric mucosal perfusion was measured by tonometry at the same time points.

RESULTS

Dobutamine and norepinephrine administration improved cardiac index. Vascular resistance was increased by norepinephrine and vasopressin administration but decreased by dobutamine at the high infusion rate. Blood pressure was increased by all treatments but was significantly higher during the high infusion rate of norepinephrine. Oxygen delivery was significantly increased by norepinephrine and dobutamine administration; O2 consumption decreased with dobutamine. The O2 extraction ratio was decreased following norepinephrine and dobutamine treatments. The gastric to arterial CO2 gap was significantly increased during administration of vasopressin at the high infusion rate.

CONCLUSION AND CLINICAL RELEVANCE

Norepinephrine and dobutamine are better alternatives than vasopressin for restoring cardiovascular function and maintaining splanchnic circulation during isoflurane-induced hypotension in neonatal foals.

Correlation of oxygen delivery with central venous oxygen saturation, mean arterial pressure and heart rate in piglets

BACKGROUND

Accurate assessment and monitoring of the cardiocirculatory function is essential during major pediatric and pediatric cardiac surgery. Invasive monitoring of cardiac output and oxygen delivery (DO(2)) is expensive and sometimes associated with adverse events. Measurement of central venous oxygen saturation (ScvO(2)) is less invasive and may reflect the DO(2). Therefore, we investigated the correlation of ScvO(2) with cardiac index (CI) and DO(2) and in comparison the more common monitored parameters heart rate (HR) and mean arterial pressure (MAP) with DO(2) in an animal experimental setting.

METHODS

In five fasted, anesthetized and mechanically ventilated piglets CI (transpulmonary thermodilution), venous and arterial blood gases, HR and MAP was measured during normal conditions, volume loading, inotropic support, and exsanguination.

RESULTS

In the five piglets 168 measurements could be performed. In a wide hemodynamic range (CI 22-335 ml x kg(-1) min(-1)) we found significant correlations of ScvO(2) with DO(2)) (r(2) = 0.91, P < 0.0001) and CI (r(2) = 0.88, P < 0.0001) and also between DO(2) and MAP (r = 0.86, P < 0.0001) and HR (r = 0.19, P < 0.05).

CONCLUSIONS

ScvO(2) is a better parameter for indirect estimation of DO(2) than MAP and heart rate. Measurement of ScvO(2) is simple and does not necessitate additional invasive techniques. In the clinical setting ScvO(2) should be used in combination with other standard vital parameters, i.e. MAP, central venous pressure, lactate, base excess, and urine output.

Continuous central venous oximetry monitoring

Traditionally, bedside evaluation of tissue oxygenation has been accomplished by assessing mixed venous oxygen saturation (SvO2) using a fiberoptic pulmonary artery catheter. It may not always be feasible to place a pulmonary artery catheter early in resuscitation or outside of the ICU. Introduction of continuous central venous oxygen saturation (ScvO2) monitoring that uses a modified, central venous catheter is making it possible to evaluate ongoing tissue oxygenation in nontraditional settings, such as the Emergency Department. Continuous measurement of ScvO2 will alert the clinician to major derangements in oxygen balance, which allows for timely implementation of appropriate interventions.

Severe sepsis and septic shock: review of the literature and emergency department management guidelines

Severe sepsis and septic shock are as common and lethal as other acute life-threatening conditions that emergency physicians routinely confront such as acute myocardial infarction, stroke, and trauma. Recent studies have led to a better understanding of the pathogenic mechanisms and the development of new or newly applied therapies. These therapies place early and aggressive management of severe sepsis and septic shock as integral to improving outcome. This independent review of the literature examines the recent pathogenic, diagnostic, and therapeutic advances in severe sepsis and septic shock for adults, with particular relevance to emergency practice. Recommendations are provided for therapies that have been shown to improve outcomes, including early goal-directed therapy, early and appropriate antimicrobials, source control, recombinant human activated protein C, corticosteroids, and low tidal volume mechanical ventilation.

Evolutive standard base excess and serum lactate level in severe sepsis and septic shock patients resuscitated with early goal-directed therapy: still outcome markers?

