VENTILATION/PERFUSION RELATIONSHIPS Listen to Audio ventilation perfusion intrapulmonay shunts anesthetic effects on gas exhange VENTILATION General Distribution of ventilation Time constants General -usually measured in terms of Minute Ventilation which is the sum of all gases exhaled in one minute -assuming tidal volume is constant than MV= respiratory rate x tidal volume -average adult at rest: MV = 5L/min. Dead space -anatomicdead space part of the tidal volume which does not participate in alveolar gas exchange -alveolar dead space volume of gas which ventilates nonperfused alvoleli ( ex. pulmonary embolism) -physiologic dead space sum total of both anatomic and alveolar dead spaces Factors that effect dead space -posture uprght posture has more dead space -age increased age associated with increased dead space -artificial airway may decrease dead space -postion of artificial airway neck extension may increase dead space -positive pressure ventilation increases dead space -drugs ex. anticholingergics may increase dead space -pathology ex. pumlonary embolism, hypotension may increase dead space Calculating dead space -bohr equation used to calculate the ratio of dead space to tidal volume Vd/Vt = PAC02 - PEC02 / PAC02 ex. average adult Vt = 450 ml (6ml/kg) nl Vd/Vt = 0.33 Vd/450 = 0.33 Vd = 150 ml alveolar ventilation the volume of gas which participates in gas exchange within the alveoli in 1 minute Distribution of ventilation -uneven distribution of alveolar ventilation regardless of body position -right lung recieves greater ventilation than the left lung (53% vs 47%) -lower dependant areas of the lung are generally better ventilated compared to upper nondependant areas of the lung -increased ventilation in lower dependant areas due to a gravitational induced gradient in the intrapleural pressure Alveoli general properties of alveolar compliance -pleural pressure decreases about 1 cm H20 with every 3 cm decrease in lung height -the decreasing pleural pressure creates different compliance values for alveoli depending on the location of the lung -alveoli in upper regions are less compliant and alveoli in lower regions are more compliant (as described below) Alveoli in upper lung areas -nearly maximally inflated even at the begining of inspiration -relatively noncompliant and therefore allow for little more expansion upon inspiration Alveoli in lower lung areas -generally smaller and compressed before the start of inspiration -lower transpulmonary pressure within the alveoli -more compliant therefore allow for greater expansion during inspiration Factors affecting complete alveolar filling (ventilation) -airway resistance -compliance properties of alveoli -inspiratory time which is affected by respiratory rate and time for expiration Time constants -used to mathematically describe lung inflation -time constant = Total compliance x Airway Resistance Variations of time constants ex. normal lung breathing spontaneously at rapid respiratory rates rapid shallow breathing revereses the normal distribution of ventilation upper nondependant lung areas are ventilated better than the lower dependant lung areas Pulmonay perfusion -distribution of pulmonary perfusion -ventilation/perfusion ratios Distribution of pulmonary perfusion -uneven distribution of pulmonary blood flow regardless of body position -lower dependent portions of the lung recieve greater pulmonary blood flow than upper nondependent portions of lung -uneven distribution due to gravitational gradient of 1cmH20 / cm lung height -gravity has a significant influence on pulmonary blood flow due to a low pressure pulmonary circulation Lung zones - theoretical zones created based upon pressure gradients (PA, Pa, Pv) zone 1: PA > Pa > Pv upper lung : dead space due to alveolar pressure occluding pulmonary capillaries zone 2: Pa > PA > Pv middle lung: intermittent pulmonary capillary flow varying with respirations (a-A pressure grad) zone 3: Pa > Pv > PA lower lung continuous pulmonary capillary flow proportional to art-venous pressure grad. zone 4: Pisf> Pa > Pv> PA Ventilation/perfusion ratios -normal ventilation = 4L/min -normal perfusion = 5L/min -normal V/Q ratio = 0.8 normal range of V/Q ratios: 0.3 - 3.0 -V/Q ratio = infinity represents dead space: ventilation with no perfusion (high ventilation / 0 perfusion = infinity) -V/Q ratio = Zero represents shunting : perfusion of nonventilated alveoli ( 0 ventilation/ nl perfusion= zero) V/Q ratios in lung regions -relates to the effeciency which lung units can resaturate venous blood and eliminate carbon dioxide -both ventilation and perfusion increase from top to bottom of lung regions -perfusion increases at a greater rate than ventilation from top to bottom of lung regions -because of the greater increase of perfusion than ventilation from top to bottom...differing V/Q values ex. pulmonary venous blood (mixed venous blood) has low V/Q ratios - depresses arterial oxygen tension - increases arterial CO2 tension Upper nondependant region: -dead space -has a higher V/Q ratio -although both ventilation and perfusion are decreased in the upper region of the lung, ventilation is greater than perfusion -ventilation > perfusion creates a higher V/Q ratio Lower dependant region: -intrapulmonary shunting -has a lower V/Q ratio -although both ventilation and perfusion are increased in the lower region of the lung, perfusion is greater than ventilation -perfusion > ventilation creates a lower V/Q ratio Intrapulmonary shunts Right to left shunt -desaturated (mixed venous) blood from the right heart returns to the left heart without having been re-oxgentated from the lung -decreases arterial oxygen tension (PaO2) -hypoxemia with profound right to left shunts Left to right shunt -oxygenated blood from the left heart mixed with deoxygenated blood from the right heart -generally do not cause hypoxemia Anatomic shunts -where lung units V/Q ratios = 0 -cannot be corrected with increased inspired concentration of O2 (FIO2) Relative shunts -where lung units V/Q is low but finite -usually can be corrected with increased inspired concentration of O2 (FIO2) Venous admixture -amount of mixed venous blood required to be mixed with pulmonary end-capillary blood to account for the diffence in oxygen tension Shunt fraction: normal physiologic shunting < 5% Qs/Qt : the fraction of shunted blood compared to total cardiac output Sources of shunting -deep bronchial viens -pulmonary veins -thebesian circulation in the heart -relative intrapulmonary shunts Anesthetic effects on gas exchange -increased dead space most commonly with mechanical ventilation than with spontaneous ventilation -hypoventilation -increased intrapulmonary shunting -increased scatter of V/Q ratios -may increase Qs/Qt to 5-10% possible from atelectasis and collapse of airways, elderly most affected (Qs/Qt upto 30-40%) -inhibition of hypoxic pulmonary vasoconstriction -possible increases in absolute shunt with FIO2>50% -absortion atelectasis