Respiratory Failure
PKGhatak, MD
Lungs are vital for survival. Disease, congenital abnormality, advanced age, or environmental condition produce changes in the structural and functional state of the lung to a degree that sustaining life becomes precarious and the condition is called respiratory insufficiency. When respiratory insufficiency, producing a low partial pressure of oxygen of arterial blood at or below PaO2 50 mmHg or partial pressure of arterial blood carbon dioxide PaCO2 of 50 mmHg or above, usually both at the same time, is known as respiratory failure.
Lack of Oxygen: It is widely understood that oxygen is essential for cellular respiration and normal bodily functions. Brain cells are particularly venerable to low O2, next are the heart, kidneys and liver.
Why CO2 accumulation is bad: CO2 when dissolves in the blood forms an organic acid CO2 + H2O = H2CO3. The H2CO3 immediately breaks down to H2CO3 = H +HCO3. [H+] enters the cells (RBC) and K[+] is pushed out from the RBC into plasma and excreted in urine; the [HCO3-] combines with Na[+] inside the RBC and forms Na + HCO3 = NaHCO3. NaHCO3 reenters the plasma of blood. The enzyme carbonic anhydrase helps this reaction both ways. By this action, the blood manages to keep the pH at an optimal 7.4. When this Bicarbonate buffer system is overwhelmed by the rapid accumulation of CO2, two other blood buffers namely Phosphate buffer and Protein buffer try to maintain pH. The more CO2 accumulates the pH becomes lower and lower 7.3 -> 7.2-> 7.1. And at this acid pH most cells function poorly and soon will die if the condition persists.
Effects of high CO2 in alveolar O2: Air entering the lung becomes fully saturated with water vapor. That means air pressure is 760-47 = 713 mmHg. The partial pressure of alveolar air O2, PAO2 is 20% of 713 = 160 mmHg. O2 is diluted further by CO2, as CO2 enters the alveoli from blood. Normal PaCO2 is 40 mm Hg. The O2 pressure now is 160 - 40 = 120 mmHg and arterial PaO2 is 110 mmHg (120 to 110) happens during the transition called A-a gradient. Oxygen saturation at PaO2 110 mmHg is 100 %.
In acute respiratory failure, assume that the CO2 is 70 mmHg, and the PAO2 will be 160-70=90 mmHg. At PAO2 90 mmHg arterial blood oxygen saturation is 87 %. That is hypoxemia. (the normal O2 saturation of arterial blood is 93 to 100 %).
A simplified version of the lung is like a bellow seen in a blacksmith workshop. Lungs expand, taking in the fresh air and blowing out stale air by breathing out. Lungs are encased in an elastic box made up of vertebrae, ribs, intercostal and abdominal wall muscles and a muscular diaphragm. The respiratory center is situated in the hindbrain (actually three in number, inspiratory, exploratory in the medulla and pneumotactic center in the pons). The aortic body monitors O2, CO2 and pH of the blood and Carotid bodies monitor O2. The motor nerve for the diaphragm is the phrenic nerve, the neurons are situated in the cervical 4th and a few in C3 and C5 spinal cord segments and spinal motor nerves for the respiratory muscles are supplied by the thoracic T1 to T12 and lumber L2 segments of the spinal cord.
The heart pumps blood to the lung alveoli to pick up oxygen before the oxygen can be distributed to all living tissues of the body and at the same time accumulated waste product of energy generation is carbon dioxide and which must to eliminated. Like a heat exchanger, the lung capillaries exchange CO2 of blood for O2 of the air in the alveoli. The transfer of gases is governed by laws of differential pressure gradient and solubility coefficient. O2 has a high affinity for hemoglobin and CO2 is forced out by an enzyme carbonic anhydrase.
Like a well lubricated machine, the airways of the lungs are kept moist and free of debris by mucus, and the airspace inside of the alveoli is kept open by surfactant. Any malfunction, of the above components or a combination of some of them, will cause respiratory insufficiency that may eventually lead to respiratory failure. Respiratory failure may develop suddenly as in strangulation or drowning called Acute Respiratory failure or after a prolonged period of an illness called Chronic Respiratory Failure as seen in pulmonary emphysema or kyphoscoliosis.
