Hypotension during gradual blood loss: waveform variables response and absence of tachycardia

Oxygen saturation in intraosseous sternal blood measured by CO-oximetry and evaluated non-invasively during hypovolaemia and hypoxia - a porcine experimental study

PURPOSE

This study intended to determine, and non-invasively evaluate, sternal intraosseous oxygen saturation (SsO₂) and study its variation during provoked hypoxia or hypovolaemia. Furthermore, the relation between SsO₂ and arterial (SaO₂) or mixed venous oxygen saturation (SvO₂) was investigated.

METHODS

Sixteen anaesthetised male pigs underwent exsanguination to a mean arterial pressure of 50 mmHg. After resuscitation and stabilisation, hypoxia was induced with hypoxic gas mixtures (air/N₂). Repeated blood samples from sternal intraosseous cannulation were compared to arterial and pulmonary artery blood samples. Reflection spectrophotometry measurements by a non-invasive sternal probe were performed continuously.

RESULTS

At baseline SaO₂ was 97.0% (IQR 0.2), SsO₂ 73.2% (IQR 19.6) and SvO₂ 52.3% (IQR 12.4). During hypovolaemia, SsO₂ and SvO₂ decreased to 58.9% (IQR 16.9) and 38.1% (IQR 12.5), respectively, p < 0.05 for both, whereas SaO₂ remained unaltered (p = 0.44). During hypoxia all saturations decreased; SaO₂ 71.5% (IQR 5.2), SsO₂ 39.0% (IQR 6.9) and SvO₂ 22.6% (IQR 11.4) (p < 0.01), respectively. For hypovolaemia, the sternal probe red/infrared absorption ratio (SQV) increased significantly from baseline (indicating a reduction in oxygen saturation) + 5.1% (IQR 7.4), p < 0.001 and for hypoxia + 19.9% (IQR 14.8), p = 0.001, respectively.

CONCLUSION

Sternal blood has an oxygen saturation suggesting a mixture of venous and arterial blood. Changes in SsO₂ relate well with changes in SvO₂ during hypovolaemia or hypoxia. Further studies on the feasibility of using non-invasive measurement of changes in SsO₂ to estimate changes in SvO₂ are warranted.

Acute Hemodilution is Safe in Patients with Comorbid Illness Undergoing Partial Hepatectomy

Despite increased use of acute hemodilution (AH) to reduce perioperative blood transfusion in patients undergoing partial hepatectomy, there is a lack of data on safety in those with comorbid medical illness. We performed a retrospective review of 96 patients undergoing partial hepatectomy with AH and low central venous pressure (CVP) anesthesia. The cohort was compared with 63 patients undergoing partial hepatectomy using standard anesthetic management (SA) and low CVP anesthesia. All were American Society of Anesthesiologists physical status 3 or 4. 58 per cent were baseline hypertensive on medication. Hepatic resection was predominantly performed for metastatic colorectal (41%) and primary hepatic (32%) cancer. Forty per cent underwent major hepatectomy. The mean volume of blood removed for AH was 497 mL (range, 0 to 1 L). Most achieved low CVP (AH 90% vs SA 84%, P = 0.3). Blood loss was lower with AH (mean 480 mL vs 904 mL, P < 0.001). Blood transfusion rate was 74 per cent lower with AH ( P < 0.001). There was no difference in cardiac, respiratory, renal, or overall complications with AH compared with SA. Acute hemodilution is well tolerated by patients with comorbid illness undergoing partial hepatectomy, favoring ongoing use and further study.

Postoperative ward monitoring - Why and what now?

The postoperative ward is considered an ideal nursing environment for stable patients transitioning out of the hospital. However, approximately half of all in-hospital cardiorespiratory arrests occur here and are associated with poor outcomes. Current monitoring practices on the hospital ward mandate intermittent vital sign checks. Subtle changes in vital signs often occur at least 8-12 h before an acute event, and continuous monitoring of vital signs would allow for effective therapeutic interventions and potentially avoid an imminent cardiorespiratory arrest event. It seems tempting to apply continuous monitoring to every patient on the ward, but inherent challenges such as artifacts and alarm fatigue need to be considered. This review looks to the future where a continuous, smarter, and portable platform for monitoring of vital signs on the hospital ward will be accompanied with a central monitoring platform and machine learning-based pattern detection solutions to improve safety for hospitalized patients.

