![]() The changes in pH are caused by an imbalance in the CO 2 (respiratory) or HCO 3– (metabolic). Broadly speaking the causes can be either metabolic or respiratory. We need to consider the driving force behind the change in pH. So we need to ask ourselves, is the pH normal, acidotic or alkalotic? Therefore, paying close attention to pH abnormalities is essential. Seemingly small abnormalities in pH have very significant and wide-spanning effects on the physiology of the human body. Drugs acting on the respiratory centre reducing overall ventilation (e.g.Reduced strength of the respiratory muscles (e.g.Reduced compliance of the lung tissue/chest wall (e.g.Increased resistance as a result of airway obstruction (e.g.Hypoventilation can occur for a number of reasons including: It occurs as a result of alveolar hypoventilation, which prevents the patient from being able to adequately oxygenate and eliminate CO 2 from their blood. Type 1 respiratory failure involves hypoxaemia (PaO 2 6.0 kPa / 45mmHg). patients with chronic obstructive pulmonary disease).³ Hypoxaemia They are suitable for all patients needing a known concentration of oxygen, but 24% and 28% Venturi masks are particularly suited to those at risk of carbon dioxide retention (e.g. Venturi masks are available in the following concentrations: 24%, 28%, 35%, 40% and 60%. Venturi masksĪ Venturi mask will give an accurate concentration of oxygen to the patient regardless of the oxygen flow rate (the minimum suggested flow rate is written on each). These masks are most suitable for trauma and emergency use where carbon dioxide retention is unlikely. Reservoir masks deliver oxygen at concentrations between 60% and 90% when used at a flow rate of 10–15 l/min.³ The concentration is not accurate and will depend on the flow of oxygen as well as the patient’s breathing pattern. ![]() These masks should not be used with flow rates less than 5L/min.³ Reservoir mask (also known as a non-rebreather mask) Simple face masks can deliver a maximum FiO 2 of approximately 40%-60% at a flow rate of 15L/min. The oxygen delivery of simple face masks is highly variable depending upon oxygen flow rate, the quality of the mask fit, the patient’s respiratory rate and their tidal volume. Below are some guides to various oxygen flow rates and the approximate percentage of oxygen delivered: 4 2 Nasal cannulaeĪs with all oxygen delivery devices, there is a significant amount of variability depending on the patient’s breathing rate, depth and how well the oxygen delivery device is fitted. Below is a quick reference guide, providing some approximate values for the various oxygen delivery devices and flow rates you’ll come across in practice. Oxygen delivery devices and flow ratesĪ common question is “What percentage of oxygen does this device deliver at a given flow rate?”. If the patient is receiving oxygen therapy their PaO 2 should be approximately 10kPa less than the % inspired concentration FiO 2 (so a patient on 40% oxygen would be expected to have a PaO 2 of approximately 30kPa /225mmHg). PaO 2 should be >10 kPa (75mmHg) when oxygenating on room air in a healthy patient. Your first question when looking at the ABG should be “Is this patient hypoxic?” as hypoxia is the most immediate threat to life.
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