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Click Here for a Hypoxia Flash Presentation 1. Introduction Hypoxia is a general term used to describe inadequate oxygenation. Tissue hypoxia may be caused either by inadequate oxygen content of the blood, defined as hypoxemia, or by inadequate tissue perfusion, as in low cardiac output states. In some conditions both may be present. Hypoxia is a common problem encountered in surgical practice and there are numerous causes. Delay in finding and recognizing the cause of the hypoxia may result in serious morbidity and mortality. This module will help you to understand the pathophysiology of the condition and enable you to develop a differential diagnosis with a view to appropriate management.
2. Aims After working through this program and applying it to your clinical practice you should be able to:
3. Examples in Practice Examples in practice We have presented you with seven case studies which illustrate conditions you may commonly come across in your surgical practice. We suggest you read each of these clinical scenarios and the accompanying key issues. You will be questioned on these illustrative cases throughout the chapter.
4. What you should think about 4.1 Pathophysiology If you are to be successful in your management of hypoxic patients you must have a sound knowledge of the relevant pathophysiological principles. Disturbances of oxygen transport leading to hypoxia may be considered under the following five headings:
..Can you allocate the cases in section 3 to these categories? Reading the next five sections should assist you in this task. 4.1.1 Reduced alveolar oxygen tension Alveolar oxygen tension (PO2) may be reduced in a variety of circumstances. During transport by air, cabin pressure is usually maintained at about the equivalent of 1500 m above sea level. This will reduce the alveolar PO2 from 104 mmHg at sea level to about 86 mmHg. At altitudes above 4000 m the markedly reduced inspired oxygen tension may cause the clinical condition of altitude sickness which may be rapidly fatal. In clinical practice, alveolar PO2 may be reduced by the presence of other gases such as nitrous oxide excreted following a general anesthetic. Is it safe to transport critically-ill hypoxic patients by air? Increasing the FiO2 to 0.25 will return the alveolar PO2 to normal and therefore oxygen administration may compensate for the changes due to reduced pressure. Changes in pressure may have other potentially dangerous effects. It is essential to insert a chest drain in any patient who could possibly have a pneumothorax, as decompression may dramatically increase the volume of air in the pleural space. “Accurate and rapid identification of respiratory failure is absolutely vital in patient management - untreated, it may be rapidly fatal.” A senior surgeon 4.1.2 Respiratory failure Respiratory failure is defined as an arterial oxygen tension (PaO2) at sea level of less than 60 mmHg. In type I respiratory failure the arterial carbon dioxide tension (PaCO2) is less than 50 mmHg and in type II respiratory failure it is higher than 50 mmHg. Adequate oxygenation of the blood depends on an appropriate matching of the ventilation and perfusion to any area of the lungs. An abnormality of the ratio of ventilation to perfusion is known as a V/Q mismatch and may result in hypoxia. Cyanosis is an indication that at least 50 g/l of hemoglobin are unsaturated. Peripheral cyanosis may result from any cause of poor peripheral perfusion (cold, shock, polycythemia) in addition to hypoxemia, but central cyanosis (of oral mucous membranes) is indicative of a reduced PaO2. Normal arterial blood gases:
It is essential to know the normal values of arterial blood gases and be able to interpret the abnormalities. Hypercapnia (raised PCO2) may complicate hypoxia in surgical patients, particularly in the postoperative period. Think about the mechanism by which the body compensates for raised PCO2. When the body retains CO2 a respiratory acidosis develops. In the first instance, blood pH is restored by means of the normal physiological buffering mechanisms. In the longer term, the kidney excretes an increased amount of H+ into the tubular lumen. This results in correction of the acidosis by increasing the plasma HCO3 - concentration. In patients with chronic CO2 retention the plasma HCO3 - may rise to 35 or higher. This contrasts with the findings in patients with a metabolic acidosis in whom the HCO3 - is lower than normal. In patients with acute ventilatory failure the plasma HCO3 - concentration is usually normal as the renal compensatory mechanisms are much slower than the respiratory compensation. Respiratory failure has five principal causes:
Cerebral Which of these categories may be involved in the development of hypoxia in case 7?
