Mixed venous oxygen saturation monitoring revisited : thoughts for critical care nursing practice

Background Less invasive methods of determining cardiac output are now readily available. Using indicator dilution technique, for example has made it easier to continuously measure cardiac output because it uses the existing intra-arterial line. Therefore gone is the need for a pulmonary artery floatation catheter and with it the ability to measure left atrial and left ventricular work indices as well the ability to monitor and measure a mixed venous saturation (SvO2). Purpose The aim of this paper is to put forward the notion that SvO2 provides valuable information about oxygen consumption and venous reserve; important measures in the critically ill to ensure oxygen supply meets cellular demand. In an attempt to portray this, a simplified example of the septic patient is offered to highlight the changing pathophysiological sequelae of the inflammatory process and its importance for monitoring SvO2. Relevance to clinical practice SvO2 monitoring, it could be argued, provides the gold standard for assessing arterial and venous oxygen indices in the critically ill. For the bedside ICU nurse the plethora of information inherent in SvO2 monitoring could provide them with important data that will assist in averting potential problems with oxygen delivery and consumption. However, it has been suggested that central venous saturation (ScvO2) might be an attractive alternative to SvO2 because of its less invasiveness and ease of obtaining a sample for analysis. There are problems with this approach and these are to do with where the catheter tip is sited and the nature of the venous admixture at this site. Studies have shown that ScvO2 is less accurate than SvO2 and should not be used as a sole guiding variable for decision-making. These studies have demonstrated that there is an unacceptably wide range in variance between ScvO2 and SvO2 and this is dependent on the presenting disease, in some cases SvO2 will be significantly lower than ScvO2. Conclusion Whilst newer technologies have been developed to continuously measure cardiac output, SvO2 monitoring is still an important adjunct to clinical decision-making in the ICU. Given the information that it provides, seeking alternatives such as ScvO2 or blood samples obtained from femorally placed central venous lines, can unnecessarily lead to inappropriate treatment being given or withheld. Instead when using ScvO2, trending of this variable should provide clinical determinates that are useable for the bedside ICU nurse, remembering that in most conditions SvO2 will be approximately 16% lower.

Effects of dissolved oxygen availability and culture biomass at induction upon the intracellular expression of monoamine oxidase by recombinant E. coli in fed batch bioprocesses

The effects of oxygen availability and induction culture biomass upon production of an industrially important monoamine oxidase (MAO) were investigated in fed-batch cultures of a recombinant E. coli. For each induction cell biomass 2 different oxygenation methods were used, aeration and oxygen enriched air. Induction at higher biomass levels increased the culture demand for oxygen, leading to fermentative metabolism and accumulation of high levels of acetate in the aerated cultures. Paradoxically, despite an almost eight fold increase in acetate accumulation to levels widely reported to be highly detrimental to protein production, when induction wet cell weight (WCW) rose from 100% to 137.5%, MAO specific activity in these aerated processes showed a 3 fold increase. By contrast, for oxygenated cultures induced at WCW's 100% and 137.5% specific activity levels were broadly similar, but fell rapidly after the maxima were reached. Induction at high biomass levels (WCW 175%) led to very low levels of specific MAO activity relative to induction at lower WCW's in both aerated and oxygenated cultures. Oxygen enrichment of these cultures was a useful strategy for boosting specific growth rates, but did not have positive effects upon specific enzyme activity. Based upon our findings, consideration of the amino acid composition of MAO and previous studies on related enzymes, we propose that this effect is due to oxidative damage to the MAO enzyme itself during these highly aerobic processes. Thus, the optimal process for MAO production is aerated, not oxygenated, and induced at moderate cell density, and clearly represents a compromise between oxygen supply effects on specific growth rate/induction cell density, acetate accumulation, and high specific MAO activity. This work shows that the negative effects of oxygen previously reported in free enzyme preparations, are not limited to these acellular environments but are also discernible in the sheltered environment of the cytosol of E. coli cells.

Understanding intraaortic balloon pumping

Intraaortic balloon pumping (IABP) is an established treatment for the support of a failing heart (Christenson, Simonet et al. 1997). It is a process undertaken in most level two and three intensive care units. Despite IABP appearing complex, the principles are straightforward. A sausage shaped intraaortic balloon (IAB) about 250 millimetres long and 15 millimetres in diameter, is placed in the descending aorta and attached to an external pump. The external pump then inflates and deflates the IAB in synchrony with cardiac contraction. The primary purpose of this is the support of a compromised heart with a simultaneous increase in myocardial oxygen supply, and decrease in myocardial oxygen demand (Overwalder, 1999). As a nurse it is worthwhile understanding the principles of IABP. As a hospital intervention, it’s exposure to nursing is high.

Comparison between electrically evoked and voluntary isometric contractions for biceps brachii muscle oxidative metabolism using near-infrared spectroscopy

This study compared voluntary (VOL) and electrically evoked isometric contractions by muscle stimulation (EMS) for changes in biceps brachii muscle oxygenation (tissue oxygenation index, ΔTOI) and total haemoglobin concentration (ΔtHb = oxygenated haemoglobin + deoxygenated haemoglobin) determined by near-infrared spectroscopy. Twelve men performed EMS with one arm followed 24 h later by VOL with the contralateral arm, consisting of 30 repeated (1-s contraction, 1-s relaxation) isometric contractions at 30% of maximal voluntary contraction (MVC) for the first 60 s, and maximal intensity contractions thereafter (MVC for VOL and maximal tolerable current at 30 Hz for EMS) until MVC decreased ∼30% of pre-exercise MVC. During the 30 contractions at 30% MVC, ΔTOI decrease was significantly (P < 0.05) greater and ∼tHb was significantly (P < 0.05) lower for EMS than VOL, suggesting that the metabolic demand for oxygen in EMS is greater than VOL at the same torque level. However, during maximal intensity contractions, although EMS torque (∼40% of VOL) was significantly (P < 0.05) lower than VOL, ΔTOI was similar and ΔtHb was significantly (P < 0.05) lower for EMS than VOL towards the end, without significant differences between the two sessions in the recovery period. It is concluded that the oxygen demand of the activated biceps brachii muscle in EMS is comparable to VOL at maximal intensity. © Springer-Verlag 2009.