PURPOSE

To compare the evolution of standard base excess and serum lactate level between surviving and non surviving patients with severe sepsis and septic shock resuscitated with early goal-directed therapy.

METHODS

This is a retrospective study in an intensive care unit of a university tertiary hospital where 65 consecutive severe sepsis and septic shock patients were observed without any intervention in the treatment by the authors of this report.

RESULTS

In our study, the mortality of severe sepsis and septic shock patients was 38%. The central venous oxygen saturation of both groups was above 70% after the resuscitative period, excluding the second day of the non survivors group (69.8%). After the second day, the central venous oxygen saturation was significantly higher in the survivors group (P < .001). Standard base excess was initially low in both groups, but from the second day on, the correction of standard base excess was significantly more successful and linear in the survivor group (P < .001). Lactate levels were similar during the evolution of both groups.

CONCLUSIONS

Although evolutive standard base excess and serum lactate level are still outcome markers in severe sepsis and septic shock patients resuscitated with early goal-directed therapy, other studies must be performed to clarify if hemodynamic interventions based on standard base excess and serum lactate level could be reliable to improve clinical outcomes in severe sepsis and septic shock patients.

Evaluation of tissue saturation as a noninvasive measure of mixed venous saturation in children

BACKGROUND

Mixed venous saturation (S & OV0456;o2) is an important measurement that helps guide the care of critically ill patients. Invasive S & OV0456;o2 assessment in infants and children is often avoided because of the inherent risks. A noninvasive tissue saturation (S to 2) monitor has recently been developed that uses near-infrared spectroscopy to measure oxyhemoglobin saturation in muscle. In adult and animal studies, S to 2 correlated with oxygen delivery and S & OV0456;o2. There has been no evaluation in pediatric patients.

OBJECTIVE

To evaluate tissue saturation as a noninvasive measure of mixed venous saturation in children.

DESIGN

A prospective observational study.

SETTING

Catheterization laboratory in a tertiary care children's medical center.

PATIENTS

We studied 98 children (49 without intracardiac mixing and 49 with intracardiac mixing) <or=12 yrs of age who underwent cardiac catheterization. Under general anesthesia, we compared S to 2 measured over the deltoid muscle with superior vena cava saturation in both groups and S to 2 with pulmonary artery saturation in patients without intracardiac mixing. Paired measurements were analyzed for bias, precision, and correlation via Bland-Altman plot and linear regression.

RESULTS

No meaningful correlation was found between S to 2 and superior vena cava saturation or pulmonary artery saturation. Bland-Altman analyses of S to 2 with superior vena cava saturation yielded bias values of -6.67 +/- 37.33% in patients with intracardiac mixing and -0.82 +/- 41.31% in patients without mixing. Bland-Altman analysis of S to 2 with pulmonary artery saturation yielded a bias of 3.61 +/- 41.32% in patients without mixing. Differences between noninvasive and invasive measurements were greatest in smaller children.

CONCLUSION

Noninvasive tissue saturation over the deltoid does not correlate with S & OV0456;o2 in children. It is possible that more precise probe spacing, coupled with optimal muscle-mass location, could result in more accurate measures in future investigations.

Evaluation of blood transfusion effects on mixed venous oxygen saturation and lactate levels in patients with SIRS/sepsis

PURPOSE

To evaluate the effects of red blood cell transfusion in patients with SIRS/sepsis who presented hemoglobin levels under 9.0 g/dL at intensive care unit admission, using two parameters of organ perfusion: mixed venous oxygen saturation and serum lactate levels.

METHODS

All patients admitted to the intensive care unit with SIRS/sepsis, as defined by Consensus Conference in 1992, and hemoglobin levels under 9.0 g/dL were included. Hemoglobin levels, mixed venous oxygen saturation, and lactate levels were collected before red blood cell transfusion (pre-T) and up to 1 hour after transfusion (post-T). These variables were analyzed through a paired t test, and results were considered significant if P < .05.

