Answer:
This is the banking of red cell units from the patient before planned surgery.
PAD was stimulated by concerns about viral transmission by donor blood, especially during the HIV epidemic of the early 1980s. With a red cell storage-life of 35 days at 4°C, most healthy adult patients can donate up to three red cell units before elective surgery. Patients may be given iron supplements, sometimes with erythropoietin, to prevent anaemia or allow more donations to be collected. The Blood Safety and Quality Regulations (BSQR, 2005) require that donations for PAD must be performed in a licensed blood establishment, rather than a routine hospital setting. The donations must be processed and tested in the same way as donor blood and are subject to the same requirements for traceability.
Given the current remote risk of viral transfusion-transmitted infection by donor blood in developed countries, the rationale, safety and cost-effectiveness of routine PAD has been severely questioned (see 2007 British Committee for Standards in Haematology (BCSH) Guidelines for Policies on Alternatives to Allogeneic Blood Transfusion. 1. Predeposit Autologous Blood Donation and Transfusion – https://b-s-h.org.uk) and the procedure is now rarely performed in the UK. Although PAD may reduce exposure to donor blood, it does not reduce overall exposure to transfusion procedures or protect against wrong blood into patient episodes due to identification errors at collection from the blood bank or at the bedside. Indeed, the availability of autologous blood may increase the risk of unnecessary transfusion. Most Jehovah’s Witnesses will decline PAD (see Chapter 12). Clinical trials of PAD are mainly small and of low quality and do not provide strong evidence that the risks outweigh the benefits. The BCSH guideline on PAD only recommends its use in ‘exceptional circumstances’, and lists the following indications for PAD:
Patients with rare blood groups or multiple blood group antibodies where compatible allogeneic (donor) blood is difficult to obtain.
Patients at serious psychiatric risk because of anxiety about exposure to donor blood.
Patients who refuse to consent to donor blood transfusion but will accept PAD.
Children undergoing scoliosis surgery (in practice, most specialist units now use other blood conservation measures).
PAD should only be considered in surgery with a significant likelihood of requiring transfusion, operation dates must be guaranteed and the patient’s ability to donate safely must be assessed by a ‘competent clinician’, usually a transfusion medicine specialist. Adverse events and reactions associated with PAD (or other autologous transfusion systems) should be reported to the Serious Hazards of Transfusion (SHOT) haemovigilance scheme and the Medicines and Healthcare Products Regulatory Agency (MHRA).
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Depth perception is the ability to see the world in 3 dimensions and by doing so to be able to estimate the distance between objects.
Retinal disparity<span> is defined as the way that your left eye and your right eye view slightly different images. </span>
Convergence refers to the eyes' disposition to rotate inward toward each other in a coordinated manner in order to focus effectively on nearby objects. <span>
</span>Disparity and convergence are the two important things that help us to judge distance. W<span>hen looking at a close-up object, your eyes angle inwards towards each other (you become slightly cross-eyed). The extra effort used by the muscles on the outside of each eye gives a clue to the brain about how far away the object is. This is convergence.</span>
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Explanation
