1 / 16

VENTILATION

VENTILATION. CHAPTER 4 DR. CARLOS ORTIZ BIO-208. PARTIAL PRESSURES OF RESPIRATORY GASES.

kiley
Télécharger la présentation

VENTILATION

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. VENTILATION CHAPTER 4 DR. CARLOS ORTIZ BIO-208

  2. PARTIAL PRESSURES OF RESPIRATORY GASES • AIR IS A GAS MIXTURE OF MOSTLY N2 AND O2. THIS TRACES OF ARGON, CARBON DIOXIDE, AND OTHER GASES. THE TOTAL COMBINED PRESSURE EXERTED BY THESE GASES, THE BAROMETRIC PRESSURE(PB) CAN BE MEASURED BY A Hg BAROMETER. AT SEA LEVEL THE PB IS EQUIVALENT TO THE WEIGHT OF A MERCURY COLUMN OF 760 mmHg. THUS STANDARD PB IS 760 mmHg. • ACCORDING TO DALTON’S LAW , THE PRESSURE OF EACH GAS COMPRISING AIR IS INDEPENDENT OF THE OTHER GAS PRESSURE AND EXERTS A PARTIAL PRESSURE PROPORTIONAL TO ITS FRACTIONAL CONCENTRATION IN AIR. • OXYGEN CONSTITUTES 20.93% OF DRY ATMOSPHERIC AIR, ITS PARTIAL PRESSURE (PO2) AT SEA LEVEL IS AS FOLLOWS: • PO2= 0.2093 X760 =159 mm Hg

  3. PARTIAL PRESSURES OF RESPIRATORY GASES • IN PHYSIOLOGICAL CALCULATIONS THE PCO2 OF INSPIRED AIR IS CONSIDERED O. • INSPIRING AIR HEATS IT TO BODY TEMPERATURE (37ºC) AND SATURATES IT WITH WATER VAPOR ( 100% RELATIVE HUMIDITY). THE WATER PRESSURE AT 37ºC AND 100% RH IS 47mmHg. THEREFORE 47 MOST BE SUBSTRUCTED FROM PB TO CALCULATE PGAS IN THE LUNG OR BLOOD. • IN THE TRACHEA THE PO2 IS CALCULATED AS FOLLOWS: • P02= 0.2093 X ( 760 -47) = 149 mmHg.

  4. CLASIFICATION OF VENTILATION • MINUTE OR TOTAL VENTILATION • MINUTE VENTILATION(VE) IS USUALLY DEFINED AS THE VOLUME OF FRESH AIR ENTERING THE LUNG EACH MINUTE. IN CLINICAL PRACTICE, VE IS USUALLY MEASURED BY ADDING THE COLLECTIVE EXHALED VTS OBTAINED OVER 1 MINUTE. VE IS CALCULATED AS FOLLOWS: • VE= VT x RR • FOR EXAMPLE : VE= 500 mL x 12/ min = 6000mL OR 6.O L/min • NOT ALL THE VE REACHES THE ALVEOLI BECAUSE THE LAST PART OF EACH INSPIRATION STAYS IN THE CONDUCTING AIRWAYS TO BE REMOVED WITH THE NEXT EXPIRATION. THUS PART OF THE VE VENTILATES ALVEOLI, ALVEOLARVENTILATION(VA). THE REMAINING PART VENTILATES CONDUCTING AIRWAYS, DEAD-SPACE VENTILTION(VD)

  5. DEAD SPACE VENTILATION • ANATOMICAL DEAD SPACE • THE CONDUCTING AIRWAYS FROM THE MOUTH AND NOSE DOWN TO AND INCLUDING TERMINAL BRONCHIOLES CONSTITUTE ANATOMICAL DEAD SPACE(VDANAT) . • DEAD SPACE IS WASTED VENTILATION AND IT TAKES NO PART IN RESPIRATION. DEAD SPACE IS DEFINED AS LUNG AREAS THAT ARE VENTILATED BUT NOT PERFUSED BY THE PULMONARY CIRCULATION. • VDANAT DOES NOT CHANGE UNLESS SURGERY REMOVES PART OF A LUNG OR UNLESS AN ARTIFICIAL AIRWAY (ET TUBE) BYPASSES UPPER AIRWAY DEAD SPACE. • FOR CLINICAL MONITORING PURPOSES, VDANAT VOLUME IS CONSIDERED CONSTANT AT ANY TIME. • MEASURING VDANAT • VDANAT IS RELATED TO LUNG SIZE; IN NORMAL ADULTS, ANATOMICAL DEAD SPACE IS APROXIMATELY 1mL PER POUND OF IDEAL BODY WEIGHT.