RESULTS

Twenty-nine patients (17 male, 12 female) with ages of 61.9 +/- 15.1 (mean +/- SD) years (range, 21-85 years) and a mean APACHE II score of 12.5 +/- 3.75 (7-21) were transfused with a mean of 1.41 packed red cell units. A significant increase in hemoglobin levels was reached by blood transfusion, from 8.14 +/- 0.64 g/dL (pre-T) to 9.4 +/- 0.33 g/dL (post-T), with P <.001. However, this was not accompanied by a significant change in lactate levels, from 1.87 +/- 1.22 mmol/l (pre-T) to 1.56 +/- 0.28 mmol/l (post-T), with P =.28, or in mixed venous oxygen saturation, from 64.3 +/- 8.52% (pre-T) to 67.4 +/- 6.74% (post-T), with P = .13. The results were similar even in patients with hemoglobin levels under 8.0 g/dL (n = 9). These results suggest that red blood cell transfusions, in spite of leading to a significant increase in hemoglobin levels, are not associated with an improvement in tissue oxygenation in patients with SIRS/sepsis and hemoglobin levels < 9 g/dL.

Clinical review: New technologies -- venturing out of the intensive care unit

The delivery of critical care is no longer limited to the intensive care unit. The information gained by utilization of new technologies has proven beneficial in some populations. Research into earlier and more widespread use of these modalities may prove to be of even greater benefit to critically ill patients.

Equipment review: the success of early goal-directed therapy for septic shock prompts evaluation of current approaches for monitoring the adequacy of resuscitation

A recent trial utilizing central venous oxygen saturation (SCVO₂) as a resuscitation marker in patients with sepsis has resulted in its inclusion in the Surviving Sepsis Campaign guidelines. We review the evidence behind SCVO₂ and its relationship to previous trials of goal-directed therapy. We compare SCVO₂ to other tools for assessing the adequacy of resuscitation including physical examination, biochemical markers, pulmonary artery catheterization, esophageal Doppler, pulse contour analysis, echocardiography, pulse pressure variation, and tissue capnometry. It is unlikely that any single technology can improve outcome if isolated from an organized pattern of early recognition, algorithmic resuscitation, and frequent reassessment. This article includes a response to the journal's Health Technology Assessment questionnaire by the manufacturer of the SCVO₂ catheter.

Haemodynamic changes during positive-pressure ventilation in children

BACKGROUND

Positive-pressure ventilation may alter cardiac function. Our objective was to determine with the use of impedance cardiography (ICG) whether altering airway pressure modifies the central haemodynamics in mechanically ventilated children with no pulmonary pathology. Central venous saturation (S(cv)O(2)) was measured as an indicator of tissue perfusion.

METHODS

Twelve children between 7 and 65 months of age, requiring mechanical ventilation as a consequence of a non-pulmonary disease, were enrolled in the study. All patients had a central venous line as a part of their routine management. Using pressure controlled ventilation (PCV) the baseline PEEP value of 5 cmH(2)O (P(b5)) was increased to 10 cmH(2)O (P(i10)) and then to 15 cmH(2)O (P(i15)). After P(i15), PEEP was decreased to 10 (P(d10)) and then to 5 cmH(2)O (P(d5)). Each time period lasted 5 min heart rate (HR), mean arterial blood pressure (MABP), central venous pressure (CVP), end-tidal carbon dioxide (ETCO(2)), mean airway pressure (P(aw)), stroke volume index (SVI), cardiac index (CI) and central venous oxygen saturation (S(cv)O(2)) were recorded at the end of the five periods.

RESULTS

The values of CI did not change when 10 and 15 cmH(2)O of PEEP were applied. Elevation of PEEP and thus P(aw) caused slight but not significant reductions in SVI and S(cv)O(2) as compared to the baseline (T(b5)). After reducing PEEP in T(d5) we found statistically significant elevations of SVI and CI, as compared to T(i15) heart rate, ETCO(2) and MABP remained unchanged.

CONCLUSION

We did not find significant haemodynamic changes following PEEP elevation in ventilated children, as measured using impedance cardiography. Reducing the value of PEEP to 5 cmH(2)O resulted in statistically significant SVI elevations. The values of S(cv)O(2) remained unaffected.

