Ultrasound imaging is commonly used in the Intensive Care Unit (ICU) to accurately and rapidly assess many pathologies such as pneumothorax, pulmonary oedema, hydronephrosis, haemoperitoneum, and deep vein thrombosis. Ultrasound imaging can also be used to assess the diaphragm in patients on mechanical ventilation. Ultrasound assessment of inspiratory muscle effort may help to identify patients at risk for self-inflicted lung injury (SILI) and improve dyssynchrony with the ventilator. As I am learning how to use the ultrasound to assess the diaphragmatic function, I thought I would write this blog to discuss the advantages of using ultrasound imaging, how to use ultrasound to assess inspiratory muscle effort and transpulmonary pressure, and highlight the clinical implications on diaphragm- and lung-protective strategy.
Before we go further, let's see how many of us are using the ultrasound to assess the thickness fraction of the diaphragm (TFdi) and apply it in the management of patients who need protection of the lungs and diaphragm during mechanical ventilation for ARDS. Please answer this question, you will be able to see the results of the poll once you place your answer:
Do you use the ultrasound to assess the the thickness fraction of the diaphragm?
- Yes
- No
- Never heard of it!
What are the advantages of using ultrasound in the ICU?
There are several advantages of using ultrasound in the Intensive Care Unit. Firstly, ultrasound is non-invasive and non-ionising. This makes it a safe and comfortable tool to use in the assessment of respiratory muscles. Secondly, ultrasound allows for real-time visualisation of the diaphragm and other respiratory muscles. This aids clinicians in the accurate assessment of ventilator dyssynchrony and consequent setting adjustments when necessary. Thirdly, clinicians can acquire the skills easily with short training and a low cost. Finally, ultrasound is widely available in the ICU and does not require a special preparation for the patient.
How to use the ultrasound to identify the diaphragm?
The linear transducer with high frequency (6-12 MHz) is placed between the anterior and midaxillary line at the 8th and 9th ribs (left image). The diaphragm can be identified between the two rib shadows separating the thorax from the abdomen. The diaphragm appears as a three-layer structure with two hyperechoic outer layers representing the pleura and peritoneum, and hypoechoic inner layer representing the diaphragmatic muscle (middle image). The M-mode shows the movement of the two layers at a certain point where we can differentiate between inspiration and expiration (right image). This view is suitable to show the shortening and thickening of the muscle, therefore able to assess the thickness fraction of the diaphragm [1].
What is the thickness fraction of the diaphragm (TFdi)?
Ultrasound can be used to assess inspiratory muscle efforts by measuring the thickness fraction of the diaphragm and the diaphragm excursion. The thickness fraction of the diaphragm (TFdi) is the ratio of the thickness of the diaphragm in inspiration compared to expiration. The thickness of the diaphragm is at the maximum during inspiration with shortening of the fibers and at minimum during relaxation in expiration. Therefore, much information can be inferred about the inspiratory efforts in patients on mechanical ventilation. TFdi provides an index of diaphragmatic contractility and correlates reasonably well with inspiratory efforts. High thickness fraction indicates high inspiratory efforts and likely large swings in pleural pressures which may signifies the transpulmonary pressure and cause more stress on the baby lung!
The normal thickness of the diaphragm in healthy people is 1.7 ± 0.2 and it is variable between supine, sitting, and upright position. In the ICU, the diaphragm is usually thicker than normal at 2.4 ± 0.8. TFdi can be measured using the B-mode or the M-mode: TFdi = (end-inspiratory thickness − end-expiratory thickness)/end-expiratory thickness × 100%. The normal value is around 15-30% in the ICU.
The diaphragm excursion is the movement of the diaphragm from its resting position (expiration) to its maximal contraction position (inspiration). Images can be obtained with the low frequency phased-array or curved-array (“abdominal”) probe (2–5 MHz) positioned just below the costal arch at the midclavicular line,. These measurements can be used to assess the strength of the inspiratory muscles and determine if they are adequate for weaning from mechanical ventilation. A value less than 11 mm may indicate weakness of respiratory muscles and failure of weaning.
For more reading on respiratory muscle ultrasonography, I advise to go to this article as the authors explained the methods, principles, and clinical applications of ultrasound in ICU and ED patients.
How can we as intensivists use TFdi?
Over the past three years, there have been more work done on understanding the role of spontaneous inspiratory efforts on mechanical ventilation in patients with ARDS. Lung protective strategy on mechanical ventilation is well-established and has been supported with hundreds of studies. However, diaphragmatic protective ventilation has recently emerged as a potential complementary therapeutic strategy in addition to lung protection, together are named lung and diaphragm protective ventilation. This post is not meant to detail this strategy, however, TFdi is one of the monitoring tools that can help protecting the diaphragm, Tfdi >30-40% may indicate high inspiratory efforts with risk of underassistance myotrauma and require better sedation, and a values <15% indicates low inspiratory efforts with risk of overassitance myotrauma. For more information on Lung- and Diaphragm-Protective Ventilation, please refer to these articles [2] and [3].
Conclusion
The use of ultrasound to assess the diaphragm has recently gained popularity as a way to noninvasively assess inspiratory muscles function. Several studies have shown the utility of ultrasound in the protection of the diaphragm among other tools. Given the potential clinical importance of this modality, intensivists should be encouraged to learn how to perform and interpret ultrasound examinations of the diaphragm.
REFERENCES
1. Tuinman, P.R., Jonkman, A.H., Dres, M. et al. Respiratory muscle ultrasonography: methodology, basic and advanced principles and clinical applications in ICU and ED patients—a narrative review. Intensive Care Med 46, 594–605 (2020).
2. Goligher EC, Dres M, Patel BK, Sahetya SK, Beitler JR, Telias I, Yoshida T, Vaporidi K, Grieco DL, Schepens T, Grasselli G, Spadaro S, Dianti J, Amato M, Bellani G, Demoule A, Fan E, Ferguson ND, Georgopoulos D, Guérin C, Khemani RG, Laghi F, Mercat A, Mojoli F, Ottenheijm CAC, Jaber S, Heunks L, Mancebo J, Mauri T, Pesenti A, Brochard L. Lung- and Diaphragm-Protective Ventilation. Am J Respir Crit Care Med. 2020 Oct 1;202(7):950-961. doi: 10.1164/rccm.202003-0655CP. PMID: 32516052; PMCID: PMC7710325.
3. Goligher, E.C., Jonkman, A.H., Dianti, J. et al. Clinical strategies for implementing lung and diaphragm-protective ventilation: avoiding insufficient and excessive effort. Intensive Care Med. 46, 2314–2326 (2020)