Anything which interferes with the central control of respiration may cause respiratory failure (see diagram below).
Possible causes include hypoxia, hypercapnia, pharmacological agents and conditions causing cerebral injury. Morphine is a very useful analgesic but may have profound respiratory depressant effects. It must be used in a dose carefully titrated to its optimum clinical effect in the elderly and in those with impaired hepatic or renal function. Reduced consciousness with overdosage may predispose to aspiration pneumonia. The analgesic and respiratory depressant effects may both be reversed with naloxone. Remind yourself of the drugs which affect the respiratory center In case 3, why was the patient hypoxic? Shallow respiration suggests inadequate reversal of neuromuscular clocking agents (NMBAs), inadequate recovery from anesthesia or opiate overdose.
Mechanical Ventilation may be inhibited by any condition which affects the function of the respiratory muscles, the mobility of the chest wall or the expansion of the lung. These may include factors affecting the neuromuscular function of the respiratory muscles, abnormalities of the chest wall (flail chest), restricted movements due to pain and space occupying lesions in the pleural space (pneumo-, hemo- or hydro-thorax). At what spinal level will cord transection cause apnea? Identify on the diagram opposite which of the three levels indicated would be associated with apnea. The highest of the three levels causes apnea as the phrenic nerve arises from spinal segments C3, 4 and 5. Airway obstruction Partial airway obstruction increases the work of breathing and total obstruction rapidly results in asphyxia. Potential causes include:
In case 7, why did the patient become so rapidly hypoxic? He had sustained a facial injury which produced an airway obstruction and had also sustained a blunt injury to his chest which could result in hypoventilation due to pain or flail chest. He could also have a pneumothorax or hemothorax. Abnormalities of the pulmonary vasculature Factors which alter the perfusion pressure and the permeability of the pulmonary circulation will cause changes in V/Q matching and in the interstitial fluid volume. These include:
The formation of edema is governed by Starling’s law of capillary function which is demonstrated here:
kPa x 7.5 = mmHg A valuable invasive investigation in the assessment of the cardiac and pulmonary vascular function is the use of the pulmonary artery flotation catheter (PAFC). This is used to measure the pulmonary artery pressure and the pulmonary capillary wedge pressure (PCWP). The Adult Respiratory Distress Syndrome (ARDS) This is a condition of rapid onset which has a number of triggering events and is characterized by profound hypoxemia, non-cardiogenic pulmonary edema, reduced pulmonary compliance and right to left shunt (see 4.1.3). In case 4, what will the patient's pulmonary capillary wedge pressure be? His PCWP is likely to be raised above 15 mmHg. Pulmonary edema due to cardiac failure and ARDS presents with similar clinical features but must be differentiated. In pulmonary edema associated with cardiac failure the pulmonary capillary wedge pressure (PCWP) is greater than 15 mmHg whereas in ARDS, where the defect is predominantly due to increased capillary permeability, the pressure is less than 12 mmHg. The common causes include:
The pathological features detected in the lungs vary according to the etiological factors but the usual findings are:
These features are produced by an increased pulmonary vascular resistance and increased capillary permeability. The accumulated neutrophils release proteases which further increase the permeability of the capillary, resulting in exudate of protein rich fluids. Other contributory factors include endotoxin, prostaglandins, leukotrienes, activated complement, oxygen free radicals released from leucocytes and neurogenic effects. The patient therefore develops areas of lung which are ventilated but not perfused and other areas which are perfused but not ventilated. This causes major changes in the V/Q ratio. The increased interstitial fluid volume acts as a diffusion barrier and reduces pulmonary compliance. In case 2, why is she hypoxic? Septicemia has caused the pathological changes described above resulting in ARDS. Inhalation injury Airway burns produce serious respiratory problems due to a combination of thermal injury and inhaled toxins. Pulmonary embolism (PE) Embolism of the pulmonary circulation with thrombus results in a major alteration