Noninvasive assessment of cardiac output

Improved outcome from shock depends on early detection and correction of circulatory abnormalities. Global cardiac output and oxygen delivery must be adequate and distributed appropriately to meet metabolic demands to prevent the development of multiple organ system dysfunction, prolonged morbidity, and death. Circulatory assessment using standard monitors gives incomplete and sometimes misleading information. This article focuses on the available and emerging technologies that emphasize assessment of blood flow and regional tissue oxygenation.

Diagnostic measures to evaluate oxygenation in critically ill adults: implications and limitations

Accurate assessment and treatment of disturbances in oxygenation are crucial to optimal outcomes in critically ill patients. Oxygenation is dependent upon adequate pulmonary gas exchange, oxygen delivery, and oxygen consumption. Each of these physiologic processes may vary independently in response to pathophysiologic conditions and therapeutic interventions. The author reviews diagnostic measures available to evaluate pulmonary gas exchange, oxygen delivery, and oxygen consumption in critically ill patients. Currently available tools and their potential value as well as key methodological limitations are addressed. Failure on behalf of clinicians to fully appreciate these limitations can lead to misdiagnoses and inappropriate treatment. The aim of this article is to help advanced practice nurses more fully understand the implications and limitations of these diagnostic measures to ensure accurate assessment and treatment of disturbances in oxygenation.

Lack of Equivalence Between Central and Mixed Venous Oxygen Saturation

STUDY OBJECTIVE

We compared paired samples of central venous O(2) saturation (Scvo(2)) and mixed venous O(2) saturation (Svo(2)) to test the hypothesis that Scvo(2) is equivalent to Svo(2). We also compared O(2) consumption (Vo(2)) computed with Scvo(2) (Vo(2)cv) to that computed with Svo(2) (Vo(2)v).

DESIGN

Prospective, sequential, observational study.

SETTING

Combined medical-surgical ICU.

PATIENTS

Fifty-three individuals > 18 years of age of either sex who required a pulmonary artery catheter (PAC) to guide fluid therapy. Subjects were identified as postsurgical (32 patients) or medical (21 patients) according to their ICU admission diagnosis.

INTERVENTIONS

A PAC was inserted through the internal jugular or subclavian veins. Care was taken to place the PAC proximal port approximately 3 cm above the tricuspid valve. Blood samples were drawn from the proximal and distal ports in random order. An arterial blood sample also was drawn.

MEASUREMENTS

Cardiac output in triplicate, systemic pressure, and central pressure. We analyzed blood samples for hemoglobin concentration and O(2) saturation (So(2)). Data were compared by correlation analysis and by the method of Bland and Altman.

RESULTS

Svo(2) was consistently lower than Scvo(2) (p < 0.0001), with a mean (+/-SD) bias of -5.2 +/- 5.1%. Similar differences in Scvo(2) and Svo(2) were present within each subgroup (p < 0.001). A lower Svo(2) resulted in Vo(2)v values that were higher than the Vo(2)cv values for all patients in the study (mean Vo(2)v, 236.7 +/- 103.4 mL/min; mean Vo(2)cv, 191.1 +/- 84.0 mL/min; p < 0.001) as well as for patients within each subgroup (p < 0.001).

CONCLUSIONS

Measurements of Scvo(2) and Svo(2) were not equivalent in this sample of critically ill patients. Moreover, substituting Scvo(2) for Svo(2) in the calculation of Vo(2) produced unacceptably large errors. The decrease in So(2) between Scvo(2) to Svo(2) may result from the mixing of atrial and coronary sinus blood. As such, this difference may be a marker of myocardial O(2) consumption.

Monitoring the critically ill equine patient

Measurements of physiologic parameters, such as blood pressure or lactate concentration, are useful to detect occult derangements, such as tissue hypoxia and dysoxia. These tools are also useful in determining the effects of therapy. Monitoring techniques are now widely available for the intensive management of critically ill horses and foals. A number of these have evolved into noninvasive or minimally invasive devices and procedures and provide information that can be used for earlier and more dynamic therapeutic intervention. The goal of increased monitoring is to improve the level of care in the ICU; L ultimately. increased survival of critical patients is the motivation behind enhanced monitoring of physiology, with particular attention being paid to trends or alterations over time. This review highlights practical and informative monitoring tools and techniques and provides normal reference values from the literature.

