Nellcor UK Training Programme

Nellcor is a division of Tyco Healthcare

The Oxyhaemoglobin Dissociation Curve

The oxyhaemoglobin dissociation curve (ODC) is a graphic relationship between haemoglobin oxygen saturation and the partial pressure of oxygen in the blood.

The affinity of haemoglobin for oxygen produces an S-shaped curve representing the way oxygen normally loads onto, and releases from, the haemoglobin molecules. The flat upper portion represents oxygen loading of haemoglobin as blood passes through the lungs. Because the partial pressure of oxygen is high, oxygen binds with the haemoglobin molecule. However, because most haemoglobin molecules are already saturated, additional loading of oxygen onto haemoglobin will not significantly increase as partial pressure continues to increase. The steep lower portion of the curve represents the relationship at the tissue level. Haemoglobin molecules are not well saturated because they have already lost some of their oxygen to tissues. Even with minor reductions in the partial pressure of oxygen, large amounts of oxygen are off-loaded from haemoglobin molecules.

The Normal ODC

The normal ODC, as shown, represents the relationship between changes in haemoglobin saturation and partial pressure of oxygen under certain "normal" conditions.

These include a blood pH of 7.4, PaCO₂ of 40mmHG, temperature of 37^oC, and normal levels of 2,3-DPG.

2,3-DPG

2,3-DPG is a normal product of red blood cell metabolism. Because of its chemical characteristics, 2,3-DPG plays an important physiologic role in regulating affinity between haemoglobin and oxygen. Some conditions will increase the metabolic production of 2,3-DPG, resulting in decreased affinity of haemoglobin for oxygen. Other conditions, listed in the chart below, may result in lower levels of 2,3-DPG and an increased affinity of haemoglobin for oxygen.

Increased 2,3-DPG	Decreased 2,3-DPG
Residence at high altitudes	Stored blood bank
Anaemia	Hypophosphatemia
Chronic hypoxemia	Hypothyroidism
Hyperthyroidism

SHIFTING OF THE ODC

Under conditions where pH, temperature, PaCO₂and 2,3-DPG are normal, a partial pressure of oxygen at 60 mmHg corresponds with an oxygen saturation value of approximately 90%. However since many patients have altered pH, PaCO₂, temperature and/or 2,3-DPG values, this normal relationship may be altered. In this scenario, ODC is described as being "shifted." These curve shifts reflect altered affinity of haemoglobin for oxygen, which affects oxygen loading and unloading. Normal PaO₂/Sa0₂correlation is affected when the curve is shifted.

Right Shift

A right shift in the ODC decreases haemoglobin's affinity for oxygen.
Haemoglobin does not hold onto oxygen as tightly, and off-loading of oxygen is easier.

Factors causing a Right ODC Shift	Clinical Conditions Causing a Right ODC Shift
Decreased pH	Acidosis
Increased temperature	Hyperthermia
Increased PaCO₂	Alveolar hyperventilation
Increased 2,3-DPG	Anemia

Left Shift

A left shift in the ODC increases haemoglobin's affinity for oxygen.
Haemoglobin has a tighter hold on oxygen and off-loading of oxygen is more difficult.

Factors causing a Left ODC Shift	Clinical Conditions Causing a Left ODC Shift
Increased pH	Alkalosis
Decreased temperature	Hypothermia
Decreased PaCO₂	Hyperventilation
Decreased 2,3-DPG	Carboxyhemoglobinemia, hypophosphatemia, fetal hemoglobin

Hypoxaemia: The Incidence and Clinical Significance

Recently, a growing body of research has examined the incidence and significance of hypoxaemia. Although this research base continues to grow, there have been many relevant findings related to risk factors for hypoxaemia and its potential consequences.

HYPOXAEMIA: NEW TERMINOLOGY

"Hypoxaemia" refers to insufficient oxygenation of the arterial blood. Hypoxaemia may lead to "hypoxia," or insufficient oxygenation of the tissues. The consequences of hypoxia can include serious tissue damage, brain damage, and even death. A priority of any healthcare provider is to prevent hypoxaemia and ensure adequate oxygenation of patients.