in the V/Q match and may precipitate right heart failure. Fat embolism Fat embolism is more common in young men with femoral shaft fractures but may occur even in the absence of bony injury. It occurs in 2-22% of patients with long bone fractures. There are two theories about the pathology of fat embolism. The first suggests a release of marrow content into venous sinuses in the medullary cavity of the bone with subsequent embolization. The second suggests that the etiology is related to an increase in circulating free fatty acids which are toxic to endothelial cells. Defective hepatic function due to hypovolemic shock may exacerbate the effects by reducing the capacity for fatty acid clearance. Lung parenchymal disease Parenchymal disease may result in reduced lung compliance, V/Q mismatch or a barrier to diffusion. Causes include:
The diagnosis of ‘pneumonia’ in surgical patients can be controversial. Pneumonia is generally accepted to be diagnosed in a patient with pyrexia, pulmonary abnormalities on chest X-ray, and a positive sputum culture. However, many postoperative patients have pyrexia and positive sputum culture due to upper airway contamination. The pulmonary fields on X-ray may be abnormal because of atelectasis rather than infection. Definitive diagnosis may, therefore, be difficult and many patients may have the diagnosis of pneumonia made in error. In case 5, why did the patient develop postoperative pneumonia? She may have pre-existing lung disease. Because she has cancer and is malnourished, she may be immunosuppressed and have diminished respiratory muscular strength. She may be hypoventilating because of pain or opiate overdose and may have suffered aspiration of enteral contents. “Pneumonia is commonly unrecognized in the elderly who are at risk of aspiration and who are very susceptible to opiate overdose. The diagnosis may only be made when the patient is moribund.” A senior surgeon 4.1.3 Veno-arterial shunting A right to left shunt results in venous blood passing into the arterial circulation without being oxygenated. This may have a profound effect on the PaO2 and increasing the inspired oxygen concentration (FiO2) will not fully correct the reduced PaO2. This is because the shunted blood is not exposed to the raised oxygen tension. The small right to left shunt in normal individuals through the bronchial circulation is sufficient to reduce the PO2 of 104 mmHg in the pulmonary vein to 95 mmHg in the systemic arterial circulation. Many of the conditions described under respiratory failure exert their effects by producing a physiological shunt. There are however some significant pathological anatomical shunts of which the best recognized is that produced by the congenital cardiac anomaly - the Tetralogy of Fallot. 4.1.4 Defective oxygen transport The transport of oxygen is dependent on the oxygen carrying capacity of the blood and on blood flow. “In hemorrhaging patients it is essential to replace volume rapidly but the oxygen carrying capacity of the circulating fluid must not be forgotten.” A consultant anesthetist
Oxygen dissociation curve Oxygen carriage The oxygen carrying capacity of the blood is related to the hemoglobin concentration, as 97% of the oxygen is carried bound to hemoglobin. One gram of hemoglobin carries 1.34 ml of oxygen when fully saturated and therefore in a normal person with a hemoglobin of 150 g/l there will be 200 ml carried by every liter of blood. In normal individuals the hemoglobin is over 97% saturated in the arterial circulation and therefore the amount of oxygen carried is directly related to the hemoglobin concentration. Clearly patients who are anemic will tolerate desaturation less well. The uptake and release of oxygen by hemoglobin is governed by the oxygen dissociation curve as shown opposite. The oxygen dissociation curve is shifted to the right by an increase in the temperature and the concentrations of hydrogen ions, carbon dioxide and 2-3 DPG (diphosphoglycerate), an enzyme facilitating oxygen transport in red blood cells. This enables the hemoglobin to give up more oxygen in metabolically active tissues. Oxygen carriage may be abnormal in patients with abnormal hemoglobin as in the congenital hemoglobinopathies. Is the Hb dissociation curve of transfused blood shifted to the right or the left? The curve is shifted to the left by a reduced 2-3 DPG content which makes it less able to give up oxygen in the tissues. It is debatable whether this is clinically significant. It takes about 24 hours for the 2-3 DPG concentrations to be replenished. Blood flow Blood flow is governed by three components:
For the next five patients you see with major hemorrhage, note the initial hematocrit and the hematocrit following resuscitation to see how accurately the loss has been estimated and replaced. In case 1, at what level of hematocrit would you expect the patient's oxygen delivery to deteriorate? It appears from the curve that oxygen delivery begins to fall as the hematocrit drops below 30%. However the delivery may still be adequate down to 25-30% provided there are not significantly increased demands and cardiac function is adequate. The normal hematocrit is higher than that at which the curve reaches its peak and this is an inbuilt safety measure. Obviously if you operate on someone whose hematocrit is at the peak of the curve the oxygen delivery will fall as soon as you lose any blood. 4.1.5 Defective tissue oxygen uptake Defective tissue utilization of oxygen may be due to two factors:
4.2 Differential diagnosis You should now have an understanding of the pathophysiology of hypoxia and this will enable you to produce a differential diagnosis. The differential diagnosis is related to the clinical scenario in which the patient presents. The following are common scenarios. Which of the example patients are likely to have a reduced blood flow?
4.2.1 Preoperative hypoxia
4.2.2 Postoperative hypoxia Immediately after operation
Within 72 hours of operation
More than three days after operation
4.2.3 Hypoxia following trauma On arrival in the Accident and Emergency Department (A&E)
Following resuscitation
Consider the seven example cases. Can you allocate them to any of these diagnostic categories?
For the next ten patients with hypoxia that you see, try to put them into one of the diagnostic categories. Include examples of the various clinical scenarios in your portfolio. 5.What you should do When you are presented with a patient with hypoxia you should consider the situation with regard to the different categories described in section 4.2. This will then give you a working differential diagnosis with which to target your investigations. 5.1 Assessment of the hypoxic patient Assessment of the patient involves the standard routine of history, examination and investigation. 5.1.1 History In a preoperative patient a careful history of any pulmonary or cardiac problems is essential. Exercise tolerance is probably the most useful indicator of cardio-respiratory fitness. A history of smoking is a major risk factor for respiratory and cardiovascular disease. Cough, purulent sputum, wheeze and hemoptysis are important symptoms and ankle edema may be an indication of cardiac failure. Previous venous thrombo-embolism is a significant concern in patients undergoing major surgery and recent myocardial infarction (less than three months) considerably increases the risk of re-infarction. Take a careful drug history, including the frequency of use of inhalers for example. Note details of any previous operations. In trauma patients it is vital to gain some impression of the mechanism of the injury and the severity of the injuring force. “I find that frequently the patient’s drug history gives me a better indication of the severity of the respiratory condition than their symptoms.” A senior clinician 5.1.2 Examination A careful examination is fundamental to the care of any patient and may well give some indication of the cause of hypoxia. In the preoperative situation this will usually be identified by a low oxygen saturation detected by saturation monitor. Oxygen saturation monitors are routinely used to monitor patients in theatre and in the post operative period. The monitor is applied to a finger (or ear lobe or other appendage) and functions on a colorimetric assessment of the hemoglobin. It is unreliable in the absence of pulsatile flow, e.g. in patients who are cold or shocked. Most monitors will display an error warning in this situation.
Make a list of the conditions which may cause a false reading of oxygen saturation on the pulse oximeter.
The following features should be carefully assessed.
o rapid panting respiration using accessory muscles with emphysema, pneumonia, pulmonary contusion, ARDS o pursed lips and wheeze with obstructive lung disease o rapid respiration with frothy sputum in cardiac failure o slow and shallow in CNS depression o struggling, shallow breathing with neurological problems, neuromuscular blockade, pulmonary collapse and pain.
o reduced movement with neuromuscular problems and pain o hyperinflation with emphysema o |