Continuous central venous and pulmonary artery oxygen saturation monitoring in the critically ill

OBJECTIVE

To compare the course of continuously measured mixed and central venous O(2) saturations in high-risk patients and to evaluate the impact of various factors that might interfere with reflection spectrophotometry.

DESIGN AND SETTING

Prospective, descriptive study in the interdisciplinary ICU of a university hospital.

PATIENTS

32 critically ill patients with triple-lumen central vein catheters, including 29 patients requiring pulmonary artery catheterization.

INTERVENTIONS

The accuracy of fiberoptic measurements was assessed by comparison to reference co-oximeter results at regular intervals. We examined the effect on measurement accuracy of physiological variables including hematocrit, hemoglobin, pH, temperature, and the administration of various solutions via central venous catheter. Continuous parallel measurements of SvO(2) and ScvO(2) were performed in patients with each type of catheters over a total observation time of 1097 h.

RESULTS

ScvO(2) values were more accurate and stable than in vitro oximeter measurements ( r=0.96 from 150 samples, mean difference 0.15%, average drift 0.10%/day) and was not significantly affected by synchronous infusion therapy or by changes in hematocrit, hemoglobin, pH, or temperature. ScvO(2) values closely paralleled SvO(2), whether measured in vitro ( r=0.88 from 150 samples) or in vivo ( r=0.81 from 395,128 samples) but averaged about 7+/-4 saturation percentage higher. ScvO(2) changed in parallel in 90% of the 1,498 instances in which SvO(2) changed more than 5% (over an average of 43 min).

CONCLUSIONS

Continuous fiberoptic measurement of central vein O(2) saturation has potential to be a reliable and convenient tool which could rapidly warn of acute change in the oxygen supply/demand ratio of critically ill patients.

The evaluation and management of shock

Shock is an emergency that requires continuous bedside evaluation, resuscitation, and re-evaluation. The initial bedside examination allows the clinician to determine whether the patient exhibits a clinical picture that is consistent with hypovolemic, cardiogenic, or vasodilatory shock. The primary survey dictates urgent initial resuscitation that usually consists of intubation, ventilation, and volume support. Vasoactive therapy is started when the patient is well volume-resuscitated and consists of inotropic support for cardiogenic shock and pressor therapy for vasodilatory shock. The secondary survey is helpful in revealing the cause of shock and necessary to institute early definitive therapy. Early shock has a hemodynamic component, which is often easily reversed. Septic shock and prolonged shock from any cause has an inflammatory component, which is not easily reversed and leads to multiple-system organ failure (MSOF) and death. Success in treatment of shock depends on early recognition of shock and the rapid tempo of resuscitation of its hemodynamic component to prevent or minimize the inflammatory component.

Metabolic disturbances in shock, and the role of ATP-MgCl2 and sex steroids

Hemorrhage following accidental injuries is a common cause of death in the industrialized world. Moreover, the impact of elective surgery and solid organ transplantation sometimes results in low flow conditions similar to those seen following hemorrhagic shock. A shortage in O(2) availability, or hypoxia, leads to sequential changes in cell metabolism and morphology, including inflammatory responses and the expression of hypoxia-inducible transcription factor-1, which controls the cellular adaptation to hypoxia. These endogenous adaptive responses show that O(2) deprivation is not an unforeseen event for cells. The purpose of this review article is to discuss the pathophysiologic principles of shock and the metabolic alterations that cells undergo during low flow conditions. Moreover, the rationale for therapeutic intervention by administering ATP-MgCl(2) and sex steroids following shock and trauma will also be discussed.

Goal-oriented shock resuscitation for major torso trauma: what are we learning?

