Part VI: Microvascular Redistribution
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Authored by: Gary Grist, BS, RN, CCP, LCP
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Mr Gary Grist delivered a seminar on this topic at The Missouri Perfusion Society 16th Annual Meeting titled “Beyond The Fick Equation” on June 10-12, 2011
By now it should be blatantly clear that intracapillary blood flow velocity and perfused capillary density are vital variables that must be balanced in order to maintain homeostasis. If either is out of balance the results can potentially be very bad; shock, organ failure, failure to improve, and death are just some of the more severe consequences of perfusion that is out of balance even though vital signs appear normal.
Various body functions are often described and classified according to organ system; nervous system, urinary tract system, digestive system, circulatory system, etc. Each system is often described as if it functions independently of the others. In reality, they are all related, the purpose being to ensure the survival of each cell so each cell can do its part in contributing to the survival of the entire organism. If something goes out of balance the cells die and eventually the organism dies.
The Microvascular Redistribution system (MR) is a combination of effects of the various body systems; the autonomic nervous system, hormones of the glandular system, chemical sensing mechanisms, pressure sensing systems and humoral response system are all part of the MR system. The MR system ensures that perfusion is adequate for all areas of the body under normal conditions.
Under stress the MR system ensures perfusion to vital areas, sacrificing the “less” important body functions and parts. When the stress is eliminated, adequate perfusion to all areas is restored. The MR system can function globally and regionally. Globally, the MR system ensures perfusion to the entire body to prevent exhaustion (hypoxia and build-up of metabolites in the lethal corner) in the Marathon runner. Cardiac output is maximized, certainly, but perfused capillary density is kept high throughout the body to maximize oxygenation and nutritional supply to tissues and to provide for the removal of metabolites that can cause a change in tissue pH.
Regional MR control is obvious in a healing sternotomy scar. Perfused capillary density is maximized around the area of healing to flood the healing tissues with oxygen and nutrients. The high metabolic needs of healing produce lots of metabolites that must be efficiently removed from shallow, efficient tissue cylinders. Factors which may inhibit good perfused capillary density such as diabetes or poor cardiac output are going to result in a delay of healing or no healing at all. The MR system can also be evident inconsequentially, as in the blushing cheek during an embarrassing moment.
The MR system is quite susceptible to imbalance form many causes. Iatrogenic intervention can result in MR imbalance. One of these quite familiar to perfusionists is cardiopulmonary bypass (CPB). There is an ongoing debate about the need for pulsitility during CPB. Patients who can maintain perfused capillary density (PCD) on bypass probably do well without pulsitility. Patients who cannot maintain PCD without pulsitility may do poorly.
The problem is knowing which patient is which. CPB also overrides the macrovascular controls. The body’s confusion is very obvious when the arterial pressure swings wildly and systemic vascular resistance changes radically without any change in blood flow by the perfusionist. The use of drugs to increase or decrease blood pressure on CPB further compounds the problem. The development of a lethal corner is not only possible but probable when the MR system is subject to such usurpation.
Even off CPB the MR system is often subject to the radical effects of catecholamines or other inotropes. Dopamine, the wonder drug that has saved many lives, and other catecholamines are in part effective because they override the MR system. Sometimes this is helpful, sometimes it is not. In a patient with an hypoxic lethal corner dopamine can aggravate the situation while seemingly improving hemodynamics. For example, 5 u/kg/min of dopamine increases the basal metabolic rate (BMR) by 15% in a normal healthy human. But, increasing the BMR by 15% or more in a sick patient with a large lethal corner by giving 5+ u/kg/min of dopamine may be enough to push him/her over the edge.
By increasing BMR, oxygen consumption and CO2 production are increased causing the lethal corner cells to become more hypoxic and acidotic. What other types of factors effect the MR system? Just about everything; trauma, blood loss, dehydration, temperature change, drugs, infection, poisoning, surgery, anesthesia, stress, fear, anxiety, you name it! Of course, these changes are rarely life threatening to the point of requiring intervention with extracorporeal techniques.
But when they are, what is the goal of the “perfusionist”?
- Is it to provide oxygenation and normalize hemodynamics?
- Is it to ensure brain and kidney function?
These things, among many others, are necessary, but they are just effects of the perfusionist’s true goal which is to maintain or return to normal intracapillary blood flow velocity and perfused capillary density. In this way the perfusionist should complement the MR system so that tissue health can be maintained or restored to normal. Human life simply cannot exist without proper perfusion. Healing cannot take place without balance of the MR system at the microvascular level.
Cells in a large lethal corner will have trouble just surviving, let alone contributing to tissue healing.Failure to recognize the true nature of perfusion at the microvascular level reduces the effectiveness of extracorporeal perfusion techniques, especially long-term applications, to a hit-or-miss treatment. Restoring proper microvascular perfusion is as much a “treatment option” as anitbiotics or surgery, in the proper context of need. Patients are usually capable of enduring inadequate perfusion states for a few hours in the OR. But, long-term applications are different. The perfusionist who fails to understand this will lack consistency in the success of long-term extracorporeal applications.
With this posting we have discussed the four main concepts of Oxygen Pressure Field Theory; the Krogh tissue cylinder, the lethal corner, perfused capillary density and microvascular redistribution. In reality, the oxygen pressure field is not theory. Its existence has been confirmed experimentally for many years. It has been mapped and shown to change during times of increased physiologic demands and stress. The theoretical part actually concerns the “models” which have been proposed to explain how the oxygen pressure field works; the Krogh model, the lethal apple core model, the Krogh cone model, the mitochondria scatter model, etc. These remain academic arguments. However, in past postings I have alluded to practical applications of OPFT for the perfusionist.
With the next postings, the discussion will center on specific practical concepts that OPFT suggests.
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