respiratory acidosis Flashcards
what is the normal range of PaO2 in arterial blood?
10-13.5kPa
80-100mmHg
what is the normal range PaCO2 in arterial blood?
4.7-6.0kPa
35-45mmHg
what is the normal PaO2 in alveolar blood?
14kPa
105mmHg
what is the normal PaCO2 in alveolar blood?
4.8kPa
36mmHg
define respiratory acidosis
condition where build-up of CO2 in the blood shift’s pH balance and causes blood to be more acidic (pH<7.35)
what is the mechanism of respiratory acidosis?
Alveolar hypoventilation –> increased PaCO2 (hypercapnia) –> decrease in blood pH
what are the main causes of respirtory acidosis?
- Hypoventilation bc of drugs that suppress breathing (inc powerful pain meds e.g. narcotics and ‘downers’ e.g. benzodiazepines) esp when combined with alcohol
- Diseases of the airway e.g. asthma, chronic obstructive lung disease
- Diseases of the chest e.g. scoliosis which makes the lungs less efficient at filling and emptying
- Diseases affecting the nerves and muscles that drive the lungs to inflate/deflate
- Severe obesity – restricts how much the lungs can expand
what is the ratio that controls pH?
[HCO3-]/pCO2
explain how alkalosis occurs in terms of the henderson-hasselbach equation?
If HCO3- higher than ref and/or CO2 is lower than ref then pH = high –> alkalosis
explain how acidosis occurs in terms of the henderson-hasselbach equation?
If HCO3- lower than ref and/or CO2 is higher than ref then pH = low acidosis
why does CO2 rapidly diffuse across the BBB?
it’s lipid-soluble gas
why does the pH of the CSF change more rapidly?
no proteins to buffer it
why may areas of the brain become relatively hypoxaemic or hyperoxic?
Normally brain can regulate its own local pH. Global acidity of the blood disrupts the local control so areas of the brain may become relatively hypoxaemic or hyperoxic
what is the main cause of the signs and symptoms of respiratory acidosis?
primarily a result of low CSF pH
what are the symptoms of respiratory acidosis?
Headache, drowsiness, lethargy, anxiety, sleepiness, fatigue, memory loss, restlessness, and muscle weakness
what are the signs of respiratory acidosis?
Slowed breathing, gait disturbance, blunted deep tendon reflexes, disorientation, tremor, myoclonic jerks, papilledema, tachycardia, drop in blood pressure, and swelling of blood vessels in the eyes may also be present
how do you diagnose respiratory acidosis and what is the key finding?
- Provisionally diagnosed based on signs and symptoms
- Blood sample to test for pH and ABG will confirm the diagnosis
- Key finding: The pCO2 in the blood will be high, usually over 5.9 kPa (45 mm Hg).
how do you treat respiratory acidosis?
wAimed at the underlying lung disease. May include;
• Bronchodilator drugs to reverse some types of airway obstruction
• Noninvasive positive-pressure ventilation (sometimes called CPAP or BiPAP) or mechanical ventilation if needed
• Oxygen if the blood oxygen level is low
• Treatment to stop smoking
what is acute respiratory acidosis?
PaCo2 is elevated above 6.3kPa (47mmHg) with an accompanying acidemia (blood pH <7.35)
what causes acute respiratory acidosis?
• Occurs when an abrupt failure of ventilation occurs – may be caused by;
o depression of central respiratory centre by cerebral disease or drug
o Inability to ventilate normally bc of neuromuscular disease e.g. myasthenia gravis, amyotrophic lateral sclerosis, Guillain-Barré syndrome, muscular dystrophy
o Airway obstruction related to asthma or chronic obstructive pulmonary disease (COPD) exacerbation
what is chronic respiratory acidosis?
• PaCO2 is elevated above 6.3 kPa (47 mm Hg) but the blood pH is normal or near normal.
what causes chronic respiratory acidosis?
- This is bc renal (metabolic) compensation is occurring and has produced a highly elevated serum bicarbonate (HCO3− >30 mm Hg) – restores pH to near normal
- Chronic RA may be secondary to disorders like COPD
what are the multiple mechanisms of hypoventilation in COPD?
o Decreased responsiveness to hypoxia and hypercapnia
o Increased ventilation-perfusion mismatch leading to increased dead space ventilation
o Decreased diaphragm function secondary to fatigue and hyperinflation
what is chronic respiratory acidosis secondary to?
COPD
o Obesity hypoventilation syndrome
o Neuromuscular disorders e.g. amyotrophic lateral sclerosis
o Severe restrictive ventilatory defects as seen in interstitial fibrosis and thoracic deformities.
what is compensating metabolic alkalosis?
increased bicarbonate retention
what is the max level of plasma HCO3 that can be reached?
45mmol/L
how does metabolic compensation for respiratory acidosis occur?
- After a few days of respiratory acidosis, high blood H+ stimulates kidney to retain bicarbonate
- High plasma bicarbonate compensates for the high PaCO2 – brings bicarbonate/PaCO2 ration back to near normal raises pH to normal
define respiratory alkalosis?
condition where the amount of CO2 found in the blood drops to levels below the normal range. Body system becomes more alkaline
how does respiratory alkalosis occur?