  6. ALVEOLAR AND PHYSIOLOGICAL DEAD SPACE • ALVEOLAR DEAD SPACE(VDA) IS THE VOLUME CONTAINED IN NONPERFUSED ALVEOLI, OR ALVEOLI WITH NO BLOOD FLOW. VDA IS ABNORMAL. ANY FACTOR DECREASING PULMONARY BLOOD FLOW, SUCH AS EXTREMELY LOW CARDIAC OUTPUT OR A PULMONARY EMBOLUS, INCREASES VDA. • PHYSIOLOGICAL DEAD SPACE IS THE SUM OF VDANAT AND VDA. • VD= VDANAT + VDA • VD IS INCREASED NOT ONLY BY DECREASED PULMONARY BLOOD FLOW BUT ALSO BY AN INCREASED BREATHING RATE. DOUBLING THE BREATHING FREQUENCY DOUBLES THE NUMBER OF TIMES THE VT MOVES THROUGH THE VDANAT, THUS DOUBLING THE VDANAT .

  7. ALVEOLAR VENTILATION • VA IS THE AMOUNT OF GAS ENTERING OR LEAVING THE ALVEOLI PER MINUTE. VA= VE – VD. • PRACTICALLY SPEAKING , ALL CO2 IN EXHALED GAS COMES FROM VENTILATED, PERFUSED ALVEOLI. THE SOURCE OF THIS CO2 IS TISSUE METABOLISM. NORMAL AEROBIC METABOLISM PRODUCES CO2, WHICH IS CARRIED BY VENOUS BLOOD TO THE LUNGS. THE MIXED VENOUS PCO2 (PvCO2) APPROACHING THE ALVEOLI IS SEVERAL mmHg HIGHER THAN ALVEOLAR PCO2 (PACO2). THUS CO2 DIFFUSES INTO THE ALVEOLI. • THE BALANCE BETWEEN METABOLIC CO2 PRODUCTION PER MINUTE (VCO2) AND ITS RATE OF ELIMINATION (VA) DETERMINES THE PCO2 OF THE BLOOD LEAVING THE LUNG.

  8. HYPERVENTILATION AND HYPOVENTILATION • IF VA REMOVES MORE CO2 PER MINUTE THAN IS METABOLICALLY PRODUCED, ALVEOLAR AND BLOOD PCO2 DECREASE AND A STATE OF HYPERVENTILATION EXISTS. • IF VA REMOVES LESS CO2 THAN THE BODY PRODUCES, ALVEOLAR AND BLOOD PCO2 RISE AND A STATE OF HYPOVENTILATION EXISTS. • ARTERIAL BLOOD ARISING FROM THE LEFT VENTRICLE HAS THE SAME PCO2 AS THE ALVEOLI (PACO2 = PaCO2) • VA DETERMINES ARTERIAL PCO2 BECAUSE IT CONTROLS ALVEOLAR PCO2. THE PaCO2 OBTAINED CLINICALLY THROUGH ABG ANALYSIS IS THE DEFINITIVE INDEX OF VA . • HYPERVENTILATION AND HYPOVENTILATION ARE DEFINED BY VA RELATIVE TO CO2 PRODUCTION, A RELATIONSHIP THAT CAN BE KNOWN ONLY BY MEASURING PaCO2. • IF PaCO2 IS ABOVE NORMAL HYPERCAPNIA, HYPOVENTILATION EXISTS; IF PaCO2 IS BELOW NORMAL HYPOCAPNIA, HYPERVENTILATION EXISTS.

  9. VA AND PACO2 • IF VA IS INVERSELY RELATED TO PACO2; IF VA IS REDUCED BY HALF, PACO2 DOUBLES. IF VA DOUBLES, PACO2 ( AND PaCO2) IS REDUCED BY HALF. • RATIO OF DEAD SPACE TO VT • VA CAN BE CALCULATED IF IT IS KNOWN WHAT FRACTION OF THE VT IS DEAD SPACE (VD/VT). NORMALLY, ABOUT 20% TO 40% OF THE INSPIRED VT REMAINS IN CONDUCTING AIRWAYS, NEVER REACHING ALVEOLI. • SHALLOW VTS INCREASE THE VD/VT RATIO BECAUSE CONDUCTING AIRWAY VOLUME REMAINS CONSTANT. • DEEP BREATHS DECREASE THE VD/VT FOR THE SAME REASON, CAUSING A LARGER PERCENTAGE OF THE INSPIRED VOLUME TO REACH THE ALVEOLI. • THE VD/VT RATIO CAN BE CLINICALLY MEASURED THROUGH AN ANALYSIS OF MIXED-EXHALED CARBON-DIOXIDE CONCENTRATIONS. THE PHYSIOLOGICAL DEAD-SPACE EQUATION, KNOWN AS THE BOHR EQUATION, ALLOWS THE VD/VT TO BE CALCULATED.

