Understanding Intermittent Positive Pressure Ventilation (IPPV)

Intermittent positive pressure ventilation (IPPV) is a medical technique utilised to ventilate apnoeic or dispnoeic. 

When would you use IPPV? 

There are certain indications for the use of IPPV for patient care that as healthcare professionals we must look our for: 

• Respiratory depression due to certain drugs like anesthetics 
• Apnoeic patient 
• Hypoventilation (prevent respiratory acidosis) 
• Reduce respiratory effort in senior patients 
• Cardiorespiratory arrest 
•  PaC02 (partial pressure of carbon dioxide) of > 60mmHg
• Cases like thoracotomy, rupture in the thoracic cavity or diaphragm

 Potential risks of IPPV

There are several risks associated with the use of IPPV:

• Reduction in cardiac output 
• Hypotension due to reduction of venous return and hence cardiac output (MAP=CO X TPR) 
• Decrease in lung volume 
•  Hypercapnia (elevated carbon dioxide level) 
•  Hypocapnia (decreased carbon dioxide level)
• Increase pulmonary vascular resistance 
• Lung overinflation which can lead to traumatic lung injury 
• Lung underinflation which can can lead to hypoxia 
• Barotrauma to the lungs

How to avoid barotrauma

Barotrauma is a prevalent risk factor that can be prevented by following 3 strategies that help mitigate this risk. In the case of manal IPPV a pressure gauge can be added to the system to ensure that the peak airway pressure does not exceed 20cmH2O. The appropriate tidal volume should always be supplied to the patient patient, this is easier and more accurate with the use of spirometry or a Wright’s Spirometer but can also be determined by noting the amount of chest movements .The ideal inspiration: expiration ratio of 1:2.6 should be maintained to prevent inadequate venous return to the heart  

Pre-cautions 

For patients suffering from any type of pulmonary pathology or  issue prior to ventilation an important precaution must be taken,  the peak pressure should be slightly reduced but care should be taken to ensure that an appropriate minute volume ventilation is achieved.

Types of IPPV

There are two types of IPPV, manual and mechanical.  The manual IPPV is generally less expensive but more difficult to use, involving the medical professional manually squeezing the bag to deliver breaths. Calculations such as the tidal volume, minute volume and fresh gas flow rate using the formula: Minute volume (MV) = Bodyweight (kg) x Tidal volume (TV) x Respiratory rate (RR) must also be done before manual IPPV is started. It can be done with a T piece or a circle circuit. The major downside to using manual IPPV is that there is a substantially higher risk of barotrauma . 

On the contrary the mechanical IPPV is more expensive (more equipment) but generally easier to use once adequate training is undertaken. More modern ventilators are also generally more useful as they will typically combine volume ventilation, pressure ventilation and time ventilation all into one system. 

Important values 

There are some important values to consider when setting up IPPV for a patient:

• Respiratory rate should be around 8-12 bpm to begin with but can be adjusted if necessary from there 
• Tidal volume should be around 10 to 12 ml/kg but can be adjusted from there 
• Minute ventilation should be between 150 to 250 ml/kg/min
• Ideal inspiratory to expiratory ratio should be either 1:2 or 1:3 
• Peak inspiratory pressure can change from pet to pet but typically 10 to 20 cm H20 covers most animals. More specifically, Cat: 10 – 14 cm H20, Small dog: 10 – 14 cm H20 , Medium dog: 15 – 20 cm H20 ∙ Giant dog: 18 – 25 cm H20

Method for administrating manual IPPV

1. Determine the the tidal volume, minute volume and fresh gas flow rate before administering anesthetics
2. Partially close the APL valve. After that, ensure there is no overinflating in the patient’s lung, then squeeze the reservoir bag and release
3. Wait for the reservoir bag to refill (increase the flow rate if needed) and allow the patient to fully exhale
4. Keep the respiratory rate of 10–12 breaths per minute
5. After completing the surgical procedure, lower anesthetic gases and decrease the respiratory rate, with spontaneous respiration being stimulated by the increase in carbon dioxide
6. Once the patient is breathing normally, recover as normal, but monitor closely, especially when the patient is breathing room air to avoid hypoxemia
   

Monitoring the patient 

During the procedure, it is important to maintain patient’s condition within these ranges:

Figure 1.1: Above figure illustrates the ideal oxygen saturation, heart rate, respiration rate, inspiration to expiration ratio and end-tidal carbon dioxide partial pressure in pets who are being ventilated while also being under anesthesia 

Termination of IPPV method

 In order to terminate the IPPV you must first slowly decrease the respiratory rate to allow for CO2 to build up, will be detected by the brain and patients will attempt to start breathing on their own. Easier in mechanical ventilators as you can just change the settings.

Post-anesthetic

After the IPPV has been terminated we expect that the patient;s heart rate, respirtaion rate and temperature and within these parameters. 

Figure 1.2: Above figure illustrates the normal heart rate, respiration rate and temperature of pets after anesthesia.

References

‌Robertshaw D. Temperature Regulation and Thermal Environment, in Dukes' Physiology of Domestic Animals, 12th ed., Reece WO, Ed. Copyright 2004, Cornell University Press.

Reece WO, Respiration in Mammals, in Dukes' Physiology of Domestic Animals, 12th ed., Reece WO, Ed. Copyright 2004 by Cornell University Press.