Shock resuscitation is an obligatory intervention for severely injured patients who present in shock. During the past 15 years, with widespread acceptance of "damage control" surgery and early triage to the intensive care unit (ICU) to optimize resuscitation, the lives of many major trauma victims have been saved, and much has been learned about shock resuscitation. Due largely to the work of Shoemaker et al., a resuscitation strategy based on a standardized process using O(2) delivery index (DO(2)I) as an endpoint and physiologic performance goal for interventions has been developed, studied, and refined for resuscitation of shock caused by major trauma. DO(2)I >or=600 mL O(2)/min-m2 is the only resuscitation endpoint variable that has been tested in prospective randomized trials (PRTs) of trauma patient outcome. These PRTs are limited, and their results are not conclusive. Results from other investigators, including our group, using similar process and endpoints, are indicating similar performance and outcomes. We believe that DO(2)I is a useful endpoint because it integrates three important variables, ie, hemoglobin concentration [Hb], arterial hemoglobin O(2) saturation, and cardiac output. We have found DO(2)I >or=500 mL O(2)/min-m2 to be an endpoint with more general applicability, but we believe that the standardized process is more important than the specific endpoint. To standardize our process, we have developed a computerized decision support tool for shock resuscitation. This technology has provided novel data collection and has permitted refinement of the bedside process. Our data analysis indicates that the next challenge will be to develop a similar pre ICU resuscitation process that will use less invasive monitors and different endpoints. Identification of the high-risk resuscitation nonresponders early in the resuscitation process will be needed to redirect their clinical trajectories. As an endpoint for interventions for goal-directed resuscitation in the critically injured trauma patient, systemic O(2) delivery is the current state of the art and the basis for near future development of clinical processes for resuscitation of shock due to major trauma.

Bench to bedside: electrophysiologic and clinical principles of noninvasive hemodynamic monitoring using impedance cardiography

The evaluation of the hemodynamic state of the severely ill patient is a common problem in emergency medicine. While conventional vital signs offer some insight into delineating the circulatory pathophysiology, it is often impossible to determine the true clinical state from an analysis of blood pressure and heart rate alone. Cardiac output measurements by thermodilution have been the criterion standard for the evaluation of hemodynamics. However, this technology is invasive, expensive, time-consuming, and impractical for most emergency department environments. Impedance cardiography (ICG) is a noninvasive method of obtaining continuous measurements of hemodynamic data such as cardiac output that requires little technical expertise. ICG technology was first developed by NASA in the 1960s and is based on the idea that the human thorax is electrically a nonhomogeneous, bulk conductor. Variation in the impedance to flow of a high-frequency, low-magnitude alternating current across the thorax results in the generation of a measured waveform from which stroke volume can be calculated by a modification of the pulse contour method. To adequately judge the possible role of this technology in the practice of emergency medicine, it is important to have a sufficient understanding of the basic scientific principles involved as well as the clinical validity and limitations of the technique.

Monitoring cardiac function in intensive care

Systolic cardiac function results from the interaction of four interdependent factors: heart rate, preload, contractility, and afterload. Heart rate can be quantified easily at the bedside, while preload estimation has traditionally relied on invasive pressure measurements, both central venous and pulmonary artery wedge. These have significant clinical limitations; however, adult literature has highlighted the superiority of several novel preload measures. Measurement of contractility and afterload is difficult; thus in clinical practice the bedside assessment of cardiac function is represented by cardiac output. A variety of techniques are now available for cardiac output measurement in the paediatric patient. This review summarises cardiac function and cardiac output measurement in terms of methodology, interpretation, and their contribution to the concepts of oxygen delivery and consumption in the critically ill child.

Critical care and emergency medicine

The changing landscape of health care has resulted in an increase in the delivery of critical care in the emergency department. Although the emergency department duration is brief compared with the total length of hospitalization, physiologic determinants of outcome may be established before ICU admission. The care provided during the emergency department stay for critically ill patients has been shown to significantly impact the progression of organ failure and mortality. Despite these trends, formal critical care training for emergency physicians is limited. The need to maximize patient throughput in frequently overcrowded emergency departments hinders the provision of optimal care to the critically ill patient. Methodologies should be developed to examine the quality of patient care and objectively measure the impact of clinical interventions. The potential to improve outcome through educational initiatives and resource allocation should not be viewed as a burden of delivering a higher level of care in this setting but as a significant opportunity to effectively mitigate the socioeconomic consequences.

Measurement of cardiac output and tissue perfusion

Recent technologic innovations have allowed a greater scope for cardiac output measurement in critically ill children. There is a move toward both less invasive and continuous methods, several of which also offer novel measures of preload. Many of the new methods are still undergoing preliminary evaluation in the pediatric population and will be summarized in this article.