Intermittent Positive Pressure Ventilation (IPPV) for the Surgery Patient

INTERMITTENT POSITIVE PRESSURE VENTILATION (IPPV) FOR THE SURGERY PATIENT Anita Parkin AVN Dip (Surgery & ECC), VTS (Anesthesia/Analgesia), CVPP, TAE Veterinary Specialist Services, Brisbane, Queensland, Australia 

The goal of providing IPPV in the peri-operative period is to try to mimic the normal respiratory function of the patient as if it was not anesthetized. This can be done with intermittent “bagging” of the patient or with a mechanical ventilator the latter providing more precise timed volume and pressure-sensitive delivery of oxygen and anesthetic gas.

Indications 

IPPV should be initiated if the patient is not anticipated to ventilate well during surgery eg thoracotomy, use of neuromuscular blocking agents (NMBA’s), pneumothorax, decrease lung compliance etc, when intracranial pressure needs to be controlled or when the patient does not ventilate well. Anything that has an above-normal ETCO2 reading may indicate to start IPPV as hypercapnia may result in acidaemia, hyperkalaemia and reduced myocardial contractility. However, if IPPV is not carried out at the correct pressures, too much pressure can reduce cardiac output (CO), reduce mean arterial blood pressure (MAP), or cause lung trauma in the closed chest patient.

Mild hypercapnia is normal during anaesthesia, however, if ventricular arrhythmias or bradycardia (caused by sympathetic stimulation and hypoxemia) is seen with hypercapnia - IPPV should be initiated. Normal tidal volume (TV) of small animals is 8 - 15mls/kg, but 6mls/kg can be maintained for up to five hours under anaesthesia as long as there is enough pressure to maintain normal oxygenation and carbon dioxide levels. 

Terminology 

Peak Inspiratory Pressure (PIP) is generally sufficient at 12 - 15cm H2O in a closed chest patient as long as the respiratory rate (RR) is acceptable to maintain normal oxygen and carbon dioxide levels. PIP can be increased if required up to 20cm H2O to assist in delivering adequate TV to particular patients eg: patients with a distended abdomen, mass occupying the thoracic cavity or obese patients. If the chest is open, higher pressures will need to be used, and in cats generally, the pressure needs to be less. 

If there has not been enough pressure provided, lung collapse can occur within the alveoli (atelectasis), bronchi (airway closure) or capillaries (absolute/relative hypovolaemia). Too much pressure can cause a rupture of the alveoli resulting in a pneumothorax. If the lungs have been collapsed for a while, a slow increase in pressure is advised to “recruit” all lung fields gradually to avoid re-expansion pulmonary oedema from the alveoli rupturing and weeping, causing the fluid to leak out into the bronchioles. 

Positive End Expiratory Pressure (PEEP) is the pressure that remains in the lungs at the end of the expiratory phase. PEEP is very useful in keeping the alveoli “open” but it can affect the CO by maintaining constant positive pressure in the thoracic cavity, which will affect venous return to the heart if the value is set too high. For normal lungs, this can be set between 0 - 2 cmH2O. 

Inspiratory: Expiratory (I: E) ratio is the time period of inspiration to expiration, this should be set at 1:2 for all small animals to allow enough time for vascular filling. If this is set too short, it will not allow enough time for vascular filling, therefore decreasing overall CO. 

Starting ventilation 

When a patient’s ETCO2 is starting to increase and IPPV needs to be initiated, this can be done with “assisted ventilation”. This is providing a bigger breath (TV), to the patient when they breathe. This will slowly decrease their ETCO2 to a level to where the patient will not be stimulated to breathe, as the carbon dioxide levels are too low. Then “controlled ventilation” is started with a regular RR and TV to maintain normal oxygen and carbon dioxide levels, either by manual or automatic ventilation. By ensuring adequate alveolar ventilation is maintained this will assist in the uptake of inhalants and elimination of drugs and gases. 

Manual Ventilation 

Manual ventilation or “bagging” the patient is when the pop-off valve (adjustable pressure limiting (APL valve) is closed and the re-breathing bag is compressed to a certain pressure. A manometer should be used when “bagging” the patient so the operator can see the precise amount of pressure delivered to the patient, and this is generally the PIP the operator will be aiming for. Once a manual breath has been delivered the pop-off valve must then be re-opened. 

“Bagging” the patient will be providing a forced breath including the inhalant anaesthetic, therefore the patient’s depth of anaesthetic should be closely monitored to ensure the patient does not become too deep: if regular breaths are being manually delivered it would be advisable to turn down the inhalant avoiding the patient going to a deep plane of anaesthesia. 

Ambu-bag is another way of ventilating the patient. Once the patient has a secured airway the ambu bag is attached to the ET tube and by compressing the bag, a breath is delivered to the patient. These bags normally have a valve that will release so excess that pressure cannot be delivered to the patient. These are not used for delivering anesthetic gas, however they do have an oxygen port so that an oxygen line can be connected to provide oxygen-rich breaths to the patient. 

Mechanical Ventilators 

Human ventilators are obliged to meet standards for international use, whereas, veterinary specific ventilators are under no obligation. Therefore it is important for the operator to understand the design, function and troubleshooting of any ventilator before they go ahead and use one. Equally as important is to understand the indication, contraindications and physiology of IPPV prior to use. 

Veterinary ventilators can be either bellows driven (from a second gas source) or piston-driven (electronically driven). The piston-driven ventilators are more accurate in delivering the correct and consistent TV, as the variable of compressed gas pressure is not involved.

Setting up the automatic ventilator 

Before attaching a patient to a ventilator you must be very familiar with the ventilator and its settings. Firstly, check that you have the correct size bellows attached. Attach a fake lung (or another re-breathing bag) to the patient end of the circuit. This will check for any leaks, whether the machine is connected and set up correctly and gives the operator time to check the settings are appropriate for the patient (TV/pressure, RR and I:E ratio). As an extra bit of security, it is good to also attach a pressure manometer to the circuit (if one is not normally part of your anaesthetic circuit) so that you can see the different pressures administered to the patient with each breath. 

Once the machine has been connected and checked by the operator, this can now be used on the patient. Connect the patient to the anaesthetic circuit and start the ventilation. Double-check all your settings again and then monitor the patient to see if those settings are compatible to maintain normal oxygen and carbon dioxide levels and alter if required. 

Minimum continuous monitoring throughout an anaesthetic when the patient is ventilated is oxygenation, carbon dioxide and blood pressure values. These will show you how well the patient is being ventilated.

Weaning off the ventilator 

At the end of the surgery, once the drugs have been reversed (eg: neuromuscular blocking agents), the chest has been closed (if a thoracotomy has been performed) and the patient is at a plane of anaesthetic not to inhibit the respiratory function, the patient can be weaned off the ventilator. This can either be done by: slowing down the RR (but not stopping) and/or allowing an increase in carbon dioxide levels to build up. If a ventilator has been used you can decrease the RR, or take over the ventilation by hand until the patient begins spontaneous breathing - again do not stop breathing, this must be a slow transition. Turn down (off) the anaesthetic gas so the patient will begin to wake up and spontaneously start breathing or even change the patient’s position (rolling them in lateral or changing the lateral recumbency). If this process has not initiated your patient to start spontaneous breathing, a respiratory stimulant may be required.