Do intravascular hypo- and hypervolaemia result in changes in central blood volumes?

BACKGROUND

Hypovolaemia is generally believed to induce centralization of blood volume. Therefore, we evaluated whether induced hypo- and hypervolaemia result in changes in central blood volumes (pulmonary blood volume (PBV), intrathoracic blood volume (ITBV)) and we explored the effects on the distribution between these central blood volumes and circulating blood volume (Vd circ).

METHODS

Six anaesthetized, spontaneously breathing Foxhound dogs underwent random blood volume alterations in steps of 150 ml (mild) to 450 ml (moderate), either by haemorrhage, retransfusion of blood, or colloid infusion. PBV, ITBV and Vd circ were measured using (transpulmonary) dye dilution. The PBV/Vd circ ratio and the ITBV/Vd circ ratio were used as an assessment of blood volume distribution.

RESULTS

68 blood volume alterations resulted in changes in Vdcirc ranging from -33 to +31%. PBV and ITBV decreased during mild and moderate haemorrhage, while during retransfusion, PBV and ITBV increased during moderate hypervolaemia only. The PBV/Vd circ ratio remained constant during all stages of hypo- and hypervolaemia (mean values between 0.20-0.22). This was also true for the ITBV/Vd circ ratio, which remained between 0.31 and 0.32, except for moderate hypervolaemia, where it increased slightly to 0.33 (0.02), P<0.05.

CONCLUSIONS

Mild to moderate blood volume alterations result in changes of Vd circ, PBV and ITBV. The ratio between the central blood volumes and Vd circ generally remained unaltered. Therefore, it could be suggested that in anaesthetized spontaneously breathing dogs, the cardiovascular system maintains the distribution of blood between central and circulating blood volume.

Perioperative cardiovascular monitoring of high-risk patients: a consensus of 12

A significant number of surgical patients are at risk of intra- or post-operative complications or both, which are associated with increased lengths of stay, costs, and mortality. Reducing these risks is important for the individual patient but also for health-care planners and managers. Insufficient tissue perfusion and cellular oxygenation due to hypovolemia, heart dysfunction or both is one of the leading causes of perioperative complications. Adequate perioperative management guided by effective and timely hemodynamic monitoring can help reduce the risk of complications and thus potentially improve outcomes. In this review, we describe the various available hemodynamic monitoring systems and how they can best be used to guide cardiovascular and fluid management in the perioperative period in high-risk surgical patients.

Perioperative fluid therapy: a statement from the international Fluid Optimization Group

BACKGROUND

Perioperative fluid therapy remains a highly debated topic. Its purpose is to maintain or restore effective circulating blood volume during the immediate perioperative period. Maintaining effective circulating blood volume and pressure are key components of assuring adequate organ perfusion while avoiding the risks associated with either organ hypo- or hyperperfusion. Relative to perioperative fluid therapy, three inescapable conclusions exist: overhydration is bad, underhydration is bad, and what we assume about the fluid status of our patients may be incorrect. There is wide variability of practice, both between individuals and institutions. The aims of this paper are to clearly define the risks and benefits of fluid choices within the perioperative space, to describe current evidence-based methodologies for their administration, and ultimately to reduce the variability with which perioperative fluids are administered.

METHODS

Based on the abovementioned acknowledgements, a group of 72 researchers, well known within the field of fluid resuscitation, were invited, via email, to attend a meeting that was held in Chicago in 2011 to discuss perioperative fluid therapy. From the 72 invitees, 14 researchers representing 7 countries attended, and thus, the international Fluid Optimization Group (FOG) came into existence. These researches, working collaboratively, have reviewed the data from 162 different fluid resuscitation papers including both operative and intensive care unit populations. This manuscript is the result of 3 years of evidence-based, discussions, analysis, and synthesis of the currently known risks and benefits of individual fluids and the best methods for administering them.