Wound healing angiogenesis : the clinical implications of a simple mathematical model

Nonhealing wounds are a major burden for health care systems worldwide. In addition, a patient who suffers from this type of wound usually has a reduced quality of life. While the wound healing process is undoubtedly complex, in this paper we develop a deterministic mathematical model, formulated as a system of partial differential equations, that focusses on an important aspect of successful healing: oxygen supply to the wound bed by a combination of diffusion from the surrounding unwounded tissue and delivery from newly formed blood vessels. While the model equations can be solved numerically, the emphasis here is on the use of asymptotic methods to establish conditions under which new blood vessel growth can be initiated and wound-bed angiogenesis can progress. These conditions are given in terms of key model parameters including the rate of oxygen supply and its rate of consumption in the wound. We use our model to discuss the clinical use of treatments such as hyperbaric oxygen therapy, wound bed debridement, and revascularisation therapy that have the potential to initiate healing in chronic, stalled wounds.

A three species model to simulate application of hyperbaric oxygen therapy to chronic wounds

Chronic wounds are a significant socioeconomic problem for governments worldwide. Approximately 15% of people who suffer from diabetes will experience a lower-limb ulcer at some stage of their lives, and 24% of these wounds will ultimately result in amputation of the lower limb. Hyperbaric Oxygen Therapy (HBOT) has been shown to aid the healing of chronic wounds; however, the causal reasons for the improved healing remain unclear and hence current HBOT protocols remain empirical. Here we develop a three-species mathematical model of wound healing that is used to simulate the application of hyperbaric oxygen therapy in the treatment of wounds. Based on our modelling, we predict that intermittent HBOT will assist chronic wound healing while normobaric oxygen is ineffective in treating such wounds. Furthermore, treatment should continue until healing is complete, and HBOT will not stimulate healing under all circumstances, leading us to conclude that finding the right protocol for an individual patient is crucial if HBOT is to be effective. We provide constraints that depend on the model parameters for the range of HBOT protocols that will stimulate healing. More specifically, we predict that patients with a poor arterial supply of oxygen, high consumption of oxygen by the wound tissue, chronically hypoxic wounds, and/or a dysfunctional endothelial cell response to oxygen are at risk of nonresponsiveness to HBOT. The work of this paper can, in some way, highlight which patients are most likely to respond well to HBOT (for example, those with a good arterial supply), and thus has the potential to assist in improving both the success rate and hence the costeffectiveness of this therapy.

Pressure, oxygen tension and temperature in the periosteal callus during bone healing - An in vivo study in sheep

Adequate blood supply and sufficient mechanical stability are necessary for timely fracture healing. Damage to vessels impairs blood supply; hindering the transport of oxygen which is an essential metabolite for cells involved in repair. The degree of mechanical stability determines the mechanical conditions in the healing tissues. The mechanical conditions can influence tissue differentiation and may also inhibit revascularization. Knowledge of the actual conditions in a healing fracture in vivo is extremely limited. This study aimed to quantify the pressure, oxygen tension and temperature in the external callus during the early phase of bone healing. Six Merino-mix sheep underwent a tibial osteotomy. The tibia was stabilized with a standard mono-lateral external fixator. A multi-parameter catheter was placed adjacent to the osteotomy gap on the medial aspect of the tibia. Measurements of oxygen tension and temperature were performed for ten days post-op. Measurements of pressure were performed during gait on days three and seven. The ground reaction force and the interfragmentary movements were measured simultaneously. The maximum pressure during gait increased (p=0.028) from three (41.3 [29.2-44.1] mm Hg) to seven days (71.8 [61.8-84.8] mm Hg). During the same interval, there was no change (p=0.92) in the peak ground reaction force or in the interfragmentary movement (compression: p=0.59 and axial rotation: p=0.11). Oxygen tension in the haematoma (74.1 mm Hg [68.6-78.5]) was initially high post-op and decreased steadily over the first five days. The temperature increased over the first four days before reaching a plateau at approximately 38.5 degrees C on day four. This study is the first to report pressure, oxygen tension and temperature in the early callus tissues. The magnitude of pressure increased even though weight bearing and IFM remained unchanged. Oxygen tensions were initially high in the haematoma and fell gradually with a low oxygen environment first established after four to five days. This study illustrates that in bone healing the local environment for cells may not be considered constant with regard to oxygen tension, pressure and temperature.

Collaboration and Engagement Throughout The Supply Chain : The Role of Government

Collaboration is acknowledged as a key to continued growth in the Australian construction industry. Government, as a major industry client, has an important role to play with respect to fostering collaboration and ensuring the global competitiveness of the industry. The paper draws upon data collected for the Construction 2020 study and aims to demonstrate that government can a) help to break down the adversarial situation that currently exists between clients, project managers and subcontractors; and b) allow the supply chain to collaborate more effectively in terms of satisfying the relational and financial needs of all parties. Government can also provide a clear set of guidelines (backed up by a functional dispute resolution system) that will promote confidence with respect to forging relationships. Thus, the paper will discuss the way in which public policy can be more closely aligned with actual industry needs in order to promote greater collaboration.