1

How is oxygen transported?

A major function of the circulation is to carry oxygen to tissues for use in metabolism. Oxygen delivery ( ), or oxygen transport, is the product of two factors: blood flow, or cardiac output (CO), and the amount of oxygen carried in the blood, or arterial oxygen content (CaO ₂ ):

As oxygen content of blood decreases, can be maintained by a proportionate increase in CO.

Oxygen in blood exist in two forms. It is bound by Hb and dissolved in plasma. The oxygen content of 100 mL of blood is described by the following equation:

where CaO ₂ is arterial oxygen content in milliliters of oxygen per 100 mL of blood, SaO ₂ is percent of arterial Hb saturated with oxygen, and PaO ₂ is partial pressure of arterial oxygen.

Under normal conditions (e.g., absence of pulmonary disease, normal Hb, physiologic shunt of 2%–3%), arterial oxyhemoglobin saturation is approximately 97%, and the partial pressure of dissolved oxygen is approximately 100 mm Hg. CaO ₂ can then be calculated as follows:

This calculation demonstrates that the amount of oxygen dissolved in plasma (0.31 mL/100 mL blood) is negligible compared with the amount carried by Hb (19.5 mL/100 mL blood).

Because oxyhemoglobin binding factor (1.34) and SaO ₂ (0.97) are constant under normal conditions, CaO ₂ varies almost linearly with Hb concentration. Under abnormal conditions, such as obesity and term pregnancy, rapid falls in SaO ₂ during induction of anesthesia result in immediate decreases in CaO ₂ . The compensatory tachycardia may or may not be sufficient to maintain normal .

2

Describe the compensatory mechanisms for blood loss.

Blood loss results in diminished intravascular volume and reduced oxygen-carrying capacity secondary to loss of Hb. As intravascular volume decreases, compensatory vasoconstriction and tachycardia occur in an attempt to preserve CO. Continuing volume loss results in decreased CO, reducing to tissues. Restoration of intravascular volume by infusion of either colloid solution in a 1:1 ratio to blood loss or crystalloid solution in a 3:1 ratio allows normalization of CO and maintenance of hemodynamic stability.

Several mechanisms are available to respond to loss of oxygen-carrying capacity. First, CO may increase with restoration of intravascular volume, maintaining or increasing . Second, at the tissue level, oxygen extraction may increase. Normal mixed venous oxygen saturation is approximately 75%; this indicates that only 25% of available oxygen is being extracted. A substantial reserve of oxygen is available to the tissues, which can be used simply by increasing the amount extracted.

3

What is the minimum acceptable hemoglobin concentration (transfusion trigger)?

If blood loss continues during surgery, even if intravascular volume is maintained, oxygen-carrying capacity eventually falls too low to meet metabolic demands, and red cell transfusion is required. The minimum safe level of Hb, or transfusion trigger, is a question on which much attention has been focused. In the mid-twentieth century, advances in the science of transfusion medicine and the greater safety of blood banking led to routine transfusion of patients to maintain an arbitrary Hb level of >10 g/dL or Hct >30%. However, fueled largely by awareness of the role of blood transfusion in the transmission of acquired immunodeficiency syndrome (AIDS), recognition of morbidity associated with blood transfusion grew. Increasingly, attempts have been made to determine scientifically the transfusion trigger—the Hb threshold at which red cell transfusion is warranted.

From animal models and from experience with otherwise healthy Jehovah’s Witnesses, it is known that survival is possible with a Hct of 5%–6% (Hb 2 g/dL) if normovolemia is maintained. Experience with other chronically anemic patients, such as patients with renal failure, showed that Hct values in the low 20s are routinely tolerated. From these data, it is apparent that previously recommended transfusion triggers of Hb 10 g/dL and a Hct of 30% are unnecessarily restrictive.

Attempts to determine the transfusion trigger have focused in particular on patients with risk factors for cardiac disease. Maximal stress on occurs in the heart, with 70% of available oxygen extracted, as opposed to 25% for the body as a whole. If CaO ₂ decreases, the reserve for increased extraction is low, and the only available compensatory mechanism is to increase coronary blood flow. In patients with coronary artery disease, ability to increase coronary blood flow may be compromised, and the critical Hct level—the transfusion trigger—may be much higher. Similarly, patients with significant valvular heart disease or poor ventricular function as well as patients in whom CaO ₂ is limited by pulmonary disease or who are in hypermetabolic states with large oxygen extractions may have high transfusion triggers.

Although not defining a precise transfusion trigger, several studies concur that the previous transfusion trigger of Hb 10 g/dL or Hct 30% was unnecessarily high. The TRICC (Transfusion Requirements in Critical Care) trial found no significant difference in outcomes in patients assigned to a transfusion trigger of Hb 7 g/dL as opposed to Hb 10 g/dL. Certain complications (i.e., myocardial infarction, congestive heart failure) were higher in the Hb 10 g/dL group. Similarly, a study comparing transfusion thresholds in patients with cardiac risk factors undergoing surgery for hip fracture found no significant improvements in outcomes when using a transfusion threshold of Hb 8 g/dL as opposed to Hb 10 g/dL.

The safest blood transfusion is no transfusion. However, if “no blood transfusion” is not an option, potential complications of blood transfusion can be decreased by using the patient’s own blood, rather than blood from a donor.

As of this writing, the current practice guidelines of the American Society of Anesthesiologists (ASA) for perioperative blood transfusion reflect the consensus that previously recommended thresholds of Hb 10 g/dL were unnecessarily high. The precise transfusion trigger has yet to be determined. The ASA practice guidelines state that “red blood cells should usually be administered when the hemoglobin level is less than 6 g/dL and …are usually unnecessary when the level is more than 10 g/dL.” The guidelines also support the use of preadmission blood collection, acute normovolemic hemodilution, and intraoperative red blood cell recovery in situations where autologous blood is required or preferred.

4

List potential sources of autologous blood.

Physician awareness of the dangers associated with homologous blood transfusions as well as increased pressure from the public to avoid transfusions has led to an increased use of autologous blood sources. These sources are the following:

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  Preoperative autologous blood donation

  
  
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  Acute isovolemic hemodilution (AIHD)

  
  
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  Intraoperative cell salvage

  
  
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  Postoperative cell salvage

Each of the above-listed autologous blood sources has a role to play in avoidance of homologous transfusion.