Ventilation/perfusion

Electrocardiograph normal sinus rhythm Chest x-ray slightly enlarged heart Ventilation/perfusion (V/Q) scan high probability of PE... 

Patients with an oxygen saturation less than 90% (less than 95% in children, pregnant women, and patients with co-existing heart disease) should receive oxygen with the dose adjusted to keep oxygen saturation above these levels.3,12,40 Hypoxemia usually results from a ventilation/perfusion mismatch, and low oxygen levels (less than 30% of the fraction of inspired air) administered by nasal cannula or facemask are sufficient to reverse hypoxemia in most patients. 

In advanced COPD, airflow obstruction, damaged bronchioles and alveoli, and pulmonary vascular abnormalities lead to impaired gas exchange. This results in hypoxemia and eventually hypercapnia. Hypoxemia is initially present only during exercise but occurs at rest as the disease progresses. Inequality in the ventilation/perfusion ratio (VAQ) is the major mechanism behind hypoxemia in COPD. 

Ventilation-perfusion ratio (VA/Q) A comparison of the proportion of lung tissue being ventilated by inhaled air to the rate of oxygenation of pulmonary blood. 

Explain the effects of airway obstruction and obstructed blood flow on ventilation-perfusion matching... 

Ventilation-perfusion mismatch leads to hypoxemia. Reduced ventilation caused by obstructed airflow or reduced perfusion caused by obstructed blood flow leads to impaired gas exchange. Interestingly, each of these conditions is minimized by local control mechanisms that attempt to match airflow and blood flow in a given lung unit. 

Because contrast studies are expensive, invasive, and technically difficult to perform and evaluate, noninvasive tests (e.g., ultrasonography, computed tomography scans, and the ventilation-perfusion scan) are used frequently for the initial evaluation of patients with suspected VTE. 

Note that there is no difference between the ideal alveolar value and the normal arterial Pao2 of 13.3 kPa. In practice a difference of up to 2 kPa is allowable owing to ventilation-perfusion (V/Q) mismatch and shunt. 

Cyanide toxicity, overshoot hypotension, and myocardial ischaemia. Hypoxia caused by increased ventilation-perfusion mismatch due to pulmonary vasodilatation and inhibition of hypoxic pulmonary vasoconstriction. Rebound hypertension after discontinuation of SNP infusion. 

Calcium channel blockers with vasodilator effects, such as nifedipine, nicardipine, and nimodipine, will potentiate the effect of vasodilator effects of, e.g. halothane or isoflurane, potentiating any hypotension. This is especially obvious in hypertensive patients and when combined with similarly acting agents, such as sodium nitroprusside or nitroglycerin. Similarly, they also enhance the tendency of volatile anaesthetics to reduce hypoxic pulmonary vasoconstriction, which might exacerbate ventilation/perfusion mismatching during anaesthesia. 

Copper iron sulfide dust explosion 1 miner died within minutes. The other 2 experienced intense bunting of eyes, nose, and throat dyspnea diffuse pircoedial and retro sternal chest pain nausea, vomiting urinary incontinence. 3 wk after the exposure, the workers had severe airway obstruction, hypoxemia, markedly decreased exercise tolerance, ventilation-perfusion mismatch, evidence of active inflammation (positive gallium scar)... 

Q13 Chandra s blood gas composition is abnormal. One factor which contributes to this is ventilation-perfusion inequality. How is alveolar ventilation normally matched to perfusion in the lung ... 

Hypoxia can be caused by (i) decreased ambient oxygen concentration, (ii) hypoventilation, (iii) decreased diffusion across the respiratory surface and (iv) by mismatching of alveolar ventilation with perfusion. In this patient both decreased diffusion and ventilation-perfusion inequality is present. Chandra s blood is low in oxygen the hypoxia acts as a stimulus for erythropoietin release, which in turn increases red cell production. 

The reduced CO transfer factor shows that the transfer of gas from alveoli to blood is compromised this is probably due to the ventilation-perfusion inequality usually observed in chronic bronchitis, which limits the respiratory surface area available for gas exchange. 

Q7 If the embolus is quite large and obstructs a significant area of the pulmonary circulation, the affected area of lung will be underperfused or non-perfused. The area may continue to be ventilated for some time, causing a ventilation-perfusion mismatch, which leads to poor gas exchange and abnormal blood gas tensions. The lung volume in the affected area decreases, and this decrease in size can sometimes be seen on a chest X-ray. After some hours, surfactant production declines in the non-perfused area of lung and the alveoli collapse. 

Stewart, W. E., and S. M. Mastenbrook, Parametric estimation of ventilation-perfusion ratio distributions, J. Appl. Physiol Respir., Environ. Exerc. Physiol, 55, 37-51 (1983b) Errata, 56, No. 6 (1984). 

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