Common causes of Respiratory Insufficiency that may lead to Respiratory Failure.
A. Chest wall: Multiple broken rib fractures and fracture of the sternum produce a frail chest wall limiting patients' ability to inhale. Pleurisy, herpes zoster and other painful chest wall pain can produce respiratory insufficiency. Severe spinal deformities, either congenital or acquired, usually lead to respiratory failure.
B. Reduced Space in the chest cavity: Massive pleural effusion, hemorrhagic effusion, empyema makes less space available for the lungs to expand.
C. Space occupying lesions: Thymoma, Lymphoma.
D. Loss of Lung volume: Pneumonectomy, pulmonary emphysema with bullous formation, pneumothorax, extensive cavitary disease of the lung.
E. Consolidation of lungs: Pneumonia specially in elderly in nursing homes. If pneumonia is bilateral, usually viral pneumonia like COVID, the alveolar surface area available for gas exchange becomes limited, accumulated secretion in the airways prevents normal air turnover in the lungs. That results in low O2 and accumulation of CO2 respectively.
F. Airway obstruction: Obstruction of the trachea by a foreign body, tumors, hemorrhage, large peritonsillar abscess.
G. Airway disease: COPD, Asthma, cancers of main bronchus and trachea. These conditions produce limitations of air entry and exit in the lungs.
H. External pressure on major airways: Strangulation, invasive carcinoma of the thyroid gland.
I. Diseases of mucus and mucus clearance mechanism.: Cystic fibrosis, ciliary dyskinesia, bronchiectasis. Microorganisms flourish under those conditions, producing pneumonia and destruction of lung tissues.
J. Reduced capillary bed: Emphysema, recurrent pulmonary emboli, micro-platelet thrombi in covid, ATTP. Pulmonary fibrosis. These conditions prune surface areas of alveoli and capillaries making O2 transfer difficult.
K. Ventilation - Circulation Mismatch: a segment of the lung may have limitations of air entry due to airway disease or produce reduced blood circulation due to emboli, consolidation, or atelectasis. That will lead to wasted ventilation or transfer of O2.
L. Liquid filled alveolar space: Congestive heart failure, pulmonary edema, intrapulmonary hemorrhage, drowning. Air from the alveoli is pushed out by the liquids.
M. Pump failure: Acute left ventricular failure, massive pericardial effusion, or traumatic pericardial effusion.
N. Diseases of the nerves supplying respiratory muscles: Poliomyelitis, Gillian Berry syndrome, ascending polyneuritis. Succinylcholine intentional use. Botulinum poisoning. Paralysis of muscles that move the chest wall in and out results in ventilatory failure.
O. Central nervous diseases: Respiratory centers are robust and keep functioning as long the blood supply to the respiratory center remains intact. Herniation of the brain from pressure following a massive brain hemorrhage, or removal of CSF by a spinal tap in inappropriate conditions. Opioid overdose.
An abnormal condition of alveolar capillaries, severe iron deficient anemia, emboli in the lung and pulmonary fibrosis, and loss of surface area of alveoli produce low arterial oxygen, whereas, under the same circumstances CO2 easily crosses from blood to alveolar air. That is because before any gas has to combine with hemoglobin or leave hemoglobin for the air in alveoli, that gas has to dissolve in the blood first. And CO2 is 160 times more soluble in blood than O2. The only way CO2 can accumulate in blood if the exchange of air in the alveoli lags behind. That is known as Ventilatory Failure. When a low O2 in arterial blood is present it is called Hypoxemia.
Respiratory Failure.
The common causes of Chronic Respiratory Failures are - advanced COPD, Pneumonia in chronic renal, hepatic, or cardiac failure and pneumonia in the elderly in debilitated conditions. In patients surviving more than 3 days with acute respiratory failure, the kidneys start generating bicarbonate and turn acute to chronic respiratory failure, into chronic respiratory failure.
The common cause of Acute Respiratory Failure is – COPD with pneumonia, Narcotic overdose, sudden myocardial infarction, cardiac arrest and massive hemorrhage, and pulmonary edema.
***********************
No comments:
Post a Comment