  10. CAPNOMETERS ARE USED IN THE CLINICAL SETTING TO ANALYZE EXHALED CO2. CAPNOGRAPHY REFERS TO THE PCO2 CHANGES OF EXHALED VTS GRAPHYCALLY DISPLAYED AS A WAVEFORM (CAPNOGRAM). CAPNOGRAMS ALLOW THE PCO2 TO BE IDENTIFIED AT THE END OF A TIDAL EXHALATION (PETCO2), WHICH CORRESPONDS TO AVERAGE ALVEOLAR PCO2 (PACO2 = PETCO2). • THE VD/VT RATIO IS A MEASURE OF VENTILTORY EFFICIENCY. A HIGH VD/VT MEANS MUCH OF THE VE IS WASTED IN VENTILATING NONPERFUSED ALVEOLI, REQUIRING HIGH-ENERGY EXPENDITURE TO ACCOMPLISH A RELATIVELY SMALL AMOUNT OF VA.

  11. VENTILATORY PATTERN, DEAD SPACE, AND VA • THE RATE AND DEPTH OF VENTILATION AFFECT VA AND THE VD/VT RATIO. VE IS NOT A RELIABLE INDICATOR OF VA . • IN A, B AND C VDANAT AND VE ARE IDENTICAL. (VE IS 8000 ml/min AND VDANAT IS 150 ml IN EACH INSTANCE).THE FIGURE B, REPRESENTS A NORMAL VT AND RESPIRATORY RATE. DEAD SPACE VENTILATION IS THE PRODUCT OF RESPIRATORY FREQUENCY AND DEAD SPACE VOLUME ( VD= VD X F). • FIGURE B, VD EQUALS 16 MULTIPLIED BY 150, WHICH EQUALS 2400 ml/min. THE VA OF 5600 ml/min IS EQUAL TO VE MINUS VD (8000-2400=5600). FOR THE FOLLOWING DISCUSSION, IT IS ASSUMED THAT THIS VA MAINTAINS A NORMAL PaCO2 OF 40 mmHg, REPRESENTING NEITHER HYPERVENTILATION NOR HYPOVENTILATION.

  12. EFFECT OF VENTILATORY PATTERN ON ALVEOLAR AND DEAD-SPACE VENTILATION

  13. VENTILATORY PATTERN, DEAD SPACE, AND VA • FIGURE A, ILLUSTRATES THE INEFFICENCY OF RAPID(TACHYPNEA), SHALLOW (HYPOPNEA) BREATHING. THE LUNG STILL ACHIEVES A VE OF 8000 ml/min. HOWEVER, VD NECESSARILY INCREASES ( VD= 32 X 150 = 4800 ml/min, COMPARED WITH A VD OF 24OO ml/min IN FIGURE B. THIS LEAVES ONLY 3200 ml/min FOR VA, COMPARED WITH 5600 ml/min IN B. THE VD/VT INCREAES ALSO; IN B IT IS 150/500, WHICH EQUALS 30%, AND IN A IT IS 150/250=60%. • THUS 70% OF THE VE IS INVOLVED IN GAS EXCHANGE IN FIGURE B, WHEREAS ONLY 40% IS SIMILARLY INVOLVED IN FIGURE A. RAPID SHALLOW BREATHING IS A COMMON SIGNAL OF RESPIRATORY DISSTRESS AND POSSIBLE VENTILATORY FAILURE.

  14. EFFECT OF VENTILATORY PATTERN ON ALVEOLAR AND DEAD-SPACE VENTILATION

  15. VENTILATORY PATTERN, DEAD SPACE, AND VA • FIGURE C, ILLUSTRATES SLOW (BRADYPNEA), DEEP(HYPERPNEA) BREATHING, WHICH ALSO ACHIEVES A VE OF 8000 ml/min . BECAUSE VDANAT IS CONSTANT, ALL THE ADDITIONAL VT ENTERS ALVEOLI, INCREASING VA . VD IS ONLY 8 X 150, WHICH EQUALS 1200 ml/min, LEAVING 6800 ml/min FOR VA . • THE VD/VT IS 150/1000 OR EQUAL TO 15%, MEANING 85% OF THE VE PARTICIPATES IN GAS EXCHANGE. SLOW, DEEP BREATHING IS THUS THE MOST EFFICIENT VENTILATORY PATTERN IN TERMS OF THE FRACTION OF VE RECEIVED BY ALVEOLI.

More Related