Hypoxaemia can occur in an episodic or constant fashion. Episodic hypoxaemia is occasionally present and is characterized by sudden changes in Sa0₂. The degree of episodic hypoxaemia may be mild, moderate or severe. For example, patients may have an episodic hypoxaemic event while traveling from the bed to the bathroom, or while climbing stairs in their home. During these episodic events, patients may not have enough oxygen available to meet the metabolic demands required by this activity.

Patients with constant hypoxaemia are those who have consistently low oxygen saturation readings. The degree of constant hypoxaemia can vary. Constant hypoxaemia is often associated with chronic medical conditions, such as chronic obstructive lung disease or cyanotic heart disease.

PATIENTS AT RISK FOR HYPOXAEMIA

Certain patient populations are at greater risk for hypoxaemia and should be considered candidates for continuous monitoring of hypoxaemia with pulse oximetry. These patient populations include:

Patients at risk for Hypoxaemia	Risk Factors
Patients in Noncritical Care Settings (Hospitals and Alternate Care).	Patients cared for in non-critical care areas have complex medical and surgical conditions, which may directly or indirectly affect their cardio-respiratory status. Patient assessment by staff may be less frequent, and patients may be physically located away from the watchful eyes of the caregiver. A decrease in the use of monitoring technologies in these settings may result in hypoxaemia being underestimated.
Postoperative Patients	Episodic postoperative hypoxaemia can occur in the first few hours after surgery. It can also occur up to five nights postoperatively. It can also be associated with sudden unexplained death. Postoperative patients may be at risk for poor oxygenation because of residual effects of anaesthesia, pain-inhibited respiratory movement, analgesic induced respiratory depression and bed rest. Patients who have had major abdominal or thoracic surgeries, or any surgery in the upper airway area, are considered high risk. In addition, those with a significant perioperative oxygenation defect or pre-existing pulmonary disease may be at greater risk during the postoperative period.
Patients in Pain/Receiving Pain Management	Both pain and pain management can contribute to hypoxaemia. Pain can inhibit expansion of the chest wall, and interfere with patient activity and mobility. Pain management has the potential to cause respiratory depression. The Agency for Healthcare Policy and Research clinic guidelines related to pain management recommend opioids as the drug of choice, with frequent mention of respiratory depression as a serious complication of opioid use.
Confirmed Obstructive Sleep Apnoea or Morbidly Obese Patients	Patients often receive little or no assessment of their sleep habits for apnoea, yet the presence of sleep apnoea is clearly associated with the development of hypoxaemia. In the postoperative period, a rebound in REM sleep days after surgery may be a contributing factor to the development of late postoperative hypoxaemia. Since hypoventilation is a major respiratory side effect of opiate administration, patients with sleep apnoea who receive opioids for pain management may be at greater risk for apnea-associated hypoxaemia. However, patients who undergo surgery or receive opioids often have undiagnosed sleep apnoea. These patients may be at heightened risk of hypoxaemia related complications. This is especially true if they are not monitored during their postoperative care, and if they have pulmonary comorbidity.
Patients With Pre-existing Cardiopulmonary Disorders	Patients with severe cardiopulmonary disorders, who have had at least one episode of documented oxygen, increase and were treated with oxygen, are candidates for monitoring with pulse oximetry.
Patients Receiving Conscious Sedation	Sedation, with or without analagesia, may result in the loss of protective reflexes. The Joint Commission on Accreditation of Healthcare Organizations requires protocols addressing the use of pulse oximetry equipment whenever sedation is provided in a manner that may be reasonably expected to result in the loss of protective reflexes.
Neonatal, Paediatric and Elderly Patients	The typical pulmonary reserve in neonatal, paediatric and elderly patients is often decreased, especially in relation to their increased oxygen demands. These patients may desaturate more quickly than the average adult, increasing their risk for hypoxaemia and related complications.
Obstetric Patients	Pain-induced changes in respiration during labour and delivery, and desaturation from epidural morphine or other narcotics following operative delivery, contribute to the risk of hypoxaemia for the obstetric patient. This patient already has diminished pulmonary reserves.
The technology - Dependant Patient	Whether in the inpatient or home care setting, numerous patients depend on technology. Much of this technology enhances optimal oxygenation. Technology-related failures or complications, such as with invasive and non-invasive ventilators and oxygen therapy, may result in hypoxaemia.