Occurs when a person hyperventilates – produces increased alveolar respiration and expels more CO2 from circulation
• H+ and bicarbonate in the plasma react via carbonic anhydrase to make more CO2 decreased circulating H+ increased pH (alkaline)
what is the primary cause and symptom of respiratory alkalosis?
hyperventilation
what are the signs and symptoms of respiratory alkalosis in acute cases?
accompanied by dizziness, light headedness, agitation, confusions, cramps and tingling or numbing around the mouth and in the fingers and hands.
what are other signs and symptoms of respiratory alkalosis?
Muscle twitching, hyperpnea, chest pain, blurred vision, spasms, and weakness
what are signs and symptoms of extreme cases of respiratory alkalosis?
seizures, irregular heartbeats, and tetany
what are acidosis CNS symptoms?
mainly associated with fatigue e.g. drowsiness, lethargy, anxiety, fatigue, etc
what are alkalosis CNS symptoms?
mainly associated with overactivity: gait disturbance, altered deep tendon reflexes, disorientation, tremor, seizures, myoclonic jerks, etc
what are the metabolic effects of respiratory alkalosis?
- Decrease in blood CO2 causes constriction of cerebral blood vessels
- Alkalaemia shifts the haemoglobin O2 dissociation curve to the left, impairing O2 delivery to the tissue.
- pH related changes in blood Ca2+ levels increase in neuromuscular excitability
- Resp alkalosis causes reduced H+ excretion by the kidney. Some other cation must take its place – normally K+ increased K+ excretion and hypokalaemia
what happens during acute respiratory alkalosis?
• Person may lose consciousness where the ventilation rate will resume to normal
why is chronic respiratory alkalosis symptomless?
bc of metabolic compensation
• Kidney excretes increased bicarbonate to give a metabolic acidosis that compensates for the respiratory alkalosis
when does full compensation occur in chronic respiratory alkalosis?
• If the condition has been present for 7 days or more full compensation may occur
what is the minimum achievable plasma HCO3-?
12 mmol/L
what are common causes of respiratory alkalosis?
- Intracerebral hemorrhage, meningitis, stroke (altered respiratory drive)
- Salicylate and Progesterone drug usage
- Anxiety, hysteria, stress and pain
- Cirrhosis of the liver
- Sepsis
- Elevated body temperature
- Hypoxia
how is respiratory alkalosis diagnosed?
• May be suspected based on symptoms
• Blood sample to test for pH and ABGs to confirm the diagnosis
o pH will be elevated above 7.44. paCO2 in the blood will be low, usually under 35 mmHg.
how is respiratory alkalosis treated? give examples
correcting the condition causing the alkalosis
• Hyperventilation syndrome bc of anxiety - get patient to breathe into a paper bag
o By rebreathing breathed air, patient inhales more CO2 than normal
• Antimicrobials to treat pneumonia or other infections
• Other medications to treat fever, seizures, or irregular heart beats
• Treatment for poisoning if alkalosis related to drug overdose
• Use of mechanical ventilation may be necessary
what is type 1 respiratory failure?
Hypoxic (Pa O2 <60 mm Hg) with normal or low PCO2
what is type 2 respiratory failure?
Hypercapnic (PaCO2 >50 mm Hg) with or without hypoxia
what are common causes of type 1 respiratory failure?
- Ventilation/Perfusion (V/Q) mismatch
- Shunting of blood across lungs
- Poor gas exchange (alveoli filled with fluid eg in pneumonia)
- Decreased minute ventilation (MV) relative to demand
- Increased dead space ventilation (less gas to alveoli)
- Can occur with all disease of the lungs which involve fluid filling or collapse of alveoli. E.g. pulmonary edema, pneumonia, and pulmonary hemorrhage
what is the most common type of respiratory failure?
type 1
why does a patient become hypercapnic when they become hypoxic?
This is bc CO2 diffuses out more easily than O2 diffuses in – steeper concentration gradient between the blood and the alveoli
what is the ventilation perfusion ratio?
match between ventilation of a lobe and blood flow through it
what is the healthy mechanism for a poorly ventilated lobe?
capillaries and arterioles constrict = reduced blood flow
o Healthy adaptive mechanism bc it redirects blood away from poorly ventilated regions to better ventilated ones
define anion gap
difference between the concentrations of the main cation sodium* and the main anions chloride and bicarbonate in the plasma
why are there less anions than cations?
Less anions than cations bc some anions aren’t measured e.g. lactate
what causes a high anion gap?
loss of plasma bicarbonate – usually due to elevated levels of anions like lactate, beta-hydroxybutyrate and acetoacetate
• A high anion gap normally indicates metabolic acidosis
what happens if a patient’s bicarbonate is low but the cholride levels are high? when does this occur?
patient may have a normal anion gap but still be unwell
- Occurs in GI loss of bicarbonate bc of vomiting/diarrhoea or renal loss of bicarbonate bc of renal damage