RESULTS

The results of this review paper provide an overview of the components of an effective perioperative fluid administration plan and address both the physiologic principles and outcomes of fluid administration.

CONCLUSIONS

We recommend that both perioperative fluid choice and therapy be individualized. Patients should receive fluid therapy guided by predefined physiologic targets. Specifically, fluids should be administered when patients require augmentation of their perfusion and are also volume responsive. This paper provides a general approach to fluid therapy and practical recommendations.

Calculation of the Respiratory Modulation of the Photoplethysmogram (DPOP) Incorporating a Correction for Low Perfusion

DPOP quantifies respiratory modulations in the photoplethysmogram. It has been proposed as a noninvasive surrogate for pulse pressure variation (PPV) used in the prediction of the response to volume expansion in hypovolemic patients. The correlation between DPOP and PPV may degrade due to low perfusion effects. We implemented an automated DPOP algorithm with an optional correction for low perfusion. These two algorithm variants (DPOPa and DPOPb) were tested on data from 20 mechanically ventilated OR patients split into a benign "stable region" subset and a whole record "global set." Strong correlation was found between DPOP and PPV for both algorithms when applied to the stable data set: R = 0.83/0.85 for DPOPa/DPOPb. However, a marked improvement was found when applying the low perfusion correction to the global data set: R = 0.47/0.73 for DPOPa/DPOPb. Sensitivities, Specificities, and AUCs were 0.86, 0.70, and 0.88 for DPOPa/stable region; 0.89, 0.82, and 0.92 for DPOPb/stable region; 0.81, 0.61, and 0.73 for DPOPa/global region; 0.83, 0.76, and 0.86 for DPOPb/global region. An improvement was found in all results across both data sets when using the DPOPb algorithm. Further, DPOPb showed marked improvements, both in terms of its values, and correlation with PPV, for signals exhibiting low percent modulations.

Respiratory variations in the arterial pressure during mechanical ventilation reflect volume status and fluid responsiveness

Optimal fluid management is one of the main challenges in the care of the critically ill. However, the physiological parameters that are commonly monitored and used to guide fluid management are often inadequate and even misleading. From 1987 to 1989 we published four experimental studies which described a method for predicting the response of the cardiac output to fluid administration during mechanical ventilation. The method is based on the analysis of the variations in the arterial pressure in response to a mechanical breath, which serves as a repetitive hemodynamic challenge. Our studies showed that the systolic pressure variation and its components are able to reflect even small changes in the circulating blood volume. Moreover, these dynamic parameters provide information about the slope of the left ventricular function curve, and therefore predict the response to fluid administration better than static preload parameters. Many new dynamic parameters have been introduced since then, including the pulse pressure (PPV) and stroke volume (SVV) variations, and various echocardiographic and other parameters. Though seemingly different, all these parameters are based on measuring the response to a predefined preload-modifying maneuver. The clinical usefulness of these 'dynamic' parameters is limited by many confounding factors, the recognition of which is absolutely necessary for their proper use. With more than 20 years of hindsight we believe that our early studies helped pave the way for the recognition that fluid administration should ideally be preceded by the assessment of "fluid responsiveness". The introduction of dynamic parameters into clinical practice can therefore be viewed as a significant step towards a more rational approach to fluid management.

Near-infrared spectroscopy: exposing the dark (venous) side of the circulation

The safety of anesthesia has improved greatly in the past three decades. Standard perioperative monitoring, including pulse oximetry, has practically eliminated unrecognized arterial hypoxia as a cause for perioperative injury. However, most anesthesia-related cardiac arrests in children are now cardiovascular in origin, and standard monitoring is unable to detect many circulatory abnormalities. Near-infrared spectroscopy provides noninvasive continuous access to the venous side of regional circulations that can approximate organ-specific and global measures to facilitate the detection of circulatory abnormalities and drive goal-directed interventions to reduce end-organ ischemic injury.