HYPOXAEMIA: THE CLINICAL AND ECONOMIC SIGNIFICANCE

In recent years, numerous clinical studies have discussed the possible role of hypoxaemia with adverse clinical outcomes.

Hypoxaemia and Wound Healing/Infection	Oxygen plays an important role in wound healing and infection. Low tissue oxygen levels can compromise wound healing and resistance to infection, leading to prolonged hospital stays and/or healthcare costs.
Hypoxaemia and Ischaemia	Postoperative hypoxaemia has been associated with myocardial ischaemia and arrhythmias in patients, both with and without pre-existing cardiac disease. The risk of myocardial ischaemia is related to the degree and duration of hypoxaemia, and may contribute to heart attack and subsequent death.
Hypoxaemia and Celebral Function	Hypoxaemia can impair cerebral function in a number of ways. These include short-term memory loss, confusion, cognitive dysfunction or permanent impairment. In patients with pre-existing circulatory conditions, hypoxaemia can result in cerebral ischaemia or stroke, and subsequent death.
Hypoxaemia and Unfavorable Healthcare Costs	The consequences of hypoxaemia may result in delayed healing and other complications. These can increase the length of stay, rebound to more expensive areas of care, require additional healthcare resources, decrease the functional ability of the patient, and lead to costly malpractice litigation.

HYPOXAEMIA: TRADITIONAL ASSESSMENT

Although hypoxaemia is a well-recognized safety concern across the health-care continuum, it may present itself in a variety of diverse manners and make assessing patients difficult.

Once traditional method for assessing oxygen is measuring the patient's respiratory rate. However respiratory rate is more often estimated than precise. Additionally, counting respirations has been shown to have virtually no value in detecting hypoxaemia.

Another method of hypoxaemia assessment is evaluating patient skin color. While conventional wisdom holds that if patient skin color changes to a more dusky, bluish or cyanotic tone, the patient is likely hypoxaemic, there are several considerations to keep in mind. Cyanosis is in fact a late sign of hypoxaemia. Cyanosis may not be detected, even in the presence of hypoxaemia, since cyanosis is related to the amount of haemoglobin present in the blood. Patients with severe anaemia may never display signs of cyanosis, despite significant hypoxaemia. Cyanosis is often difficult to detect in patients with certain skin pigments. The cyanosis detection is considered to be subjective at best, and wide observer variability may occur. Therefore, the predictive value of cyanosis for hypoxaemia is poor.

Other assessment parameters, like respiratory effort, mentation and changes in other vital signs, are unreliable indicators of hypoxaemia. Respiratory effort may appear unlabored, even in the presence of hypoxaemia. Although changes in mentation (such as restlessness and confusion) are associated with hypoxaemia, many clinicians don't make the association, and these findings may be attributed to other causes. Changes in other vital signs, including heart rate and ECG, may or may not occur after significant hypoxaemia is present.

These assessment parameters -- including respiratory rate, color, effort, mentation and other vital signs -- usually require the clinician to assess the patient. Even if changes in these parameters result from deteriorating oxygenation status, the clinician is often not present to observe them. Traditional patient assessment activities are usually performed by clinicians when visiting the bedside or home, or at the patient's routine or episodic office visits. Because the oxygenation status of a patient can quickly change, clinicians may not be aware of changes until patient injury or death has occurred. For these reasons, common forms of hypoxaemia detection are inadequate.